Mandatory and optional structural components of a bacterial cell, their functions. Differences in the structure of the cell wall of gram-positive and gram-negative bacteria. L-forms and unculturable forms of bacteria
Bacteria are prokaryotes and differ significantly from plant and animal cells (eukaryotes). They belong to single-celled organisms and consist of a cell wall, cytoplasmic membrane, cytoplasm, nucleoid (obligatory components of a bacterial cell). Some bacteria may have flagella, capsules, and spores (optional components of the bacterial cell).
In a prokaryotic cell, the structures located outside the cytoplasmic membrane are called superficial (cell wall, capsule, flagella, villi).
The cell wall is an important structural element of the bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria, this is the outer cell membrane. Performs a number of functions: protects bacteria from osmotic shock and other damaging factors, determines their shape, participates in metabolism; in many types of pathogenic bacteria it is toxic, contains surface antigens, and also carries specific receptors for phages on the surface. The bacterial cell wall contains pores that are involved in the transport of exotoxins and other bacterial exoproteins.
The main component of the bacterial cell wall is peptidoglycan, or murein (Latin murus - wall), a supporting polymer that has a network structure and forms a rigid (hard) outer framework of the bacterial cell. Peptidoglycan has a main chain (backbone) consisting of alternating N-acetyl-M-glucosamine and N-acetylmuramic acid residues connected by 1,4-glycosidic bonds, identical tetrapeptide side chains attached to N-acetylmuramic acid molecules, and short cross-peptide chains bridges connecting polysaccharide chains.
Based on their tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. Gram-positive bacteria firmly fix the complex of gentian violet and iodine, are not subject to bleaching with ethanol and therefore do not perceive the additional dye fuchsin, remaining purple. In gram-negative bacteria, this complex is easily washed out of the cell by ethanol, and upon additional application of fuchsin, they turn red. In some bacteria, a positive Gram stain is observed only in the active growth stage. The ability of prokaryotes to be Gram stained or decolorized by ethanol is determined by the specific chemical composition and the ultrastructure of their cell walls. bacterial chlamydia trachoma
L-forms of bacteria are phenotypic modifications, or mutants, of bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan. Thus, L-forms are bacteria defective in the cell wall. They are formed under the influence of L-transforming agents - antibiotics (penicillin, polymyxin, bacitracin, vencomycin, streptomycin), amino acids (glycine, methionine, leucine, etc.), the enzyme lysozyme, ultraviolet and x-rays. Unlike protoplasts and spheroplasts, L-forms have relatively high viability and a pronounced ability to reproduce. In terms of morphological and cultural properties, they differ sharply from the original bacteria, which is due to the loss of the cell wall and changes in metabolic activity. L-form cells have a well-developed system of intracytoplasmic membranes and myelin-like structures. Due to a defect in the cell wall, they are osmotically unstable and can only be cultured in special media with high osmotic pressure; they pass through bacterial filters. There are stable and unstable L-forms of bacteria. The former are completely devoid of a rigid cell wall; they extremely rarely revert to their original bacterial forms. The latter may have elements of a cell wall, in which they are similar to spheroplasts; in the absence of the factor that caused their formation, they are reverted to the original cells.
The process of formation of L-forms is called L-transformation or L-induction. Almost all types of bacteria, including pathogenic ones (causative agents of brucellosis, tuberculosis, listeria, etc.), have the ability to undergo L-transformation.
L-shapes are given great importance in the development of chronic recurrent infections, carriage of pathogens, their long-term persistence in the body. The infectious process caused by L-forms of bacteria is characterized by atypicality, duration of course, severity of the disease, and is difficult to treat with chemotherapy.
The capsule is a mucous layer located above the cell wall of the bacterium. The substance of the capsule is clearly demarcated from the environment. The capsule is not an essential structure of the bacterial cell: its loss does not lead to the death of the bacterium.
The substance of the capsules consists of highly hydrophilic micelles, and their chemical composition is very diverse. The main components of most prokaryotic capsules are homo- or hetsropolysaccharides (entsrobacteria, etc.). In some types of bacilli, capsules are built from a polypeptide.
Capsules ensure the survival of bacteria, protecting them from mechanical damage, drying out, infection by phages, toxic substances, and in pathogenic forms - from the action of the protective forces of the macroorganism: encapsulated cells are poorly phagocytosed. In some types of bacteria, including pathogenic ones, it promotes the attachment of cells to the substrate.
Flagella are organelles of bacterial movement, represented by thin, long, thread-like structures of a protein nature.
The flagellum consists of three parts: a spiral filament, a hook and a basal body. The hook is a curved protein cylinder that acts as a flexible link between the basal body and the rigid filament of the flagellum. Basal body - complex structure, consisting of a central rod (axis) and rings.
Flagella are not vital structures of a bacterial cell: there are phase variations in bacteria, when they are present in one phase of cell development and absent in another.
Number of flagella and places of their localization in bacteria different types not the same, but stable for one species. Depending on this, the following groups of flagellated bacteria are distinguished: moiotrichs - bacteria with one polarly located flagellum; amphitrichous - bacteria with two polarly arranged flagella or having a bundle of flagella at both ends; lophotrichs - bacteria with a bundle of flagella at one end of the cell; peritrichous - bacteria with many flagella located on the sides of the cell or on its entire surface. Bacteria that do not have flagella are called atrichia.
Being organs of movement, flagella are typical of floating rod-shaped and convoluted forms of bacteria and are found only in isolated cases in cocci. They provide efficient movement in liquid media and slower movement on the surface of solid substrates.
Pili (fimbriae, villi) are straight, thin, hollow protein cylinders extending from the surface of the bacterial cell. They are formed by a specific protein - pilin, originate from the cytoplasmic membrane, are found in motile and immobile forms of bacteria and are visible only in an electron microscope. On the surface of the cell there can be from 1-2, 50-400 or more pili to several thousand.
There are two classes of pili: sexual pili (sexpili) and general pili, which are more often called fimbriae. The same bacterium can have pili of different natures. Sex pili appear on the surface of bacteria during the process of conjugation and perform the function of organelles through which genetic material (DNA) is transferred from donor to recipient.
Pili take part in the aggregation of bacteria into agglomerates, the attachment of microbes to various substrates, including cells (adhesive function), in the transport of metabolites, and also contribute to the formation of films on the surface of liquid media; cause agglutination of red blood cells.
The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an obligatory multifunctional component of the cell. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell.
Chemically, the cytoplasmic membrane is a protein-lipid complex consisting of proteins and lipids. The main part of membrane lipids is represented by phospholipids. It is built from two monomolecular protein layers, between which there is a lipid layer consisting of two rows of regularly oriented lipid molecules.
The cytoplasmic membrane serves as an osmotic barrier to the cell, controls the flow of nutrients into the cell and the release of metabolic products to the outside; it contains substrate-specific permease enzymes that carry out active selective transfer of organic and inorganic molecules.
During cell growth, the cytoplasmic membrane forms numerous invaginates that form intracytoplasmic structures of the membrane. Local membrane invaginates are called mesosomes. These structures are well expressed in gram-positive bacteria, worse in gram-negative bacteria, and poorly expressed in rickettsia and mycoplasmas.
Mesosomes, like the cytoplasmic membrane, are centers of bacterial respiratory activity, so they are sometimes called analogues of mitochondria. However, the significance of mesosomes has not yet been fully elucidated. They increase the working surface of the membranes, perhaps performing only a structural function, dividing the bacterial cell into relatively separate compartments, which creates more favorable conditions for the occurrence of enzymatic processes. In pathogenic bacteria they ensure the transport of protein molecules of exotoxins.
Cytoplasm is the contents of a bacterial cell, delimited by a cytoplasmic membrane. It consists of cytosol - a homogeneous fraction, including soluble RNA components, substrate substances, enzymes, metabolic products, and structural elements - ribosomes, intracytoplasmic membranes, inclusions and nucleoid.
Ribosomes are organelles that carry out protein biosynthesis. They consist of protein and RNA, connected into a complex by hydrogen and hydrophobic bonds.
Various types of inclusions are detected in the cytoplasm of bacteria. They can be solid, liquid or gaseous, with or without a protein membrane, and are not permanently present. A significant part of them are reserve nutrients and products of cellular metabolism. Reserve nutrients include: polysaccharides, lipids, polyphosphates, sulfur deposits, etc. Among inclusions of a polysaccharide nature, glycogen and the starch-like substance granulosa are most often found, which serve as a source of carbon and energy material. Lipids accumulate in cells in the form of granules and fat droplets. Mycobacteria accumulate waxes as reserve substances. The cells of some spirilla and others contain volutin granules formed by polyphosphates. They are characterized by metachromasia: toluidine blue and methylene blue color them violet-red. Volutin granules play the role of phosphate depots. Inclusions surrounded by a membrane also include gas vacuoles, or aerosomes; they reduce the specific gravity of cells and are found in aquatic prokaryotes.
Nucleoid is the nucleus of prokaryotes. It consists of one double-stranded DNA strand closed in a ring, which is considered as a single bacterial chromosome, or genophore.
The nucleoid in prokaryotes is not delimited from the rest of the cell by a membrane - it lacks a nuclear envelope.
The nucleoid structures include RNA polymerase, basic proteins and lack histones; the chromosome is anchored on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosome. The nucleoid does not have a mitotic apparatus, and the separation of daughter nuclei is ensured by the growth of the cytoplasmic membrane.
The bacterial core is a differentiated structure. Depending on the stage of cell development, the nucleoid can be discrete (discontinuous) and consist of individual fragments. This is due to the fact that the division of a bacterial cell in time occurs after the completion of the replication cycle of the DNA molecule and the formation of daughter chromosomes.
The nucleoid contains the bulk of the genetic information of the bacterial cell.
In addition to the nucleoid, extrachromosomal genetic elements have been found in the cells of many bacteria - plasmids, which are small circular DNA molecules capable of autonomous replication
Some bacteria are capable of forming spores at the end of the period of active growth. This is preceded by a depletion of the environment in nutrients, a change in its pH, and the accumulation of toxic metabolic products.
In terms of chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. Spores contain less water and more lipids.
In the spore state, microorganisms are metabolically inactive, withstand high temperatures (140-150 °C), exposure to chemical disinfectants and persist for a long time in the environment. High temperature resistance is associated with very low water content and high dipicolinic acid content. Once in the body of humans and animals, the spores germinate into vegetative cells. Spores are painted using a special method, which includes preheating the spores, as well as exposure to concentrated paint solutions at high temperatures.
Many types of gram-negative bacteria, including pathogenic ones (Shigella, Salmonella, Vibrio cholerae, etc.) have a special adaptive, genetically regulated state, physiologically equivalent to cysts, into which they can pass under the influence of unfavorable conditions and remain viable for up to several years. The main feature of this condition is that such bacteria do not reproduce and therefore do not form colonies on a solid nutrient medium. Such non-reproducing but viable cells are called unculturable forms of bacteria (NFB). NFB cells in an uncultured state have active metabolic systems, including electron transfer systems, protein and nucleic acid biosynthesis, and retain virulence. Their cell membrane is more viscous, the cells usually take the form of cocci and are significantly reduced in size. NFBs have a higher stability in the external environment and therefore can survive in it for a long time (for example, Vibrio cholerae in a dirty reservoir), maintaining the endemic state of a given region (reservoir).
To detect NFB, molecular genetic methods are used (DNA-DNA hybridization, CPR), as well as a simpler method of direct counting of viable cells.
For these purposes, you can also use cytochemical methods (formazan formation) or microautoradiography. The genetic mechanisms that determine the transition of bacteria into the NS and their reversion from it are not clear.
Structure and chemical composition of bacterial
cells
The general structure of a bacterial cell is shown in Figure 2. The internal organization of a bacterial cell is complex. Each systematic group of microorganisms has its own specific structural features.
Cell wall. The bacterial cell is covered with a dense membrane. This surface layer, located outside the cytoplasmic membrane, is called the cell wall (Fig. 2, 14). The wall performs protective and supporting functions, and also gives the cell a permanent, characteristic shape (for example, the shape of a rod or coccus) and represents the external skeleton of the cell. This dense shell makes bacteria similar to plant cells, which distinguishes them from animal cells, which have soft shells.
Inside the bacterial cell, the osmotic pressure is several times, and sometimes tens of times, higher than in the external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as the cell wall.
The thickness of the cell wall is 0.01-0.04 microns. It makes up from 10 to 50% of the dry mass of bacteria. The amount of material that makes up the cell wall changes during bacterial growth and usually increases with age.
The main structural component of the walls, the basis of their rigid structure in almost all bacteria studied so far is murein (a glycopeptide
mucopeptide). This is an organic compound of a complex structure, which includes nitrogen-carrying sugars - amino sugars and 4-5 amino acids. Moreover, cell wall amino acids have an unusual shape (D-stereoisomers), which is rarely found in nature.
The constituent parts of the cell wall, its components, form a complex, strong structure (Fig. 3, 4 and 5).
Using a staining method first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-positive And
gram-negative. Gram-positive organisms are able to bind some aniline dyes, such as crystal violet, and after treatment with iodine and then alcohol (or acetone) retain the iodine-dye complex. The same bacteria that are influenced ethyl alcohol this complex is destroyed (cells become discolored) and are classified as gram-negative.
The chemical composition of the cell walls of gram-positive and gram-negative bacteria is different.
In gram-positive bacteria, the composition of the cell walls includes, in addition to mucopeptides, polysaccharides (complex, high-molecular sugars), teichoic acids
(complex compounds in composition and structure, consisting of sugars, alcohols, amino acids and phosphoric acid). Polysaccharides and teichoic acids are associated with the wall framework - murein. We do not yet know what structure these components of the cell wall of gram-positive bacteria form. Using electronic photographs of thin sections (layering), no gram-positive bacteria were detected in the walls.
Probably all these substances are very tightly interconnected.
The walls of gram-negative bacteria are more complex in chemical composition; they contain a significant amount of lipids (fats) associated with proteins and sugars into complex complexes - lipoproteins and lipopolysaccharides. There is generally less murein in the cell walls of gram-negative bacteria than in gram-positive bacteria.
The wall structure of gram-negative bacteria is also more complex. Using an electron microscope, it was found that the walls of these bacteria are multilayered (Fig.
6).
The inner layer consists of murein. Above this is a wider layer of loosely packed protein molecules. This layer is in turn covered with a layer of lipopolysaccharide. The topmost layer consists of lipoproteins.
The cell wall is permeable: nutrients pass through it freely into the cell, and metabolic products exit into the cell. environment. Large molecules with high molecular weight do not pass through the shell.
Capsule. The cell wall of many bacteria is surrounded on top by a layer of mucous material - a capsule (Fig. 7). The thickness of the capsule can be many times greater than the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule.
The capsule is not an essential part of the cell; it is formed depending on the conditions in which the bacteria find themselves. It serves as a protective cover for the cell and participates in water metabolism, protecting the cell from drying out.
The chemical composition of capsules is most often polysaccharides.
Sometimes they consist of glycoproteins (complex complexes of sugars and proteins) and polypeptides (genus Bacillus), in rare cases - of fiber (genus Acetobacter).
Mucous substances secreted into the substrate by some bacteria cause, for example, the mucous-stringy consistency of spoiled milk and beer.
Cytoplasm. The entire contents of a cell, with the exception of the nucleus and cell wall, are called cytoplasm. The liquid, structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, mitochondria, plastids and other structures, as well as reserve nutrients. The cytoplasm has an extremely complex, fine structure (layered, granular). Using an electron microscope, many interesting details of the cell structure have been revealed.
The outer lipoprotoid layer of the bacterial protoplast, which has special physical and chemical properties, is called the cytoplasmic membrane (Fig.
2, 15).
Inside the cytoplasm are all vital structures and organelles.
The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products to the outside.
Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes. In addition, the synthesis of some cell components occurs in the membrane, mainly components of the cell wall and capsule.
Finally, the cytoplasmic membrane contains the most important enzymes (biological catalysts). The ordered arrangement of enzymes on membranes makes it possible to regulate their activity and prevent the destruction of some enzymes by others. Associated with the membrane are ribosomes - structural particles on which protein is synthesized.
The membrane consists of lipoproteins. It is strong enough and can ensure the temporary existence of a cell without a shell. The cytoplasmic membrane makes up up to 20% of the dry mass of the cell.
In electronic photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75A thick, consisting of a light layer
(lipids) sandwiched between two darker ones (proteins). Each layer has a width
20-30A. Such a membrane is called elementary (Table 30, Fig. 8).
There is a connection between the plasma membrane and the cell wall in the form of desmoses
- bridges. The cytoplasmic membrane often gives rise to invaginations - invaginations into the cell. These invaginations form special membrane structures in the cytoplasm called
mesosomes. Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside these membrane sacs (Fig. 2). These structures perform a variety of functions in bacteria. Some of these structures are analogues of mitochondria. Others perform the functions of the endoplasmic reticulum or Golgi apparatus. By invagination of the cytoplasmic membrane, the photosynthetic apparatus of bacteria is also formed.
After invagination of the cytoplasm, the membrane continues to grow and forms stacks (Table 30), which, by analogy with plant chloroplast granules, are called thylakoid stacks. Pigments (bacteriochlorophyll, carotenoids) and enzymes are localized in these membranes, which often fill most of the cytoplasm of the bacterial cell.
(cytochromes) that carry out the process of photosynthesis.
,
The cytoplasm of bacteria contains ribosomes, protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cage. Ribosomes consist of RNA and protein. In bacteria, many ribosomes are freely located in the cytoplasm, some of them may be associated with membranes.
Ribosomes are centers of protein synthesis in the cell. At the same time, they often connect with each other, forming aggregates called polyribosomes or polysomes.
The cytoplasm of bacterial cells often contains granules of various shapes and sizes.
However, their presence cannot be considered as some kind of permanent sign of a microorganism; it is usually largely related to the physical and chemical conditions of the environment. Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These reserve substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body finds itself in conditions less favorable in terms of nutrition.
In many bacteria, granules consist of starch or other polysaccharides - glycogen and granulosa. Some bacteria, when grown in a sugar-rich medium, have droplets of fat inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules consist of polymetaphosphate (a reserve substance containing phosphoric acid residues).
Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria are more likely to accumulate volutin under unusual nutritional conditions, such as sulfur-free media. In the cytoplasm of some sulfur bacteria there are droplets of sulfur.
In addition to various structural components, the cytoplasm consists of a liquid part - the soluble fraction. It contains proteins, various enzymes, t-RNA, some pigments and low molecular weight compounds - sugars, amino acids.
As a result of the presence of low molecular weight compounds in the cytoplasm, a difference arises in the osmotic pressure of the cellular contents and the external environment, and this pressure may be different for different microorganisms. The highest osmotic pressure is observed in gram-positive bacteria - 30 atm; in gram-negative bacteria it is much lower - 4-8 atm.
Nuclear apparatus. The nuclear substance, deoxyribonucleic acid (DNA), is localized in the central part of the cell.
,
Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have its analogue -
"nuclear equivalent" - nucleoid(see Fig. 2, 8), which is an evolutionarily more primitive form of organization of nuclear matter. Microorganisms that do not have a real nucleus, but have an analogue of it, are classified as prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, the bulk of DNA is concentrated in one or several places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The core is surrounded by a shell membrane.
In bacteria, DNA is packed less tightly, unlike true nuclei; A nucleoid does not have a membrane, a nucleolus, or a set of chromosomes. Bacterial DNA is not associated with the main proteins - histones - and is located in the nucleoid in the form of a bundle of fibrils.
Flagella. Some bacteria have appendage structures on the surface; The most widespread of them are flagella - the organs of movement of bacteria.
The flagellum is anchored under the cytoplasmic membrane using two pairs of discs.
Bacteria may have one, two, or many flagella. Their location is different: at one end of the cell, at two, over the entire surface, etc. (Fig. 9). Bacterial flagella have a diameter
0.01-0.03 microns, their length can be many times greater than the length of the cell. Bacterial flagella consist of a protein - flagellin - and are twisted helical filaments.
On the surface of some bacterial cells there are thin villi -
fimbriae.
Life of plants: in 6 volumes. - M.: Enlightenment. Edited by A. L. Takhtadzhyan, chief
Editor Corresponding Member USSR Academy of Sciences, prof. A.A. Fedorov. 1974
- Structure and chemical composition of a bacterial cell
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CYTOPLASMA (CP)
Participate in spore formation.
MESOSOMA
With excessive growth, compared with the growth of the CS, the CPM forms intussusceptions (invaginations) - mesosomes. Mesosomes are the center of energy metabolism of a prokaryotic cell. Mesosomes are analogues of eukaryotic mitochondria, but are simpler in structure.
Well-developed and complexly organized mesosomes are characteristic of Gram+ bacteria.
Bacterial cell wall
In Gram bacteria, mesosomes are less common and are simply organized (loop-shaped). Polymorphism of mesosomes is observed even in the same species of bacteria. Rickettsia does not have mesosomes.
Mesosomes vary in size, shape and location in the cell.
Mesosomes are classified according to their shape:
– – lamellar (lamellar),
- - vesicular (shaped like bubbles),
– – tubular (tubular),
- - mixed.
Mesosomes are classified according to their location in the cell:
– – formed in the zone of cell division and formation of the transverse septum,
- - to which the nucleoid is attached;
– – formed as a result of intussusception of peripheral areas of the CPM.
Functions of mesosomes:
1. Strengthen the energy metabolism of cells, since they increase the total “working” surface of the membranes.
2. Participate in secretory processes(in some Gram+ bacteria).
3. Participate in cell division. During reproduction, the nucleoid moves to the mesosome, receives energy, doubles and divides by amitosis.
Identification of mesosomes:
1. Electron microscopy.
Structure. Cytoplasm (protoplasm) is the contents of the cell, surrounded by the cytoplasm and occupying the main volume of the bacterial cell. The CP is the internal environment of the cell and is a complex colloidal system consisting of water (about 75%) and various organic compounds (proteins, RNA and DNA, lipids, carbohydrates, minerals).
The layer of protoplasm located under the CPM is denser than the rest of the mass in the center of the cell. The fraction of the cytoplasm, which has a homogeneous consistency and contains a set of soluble RNA, enzyme proteins, products and substrates of metabolic reactions, is called cytosol. The other part of the cytoplasm is represented by various structural elements: nucleoid, plasmids, ribosomes and inclusions.
Functions of the cytoplasm:
1. Contains cellular organelles.
Detection of cytoplasm:
1. Electron microscopy.
Structure. Nucleoid - equivalent to the nucleus of eukaryotes, although it differs from it in its structure and chemical composition. The nucleoid is not separated from the CP by a nuclear membrane, does not have nucleoli and histones, contains one chromosome, has a haploid (single) set of genes, and is not capable of mitotic division.
The nucleoid is located in the center of the bacterial cell and contains a double-stranded DNA molecule, a small amount of RNA and proteins. In most bacteria, a double-stranded DNA molecule with a diameter of about 2 nm, a length of about 1 m with a molecular weight of 1–3x109 Da is closed in a ring and tightly packed like a ball. Mycoplasmas have the smallest DNA molecular weight for cellular organisms (0.4–0.8×109 Da).
The DNA of prokaryotes is built in the same way as that of eukaryotes (Fig. 25).
Rice. 25. Structure of prokaryotic DNA:
A- a fragment of a DNA strand formed by alternating deoxyribose and phosphoric acid residues. The first carbon atom of deoxyribose has a nitrogen base attached to it: 1 - cytosine; 2 - guanine.
B- DNA double helix: D- deoxyribose; F - phosphate; A - adenine; T - thymine; G - guanine; C - cytosine
The DNA molecule carries many negative charges because each phosphate residue contains an ionized hydroxyl group. In eukaryotes, negative charges are neutralized by the formation of a complex of DNA with the main proteins - histones. There are no histones in prokaryotic cells, so charges are neutralized by the interaction of DNA with polyamines and Mg2+ ions.
By analogy with eukaryotic chromosomes, bacterial DNA is often referred to as a chromosome. She is presented in a cage in singular, since bacteria are haploid. However, before cell division, the number of nucleoids doubles, and during division it increases to 4 or more. Therefore, the terms “nucleoid” and “chromosome” are not always the same. When cells are exposed to certain factors (temperature, pH, ionizing radiation, salts) heavy metals, some antibiotics, etc.) the formation of many copies of the chromosome occurs. When the influence of these factors is eliminated, as well as after the transition to the stationary phase, one copy of the chromosome is found in the cells.
For a long time it was believed that there was no pattern in the distribution of DNA strands on the bacterial chromosome. Special studies have shown that prokaryotic chromosomes are a highly ordered structure. Part of the DNA in this structure is represented by a system of 20–100 independently supercoiled loops. Supercoiled loops correspond to DNA regions that are currently inactive and are located in the center of the nucleoid. Along the periphery of the nucleoid there are despiralized areas where messenger RNA (mRNA) is synthesized. Since transcription and translation processes occur simultaneously in bacteria, the same mRNA molecule can be simultaneously associated with DNA and ribosomes.
In addition to the nucleoid, the cytoplasm of a bacterial cell may contain plasmids - factors of extrachromosomal heredity in the form of additional autonomous circular molecules of double-stranded DNA with a lower molecular weight. Plasmids also encode hereditary information, but it is not vital for the bacterial cell.
Nucleiod functions:
1. Storage and transmission of hereditary information, including the synthesis of pathogenicity factors.
Nucleoid detection:
1. Electron microscopy: in electron diffraction patterns of ultrathin sections, the nucleoid appears as light zones of lower optical density with fibrillar, thread-like DNA structures (Fig. 26). Despite the absence of a nuclear membrane, the nucleoid is quite clearly demarcated from the cytoplasm.
2. Phase contrast microscopy of native preparations.
3. Light microscopy after staining with DNA-specific methods according to Feulgen, according to Pashkov or according to Romanovsky-Giemsa:
– the drug is fixed with methyl alcohol;
– Romanovsky-Giemsa dye (a mixture of equal parts of three dyes - azure, eosin and methylene blue, dissolved in methanol) is poured onto the fixed preparation for 24 hours;
– the paint is drained, the preparation is washed with distilled water, dried and microscoped: the nucleoid is stained purple and is located diffusely in the cytoplasm, colored pale pink.
Read also:
Features of the chemical composition of bacterial cells
Structure of a bacterial cell. The main differences between prokaryotes and eukaryotes. Functions of individual structural elements of a bacterial cell. Features of the chemical composition of the cell walls of gram-positive and gram-negative bacteria.
A bacterial cell consists of a cell wall, a cytoplasmic membrane, cytoplasm with inclusions, and a nucleus called the nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili. Some bacteria are capable of forming spores under unfavorable conditions.
Differences in cell structure
1) Prokaryotes do not have a nucleus, but eukaryotes do.
2) Prokaryotes have only ribosomes (small, 70S) among their organelles, while eukaryotes, in addition to ribosomes (large, 80S), have many other organelles: mitochondria, EPS, cell center, etc.
3) A prokaryotic cell is much smaller than a eukaryotic cell: 10 times in diameter, 1000 times in volume.
1) Prokaryotes have circular DNA, and eukaryotes have linear DNA
2) In prokaryotes, DNA is naked, almost not connected to proteins, and in eukaryotes, DNA is connected to proteins in a 50/50 ratio, forming a chromosome
3) In prokaryotes, DNA lies in a special region of the cytoplasm called the nucleoid, and in eukaryotes, DNA lies in the nucleus.
Permanent components of a bacterial cell.
Nucleoid is the equivalent of a prokaryotic nucleus
The cell wall is different in Gr+ and Gr– bacteria. Determines and maintains a constant shape, provides communication with the external environment, determines the antigenic specificity of bacteria, and has important immunospecific properties; disruption of cell wall synthesis leads to the formation of L-forms of bacteria.
Gr+: this coloring is associated with the content of teichoic and dipoteichoic acids in the CS, which penetrate it through and fix it in the cytoplasm. Peptidoglycan is thick and consists of a plasma membrane bound by beta-glycosidic bonds.
Gr -: a thin layer of peptidoglycans, the outer membrane is represented by lipopolysaccharide glycocoproteins, glycolipids.
CPM - consists of lipoproteins. Perceives all chemical information entering the cell. Is the main barrier. Participates in the process of replication of nucleoid and plasmids; contains a large number of enzymes; Participates in the synthesis of cell wall components.
Mesosomes are analogues of mitochondria in a bacterial cell
70S ribosomes are numerous small granules located in the cytoplasm.
PERMANENT:
Flagella: consist of the protein flagellin, originate from the central nervous system, the main function is motor.
Pili: they are responsible for attachment to the host cell
Plasmids. Capsule, Spores, Inclusions.
Main article: Supramembrane complex
The supramembrane apparatus of bacteria is represented by a cell wall, the specific organization of which serves as the basis for dividing them into two non-taxonomic groups (gram-positive and gram-negative forms) and correlates with very a large number morphofunctional, metabolic and genetic characteristics. The cell wall of prokaryotes is essentially a multifunctional organelle, extended beyond the protoplast and carrying a significant share metabolic load cells.
Cell wall of gram-positive bacteria
Cell wall structure
In gram-positive bacteria (Fig. 12, A), the cell wall has a generally simpler structure. The outer layers of the cell wall are formed by protein in complex with lipids. In some species of bacteria, a layer of surface protein globules, the shape, size and arrangement of which are species-specific, has been discovered relatively recently. Inside the cell wall, as well as directly on its surface, enzymes are placed that break down substrates into low molecular weight components, which are subsequently transported through the cytoplasmic membrane into the cell. Enzymes that synthesize extracellular polymers, such as capsular polysaccharides, are also located here.
Polysaccharide capsule
The polysaccharide capsule, which externally envelops the cell wall of a number of bacteria, has mainly an adaptive significance, and its presence is not necessary to preserve the vital activity of the cell. Thus, it ensures the attachment of cells to the surface of dense substrates, accumulates some minerals and, in pathogenic forms, prevents their phagocytosis.
Murein
Directly adjacent to the cytoplasmic membrane is a hard murein layer.
Murein, or peptidoglycan, is a copolymer of acetylglucosamine and acetylmuramic acid with oligopeptide cross-links. It is possible that the murein layer is one giant bag molecule that ensures the rigidity of the cell wall and its individual shape.
Teichoic acids
In close contact with the murein layer is the second polymer of the wall of gram-positive bacteria - teichoic acids. They are credited with the role of a cation accumulator and a regulator of ion exchange between the cell and the environment.
Cell wall of gram-negative bacteria
Cell wall structure
Compared to gram-positive forms, the cell wall of gram-negative bacteria is more complex and its physiological significance is incomparably broader. In addition to the murein layer, a second protein-lipid membrane is located closer to the surface (Fig. 12, B, C), which includes lipopolysaccharides. It is covalently linked to murein by crosslinking lipoprotein molecules. The main function of this membrane is the role of a molecular sieve; in addition, enzymes are located on its outer and inner surfaces.
3.Structure of a bacterial cell.
The space bounded by the outer and cytoplasmic membranes is called periplasmic and is a unique property of gram-negative bacteria. A whole set of enzymes are localized in its volume - phosphatases, hydrolases, nucleases, etc. They break down relatively high-molecular nutrient substrates, and also destroy their own cellular material released into the environment from the cytoplasm. To a certain extent, the periplasmic space can be likened to the lysosome of eukaryotes. In the periplasm zone, it is possible not only for the most efficient occurrence of enzymatic reactions, but also for the isolation from the cytoplasm of compounds that pose a threat to its normal functioning. Material from the site http://wiki-med.com
Functions of the bacterial cell wall
In both gram-positive and gram-negative forms, the cell wall plays the role of a molecular sieve, selectively carrying out passive transport of ions, substrates and metabolites. In bacteria that have the ability to actively move due to flagella, the cell wall is a component of the locomotor mechanism. Finally, certain sections of the cell wall are closely associated with the cytoplasmic membrane in the zone of nucleoid attachment and play an important role in its replication and segregation.
In one species of bacteria, the process of destruction of the old cell wall, which occurs during cell division, is ensured by the work of at least four systems of hydrolytic enzymes present in the cell wall in a latent state. During cell division, a natural and strictly time-sequential activation of these systems occurs, leading to the gradual destruction and exfoliation of the old (“mother”) membrane of the bacterial cell.
Material from the site http://Wiki-Med.com
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The main component of the cell wall of gram-positive bacteria is
bacterial cell wall functions
features of the structure of the bacterial cell wall
cell wall structure
characteristics of bacterial cell wall
The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, and proteins. The main component of the cell wall of these bacteria is multilayer peptidoglycan (murein, mucopeptide), accounting for 40-90% of the mass of the cell wall. Teichoic acids (from the Greek teichos - wall) are covalently bound to the peptidoglycan of the cell wall of gram-positive bacteria.
The cell wall of Gram-negative bacteria includes an outer membrane bound by a lipoprotein to an underlying layer of peptidoglycan. On ultrathin sections of bacteria, the outer membrane has the appearance of a wavy three-layer structure, similar to the inner membrane, which is called the cytoplasmic one. The main component of these membranes is a bimolecular (double) layer of lipids. The inner layer of the outer membrane is composed of phospholipids, and the outer layer contains lipopolysaccharide (LPS). The lipopolysaccharide of the outer membrane consists of three fragments: lipid A - a conservative structure, almost the same in gram-negative bacteria; core, or core, core part (lat. core - core), relatively conservative oligosaccharide structure (the most constant part of the LPS core is ketodeoxyoctonic acid); a highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences (O-antigen). The matrix proteins of the outer membrane permeate it so that protein molecules called porins line hydrophilic pores through which water and small hydrophilic molecules pass.
When the synthesis of the bacterial cell wall is disrupted under the influence of lysozyme,
penicillin, protective factors of the body, cells with a modified (often spherical) shape are formed: protoplasts - bacteria completely devoid of a cell wall; spheroplasts are bacteria with a partially preserved cell wall. Bacteria of the sphero- or protoplast type, which have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to reproduce, are called L-forms.
They are osmotically sensitive, spherical, flask-shaped cells of various sizes, including those passing through bacterial filters. Some L-forms (unstable), when the factor that led to changes in bacteria is removed, can reverse, “returning” to the original bacterial cell.
Between the outer and cytoplasmic membranes there is a periplasmic space, or periplasm, containing enzymes (proteases, lipases, phosphatases, nucleases, beta-lactomases) and components of transport systems.
In electron microscopy of ultrathin sections, the cytoplasmic membrane is a three-layer membrane (2 dark layers 2.5 nm thick are separated by a light intermediate layer). In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of phospholipids with embedded surface and integral proteins, as if penetrating through the structure of the membrane. With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complex twisted membrane structures, called mesosomes. Less complexly twisted structures are called intracytoplasmic membranes.
Cytoplasm
The cytoplasm consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes, responsible for the synthesis (translation) of proteins. Bacterial ribosomes have a size of about 20 nm and a sedimentation coefficient of 70S, in contrast to the 80S ribosomes characteristic of eukaryotic cells. Ribosomal RNAs (rRNAs) are conserved elements of bacteria (the “molecular clock” of evolution). 16S rRNA is part of the small ribosomal subunit, and 23S rRNA is part of the large ribosomal subunit. The study of 16S rRNA is the basis of gene systematics, allowing one to assess the degree of relatedness of organisms.
The cytoplasm contains various inclusions in the form of glycogen granules, polysaccharides, beta-hydroxybutyric acid and polyphosphates (volutin).
Cell wall
They are reserve substances for the nutrition and energy needs of bacteria. Volutin has an affinity for basic dyes and is easily detected using special staining methods (for example, Neisser) in the form of metachromatic granules. The characteristic arrangement of volutin granules is revealed in the diphtheria bacillus in the form of intensely stained cell poles.
Nucleoid
Nucleoid is the equivalent of a nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a ball. The nucleus of bacteria, unlike eukaryotes, does not have a nuclear envelope, nucleolus and basic proteins (histones). Typically, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring.
In addition to the nucleoid, represented by one chromosome, the bacterial cell contains extrachromosomal factors of heredity - plasmids, which are covalently closed rings of DNA.
Capsule, microcapsule, mucus
The capsule is a mucous structure more than 0.2 µm thick, firmly associated with the bacterial cell wall and having clearly defined external boundaries. The capsule is visible in imprint smears from pathological material. In pure bacterial cultures, the capsule is formed less frequently. It is detected by special methods of staining a smear (for example, according to Burri-Gins), which create a negative contrast of the substances of the capsule: ink creates a dark background around the capsule. The capsule consists of polysaccharides (exopolysaccharides), sometimes of polypeptides, for example, in the anthrax bacillus it consists of polymers of D-glutamic acid. The capsule is hydrophilic and prevents phagocytosis of bacteria. The capsule is antigenic: antibodies against the capsule cause its enlargement (capsule swelling reaction).
Many bacteria form a microcapsule - a mucous formation less than 0.2 microns thick, detectable only by electron microscopy. It is necessary to distinguish from the capsule mucoid exopolysaccharides, which do not have clear boundaries. Mucus is soluble in water.
Bacterial exopolysaccharides are involved in adhesion (sticking to substrates); they are also called glycocalyx. Besides synthesis
exopolysaccharides by bacteria, there is another mechanism for their formation: through the action of extracellular enzymes of bacteria on disaccharides. As a result, dextrans and levans are formed.
Flagella
Bacterial flagella determine the motility of the bacterial cell. Flagella are thin filaments originating from the cytoplasmic membrane and are longer than the cell itself. The thickness of the flagella is 12-20 nm, length 3-15 µm. They consist of 3 parts: a spiral filament, a hook and a basal body containing a rod with special disks (1 pair of disks in gram-positive bacteria and 2 pairs of disks in gram-negative bacteria). Flagella are attached to the cytoplasmic membrane and cell wall by discs. This creates the effect of an electric motor with a motor rod that rotates the flagellum. Flagella consist of a protein - flagellin (from flagellum - flagellum); is an H antigen. Flagellin subunits are twisted in a spiral.
Number of flagella in bacteria various types varies from one (monotrich) in Vibrio cholerae to tens and hundreds of flagella extending along the perimeter of the bacterium (peritrich) in Escherichia coli, Proteus, etc. Lophotrichs have a bundle of flagella at one end of the cell. Amphitrichy has one flagellum or a bundle of flagella at opposite ends of the cell.
Drank
Pili (fimbriae, villi) are thread-like formations, thinner and shorter (3-10 nm x 0.3-10 µm) than flagella. Pili extend from the cell surface and consist of the protein pilin, which has antigenic activity. There are pili responsible for adhesion, that is, for attaching bacteria to the affected cell, as well as pili responsible for nutrition, water-salt metabolism and sexual (F-pili), or conjugation pili. Pili are numerous - several hundred per cell. However, there are usually 1-3 sex pili per cell: they are formed by so-called “male” donor cells containing transmissible plasmids (F-, R-, Col-plasmids). Distinctive feature genital pili is the interaction with special “male” spherical bacteriophages, which are intensively adsorbed on the genital pili.
Controversy
Spores are a peculiar form of resting firmicute bacteria, i.e. bacteria
with a gram-positive type of cell wall structure. Spores are formed under unfavorable conditions for the existence of bacteria (drying, nutrient deficiency, etc.. One spore (endospore) is formed inside the bacterial cell. The formation of spores contributes to the preservation of the species and is not a method of reproduction, like fungi. Spore-forming bacteria of the genus Bacillus have spores, not exceeding the diameter of the cell. Bacteria in which the size of the spore exceeds the diameter of the cell are called clostridia, for example, bacteria of the genus Clostridium (lat. Clostridium - spindle). The spores are acid-fast, therefore they are stained red using the Aujeszky method or the Ziehl-Neelsen method, and the vegetative cell in blue.
The shape of the spores can be oval, spherical; location in the cell is terminal, i.e. at the end of the stick (in the causative agent of tetanus), subterminal - closer to the end of the stick (in the causative agents of botulinum, gas gangrene) and central (in the anthrax bacillus). The spore persists for a long time due to the presence of a multilayer shell, calcium dipicolinate, low water content and sluggish metabolic processes. Under favorable conditions, spores germinate, going through three successive stages: activation, initiation, germination.
Bacteria: habitats, structure, life processes, significance
2. b) Structure of a bacterial cell
The cell wall of bacteria determines their shape and ensures the preservation of the internal contents of the cell. Based on the characteristics of the chemical composition and structure of the cell wall, bacteria are differentiated using gram staining...
Bacterial cell wall biopolymers
Structure of a bacterial cell
The structure of bacteria is studied using electron microscopy of whole cells and their ultraviolet sections. The main structures of a bacterial cell are: cell wall, cytoplasmic membrane, cytoplasm with inclusions and nucleus...
Humoral regulation of the body
3. Features of the structure, properties and functions of cell membranes
Diversity of living cells
1.1 General plan of the structure of eukaryotic cells, which also characterizes the structure of an animal cell
A cell is a structural and functional unit of a living thing. All eukaryotic cells are characterized by the presence of the following structures: 1) The cell membrane is an organelle that limits the contents of the cell from the environment...
Diversity of living cells
1.2 Features of the structure of a plant cell
In plant cells there are organelles that are also characteristic of animals, for example, the nucleus, endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus (see Fig. 2). They lack a cell center, and the function of lysosomes is performed by vacuoles...
Diversity of living cells
1.3 Features of the structure of a mushroom cell
In most fungi, the cell structure and functions it performs are generally similar to plant cells. It consists of a hard shell and internal contents, which is the cytoplasmic system...
Diversity of living cells
1.4 General plan of the structure of prokaryotic cells, which also characterizes the structure of a bacterial cell
A prokaryotic cell is structured as follows. The main feature of these cells is the absence of a morphologically expressed nucleus, but there is a zone in which DNA is located (nucleoid).
Bacterial cell structure
Ribosomes are located in the cytoplasm...
Fundamentals of Microbiology
1. Describe the structure of a bacterial cell. Sketch the cell organelles
Bacteria include microscopic plant organisms. Most of them are single-celled organisms that do not contain chlorophyll and reproduce by division. The shape of bacteria is spherical, rod-shaped and convoluted...
Features of the visual and auditory sensory systems
13. Simple, complex and super complex cells and their functions
"Simple" and "complex" cells. Neurons that respond to simple linear stimuli (slits, edges, or dark stripes) are called “simple”, and those that respond to complex and moving stimuli are called “complex”...
Features of cell structure
1. The cell as an elementary structural unit of the body. Basic components of a cell
The cell is the basic structural and functional unit of life, bounded by a semipermeable membrane and capable of self-reproduction. In a plant cell, first of all, it is necessary to distinguish between the cell membrane and the contents...
Distribution and dynamics of the wild boar population in the Bryansk region
1.1 Structural features
The wild boar (Sus scrofa L.) is a massive animal with short, relatively thin legs. The body is relatively short, the front part is very massive, the back area of the shoulder blades is strongly raised, the neck is thick, short, almost motionless...
Structure, properties and functions of proteins
2. Functions of cell organelles
Cell organelles and their functions: 1. Cell membrane - consists of 3 layers: 1. rigid cell wall; 2. a thin layer of pectin substances; 3. thin cytoplasmic filament. The cell membrane provides mechanical support and protection...
4.1 Structural features
The thallus is a plasmodium capable of amoeba-like movements on the surface or inside the substrate. During sexual reproduction, plasmodia transform into fruiting bodies called sporocarps...
Taxonomic group of slime molds
5.1 Structural features
The vegetative body is in the form of a multinucleate protoplast, incapable of independent movement and located inside the host plant cell. Special sporulation is not formed. The wintering stage is represented by spores...
Energy system of the cell. Classification muscle tissue. Structure of sperm
Energy system of the cell. General plan of the structure of mitochondria and plastids, their functions. Hypothesis of the symbiotic origin of mitochondria and chloroplasts
Eukaryotic cells have a unique organelle, the mitochondrion, in which ATP molecules are formed through the process of oxidative phosphorylation. It is often said that mitochondria are the energy stations of the cell (Figure 1)...
The bacterial cell as a whole is structured quite simply. It is separated from the external environment by a cytoplasmic membrane and filled with cytoplasm, in which the nucleoid zone is located, including a circular DNA molecule from which the transcribed mRNA can “hang”, to which, in turn, ribosomes are attached, synthesizing protein on its matrix simultaneously with the process of synthesis itself matrices. At the same time, DNA can be associated with proteins that carry out its replication and repair. Bacterial ribosomes are smaller than eukaryotic ones and have a sedimentation coefficient of 70S. They, like eukaryotic ones, are formed by two subunits - small (30S), which includes 16S rRNA, and large - 50S, which includes 23S and 5S rRNA molecules.
The photograph obtained using transmission microscopy (Fig. 1) clearly shows a light zone in which the genetic apparatus is located and the processes of transcription and translation occur. Ribosomes are visible as small granular inclusions.
Most often, in a bacterial cell, the genome is represented by only one DNA molecule, which is closed in a ring, but there are exceptions. Some bacteria may have several DNA molecules. For example, Deinococus radiodurans, a bacterium known for its phenomenal radiation resistance and the ability to comfortably withstand radiation doses 2,000 times the lethal dose for humans, has two copies of its genomic DNA. Bacteria are known to have three or four copies. In some species the DNA may not be closed in a circle, and some Agrobacterium contain one circular and one linear DNA.
In addition to the nucleoid, genetic material can be presented in the cell in the form of additional small circular DNA molecules - plasmids. Plasmids replicate independently of the nucleoid and often contain genes useful to the cell, giving the cell, for example, resistance to antibiotics, the ability to assimilate new substrates, the ability to conjugate, and much more. Plasmids can be transferred both from mother cell to daughter cell, and by horizontal transfer they can be transferred from one cell to another.
A bacterial cell is most often surrounded not only by a membrane, but also by a cell wall, and according to the type of cell wall structure, bacteria are divided into two groups - gram-positive and gram-negative.
The cell wall of bacteria is formed by peptidoglycan - murein. At the molecular level, the murein layer is a network formed by N-acetylglucosamine and N-acetylmuramic acid molecules, linked together into long chains by β-1-4-glycosidic bonds, adjacent chains, in turn, are connected by cross peptide bridges (Fig. 2) . This creates one large network surrounding the cell.
Gram-positive bacteria have a thick cell wall located on top of the membrane. Murein is cross-linked with another type of molecules - teichoic and lipoteichoic (if they are connected to membrane lipids) acids. It is believed that these molecules give the cell wall elasticity during lateral compression and stretching, acting like springs. Because the murein layer is thick, it stains easily with the Gram stain: the cells appear bright purple because the dye (Gentian or methyl violet) is stuck in the cell wall layer.
In gram-negative bacteria, the murein layer is very thin (the exception is cyanobacteria), therefore, when staining with Gram, the violet dye is washed out, and the cells are stained with the color of the second dye (Fig. 3).
The cell wall of gram-negative bacteria is covered on top by another, outer, membrane attached to the peptidoclycan by lipoproteins. The space between the cytoplasmic membrane and the outer membrane is called the periplasm. The outer membrane contains lipopolyproteins, lipopolysaccharides (LPS), as well as proteins that form hydrophilic pores. Components of the outer membrane are often responsible for the interaction of the cell with the external environment. It contains antigens, phage receptors, molecules involved in conjugation, etc.
Since the structure of the integument differs in gram-positive and gram-negative cells (Fig. 4, top), the apparatus that anchors the flagellum in the cell integument also differs (Fig. 4, bottom).
The flagellum of Gram-positive bacteria is anchored in the membrane by two protein rings (S-ring and M-ring) and is driven by a system of proteins that, consuming energy, cause the thread to spin. In gram-negative bacteria, in addition to this structure, there are two more rings that additionally fix the flagellum in the outer membrane and cell wall.
The flagellum itself in bacteria consists of the protein flagellin, the subunits of which are connected into a helix that has a cavity inside and forms a thread. The thread is flexibly attached to the anchoring and torsion apparatus using a hook.
In addition to flagella, there may be other outgrowths called pili on the surface of bacterial cells. These are protein pili that allow bacteria to attach to various surfaces (increasing the hydrophobicity of the cell) or take part in the transport of metabolites and the conjugation process (F-pili).
A bacterial cell usually does not contain any membrane structures inside, including vesicles, but there may be various types of inclusions (reserve lipids, sulfur) and gas bubbles surrounded by a protein membrane. Without a membrane, the cell can store polysaccharide molecules, cyanophycin (as a nitrogen depot), and can also contain carboxysomes - vesicles containing the RuBisCO enzyme, necessary for the fixation of carbon dioxide in the Calvin Cycle.
In microbiology, this term means a nutrient that can be absorbed by a microorganism
This name of the groups comes from the surname of the doctor G.K. Gram, who developed a method for staining bacterial cell walls, allowing one to distinguish cells with different types cell wall structure.
Ribulose bisphosphate carboxylase/oxygenase
Prokaryotes include archaebacteria, bacteria and blue-green algae. Prokaryotes- single-celled organisms that lack a structurally formed nucleus, membrane organelles and mitosis.
Dimensions - from 1 to 15 microns. Basic forms: 1) cocci (spherical), 2) bacilli (rod-shaped), 3) vibrios (comma-shaped), 4) spirilla and spirochetes (spiral-twisted).
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.
1 - cytoplasmic membrane wound; 2 — cell wall; 3 — mucous capsule; 4 - cytoplasm; 5 - chromosomal DNA; 6 - ribosomes; 7 - meso-soma; 8 — photo-synthetic membrane wounds; 9 — switching on; 10 - burn-tiki; 11 - drank.
The bacterial cell is bounded by a membrane. The inner layer of the membrane is represented by the cytoplasmic membrane (1), above which there is a cell wall (2); in many bacteria there is a mucous capsule above the cell wall (3). The structure and functions of the cytoplasmic membrane of eukaryotic and prokaryotic cells do not differ. The membrane can form folds called mesosomes(7). They may have different shapes(bag-shaped, tubular, lamellar, etc.).
Enzymes are located on the surface of mesosomes. The cell wall is thick, dense, rigid, consists of mureina(main component) and other organic substances. Murein is a regular network of parallel polysaccharide chains linked to each other by short protein chains. Depending on the structural features of the cell wall, bacteria are divided into gram-positive(Gram stained) and gram-negative(not painted). In gram-negative bacteria, the wall is thinner, more complex, and above the murein layer there is a layer of lipids on the outside. The internal space is filled with cytoplasm (4).
The genetic material is represented by circular DNA molecules. These DNAs can be roughly divided into “chromosomal” and plasmid. “Chromosomal” DNA (5) is one, attached to a membrane, contains several thousand genes; unlike chromosomal DNA of eukaryotes, it is not linear and is not associated with proteins. The area in which this DNA is located is called nucleoid. Plasmids- extrachromosomal genetic elements. They are small circular DNA, not associated with proteins, not attached to the membrane, and contain a small number of genes. The number of plasmids may vary. The most studied plasmids are those carrying information about resistance to medicines(R-factor), taking part in the sexual process (F-factor). A plasmid that can combine with a chromosome is called episome.
The bacterial cell lacks all membrane organelles characteristic of a eukaryotic cell (mitochondria, plastids, EPS, Golgi apparatus, lysosomes).
The cytoplasm of bacteria contains 70S-type ribosomes (6) and inclusions (9). As a rule, ribosomes are assembled into polysomes. Each ribosome consists of a small (30S) and a large subunit (50S). Ribosome function: assembly of a polypeptide chain. Inclusions can be represented by lumps of starch, glycogen, volutin, and lipid droplets.
Many bacteria have flagella(10) and drank (fimbriae)(eleven). The flagella are not limited by the membrane, have a wavy shape and consist of spherical subunits of the flagellin protein. These subunits are arranged in a spiral and form a hollow cylinder with a diameter of 10-20 nm. The structure of the prokaryotic flagellum resembles one of the microtubules of the eukaryotic flagellum. The number and location of flagella may vary. Pili are straight thread-like structures on the surface of bacteria. They are thinner and shorter than flagella. They are short, hollow cylinders made of the protein pilin. Pili serve to attach bacteria to the substrate and to each other. During conjugation, special F-pili are formed, through which genetic material is transferred from one bacterial cell to another.
Sporulation in bacteria it is a way of surviving unfavorable conditions. Spores are usually formed one at a time inside the “mother cell” and are called endospores. The spores are highly resistant to radiation, extreme temperatures, drying and other factors that cause the death of vegetative cells.
Reproduction. Bacteria reproduce asexually - by dividing the “mother cell” in two. DNA replication occurs before division.
Rarely do bacteria undergo a sexual process in which recombination of genetic material occurs. It should be emphasized that in bacteria gametes are never formed, cell contents do not merge, but DNA is transferred from the donor cell to the recipient cell. There are three methods of DNA transfer: conjugation, transformation, transduction.
- unidirectional transfer of F-plasmid from a donor cell to a recipient cell in contact with each other. In this case, bacteria are connected to each other by special F-pili (F-fimbriae), through the channels of which DNA fragments are transferred. Conjugation can be divided into the following stages: 1) unwinding of the F-plasmid, 2) penetration of one of the chains of the F-plasmid into the recipient cell through the F-pilus, 3) synthesis of a complementary chain on a single-stranded DNA template (occurs as in the donor cell (F +), and in the recipient cell (F -)).
Transformation- unidirectional transfer of DNA fragments from a donor cell to a recipient cell that are not in contact with each other. In this case, the donor cell either “releases” a small fragment of DNA from itself, or the DNA enters the environment after the death of this cell. In any case, the DNA is actively absorbed by the recipient cell and integrated into its own “chromosome”.
Transduction- transfer of a DNA fragment from a donor cell to a recipient cell using bacteriophages.
Viruses
Viruses consist of nucleic acid (DNA or RNA) and proteins that form a shell around this nucleic acid, i.e. represent a nucleoprotein complex. Some viruses contain lipids and carbohydrates. Viruses always contain one type of nucleic acid - either DNA or RNA. Moreover, each of the nucleic acids can be either single-stranded or double-stranded, both linear and circular.
The size of viruses is 10-300 nm. Virus form: spherical, rod-shaped, filiform, cylindrical, etc.
Capsid— the shell of the virus, formed by protein subunits arranged in a certain way. The capsid protects the nucleic acid of the virus from various influences and ensures the deposition of the virus on the surface of the host cell. Supercapsid characteristic of complex viruses (HIV, influenza viruses, herpes). Occurs during the exit of the virus from the host cell and is a modified region of the nuclear or outer cytoplasmic membrane of the host cell.
If the virus is inside a host cell, it exists in the form of a nucleic acid. If the virus is outside the host cell, then it is a nucleoprotein complex, and this free form of existence is called virion. Viruses are highly specific, i.e. they can use a strictly defined circle of hosts for their livelihoods.
The following stages can be distinguished in the virus reproduction cycle.
- Deposition on the surface of the host cell.
- Penetration of the virus into the host cell (can enter the host cell by: a) “injection”, b) dissolution of the cell membrane by viral enzymes, c) endocytosis; Once inside the cell, the virus puts its protein-synthesizing apparatus under its own control).
- Incorporation of viral DNA into the DNA of the host cell (in RNA viruses, reverse transcription occurs before this - DNA synthesis on an RNA template).
- Transcription of viral RNA.
- Synthesis of viral proteins.
- Synthesis of viral nucleic acids.
- Self-assembly and exit of daughter viruses from the cell. Then the cell either dies or continues to exist and produce new generations of viral particles.
The human immunodeficiency virus primarily affects CD 4 lymphocytes (helpers), on the surface of which there are receptors that can bind to the surface protein of HIV. In addition, HIV penetrates the cells of the central nervous system, neuroglia, and intestines. The immune system The human body loses its protective properties and is unable to resist pathogens of various infections. The average life expectancy of an infected person is 7-10 years.
The source of infection is only a person who is a carrier of the immunodeficiency virus. AIDS is transmitted sexually, through blood and tissues containing the immunodeficiency virus, from mother to fetus.
Go to lectures No. 8" Core. Chromosomes"
Go to lectures No. 10“The concept of metabolism. Biosynthesis of proteins"
We cannot even imagine how many microorganisms constantly surround us. By holding the handrail on the bus, you have already planted about one hundred thousand bacteria on your hand; by going into a public toilet, you, again, have rewarded yourself with these microorganisms. Bacteria always and everywhere accompany humans. But there is no need to react negatively to this word, because bacteria are not only pathogenic, but also beneficial to the body.
Scientists were very surprised when they realized that some bacteria have retained their appearance for approximately a billion years. Such microorganisms were even compared to a Volkswagen car - the appearance of one of their models has not changed for 40 years, having an ideal shape.
Bacteria were among the first to appear on Earth, so they can deservedly be called long-livers. An interesting fact is that these cells do not have a formed nucleus, which is why to this day they attract a lot of attention to their structure.
What are bacteria?
Bacteria are microscopic organisms plant origin. The structure of a bacterial cell (table, diagrams exist for a clear understanding of the types of these cells) depends on its purpose.
These cells are ubiquitous because they can multiply quickly. Exist scientific evidence that in just six hours one cell can produce offspring of 250 thousand bacteria. These single-celled organisms come in many varieties that vary in shape.
Bacteria are very tenacious organisms; their spores can retain the ability to live for 30-40 years. These spores are transported by blowing wind, flowing water, and other means. Viability is maintained up to a temperature of 100 degrees and with slight frost. And yet, what structure does a bacterial cell have? The table describes the main components of bacteria; the functions of other organelles are outlined below.
Globular (cocci) bacteria
They are pathogenic in nature. Cocci are divided into groups depending on their location to each other:
- Micrococci (small). Division occurs in one plane. Arrangement in a chaotic single order. They feed on ready-made organic compounds, but do not depend on other organisms (saprophytes).
- Diplococci (double). They divide in the same plane as micrococci, but form paired cells. Outwardly they resemble beans or lancelet.
- Streptococci (in the form of a chain). The division is the same, but the cells are connected to each other and look like beads.
- Staphylococcus (grape bunch). This species divides in several planes, producing a cluster of grape-like cells.
- Tetracocci (four). Cells divide in two perpendicular planes, forming tetrads.
- Sarcinas (ligament). Such cells divide in three planes, which are mutually perpendicular to each other. Moreover, in appearance they look like bags or bales, consisting of many individuals of an even number.
Cylindrical (rod) bacteria
Rods that form spores are divided into clostridia and bacilli. In size, these bacteria are short and very short. The end sections of the sticks are rounded, thickened or cut off. Depending on the location of the bacteria, several groups are distinguished: mono-, diplo- and streptobacteria.
Spiral-shaped (convoluted) bacteria
These microscopic cells come in two types:
- Vibrios (with a single bend or generally straight).
- Spirilla (large in size, but few curls).
Filamentous bacteria. There are two groups of such forms:
- Temporary threads.
- Permanent threads.
The structural features of a bacterial cell are that during its existence it is capable of changing shape, but polymorphism is not inherited. Various factors act on the cell during metabolism in the body, as a result of which quantitative changes are observed in its appearance. But as soon as the external action stops, the cell will take on its previous image. What are the structural features of a bacterial cell can be revealed by examining it using a microscope.
Structure of a bacterial cell, membrane
The shell gives and maintains the shape of the cell and protects the internal components from damage. Due to incomplete permeability, not all substances can enter the cell, which promotes the exchange of low- and high-molecular structures between the external environment and the cell itself. Also in the wall there are various chemical reactions. Using an electron microscope, it is not difficult to study the detailed structure of a bacterial cell.
The shell base contains the polymer murein. Gram-positive bacteria have a single-layer skeleton consisting of murein. Here there are polysaccharide and lipoprotein complexes, phosphates. In gram-negative cells, the murein skeleton has many layers. The outer layer adjacent to the cell wall is the cytoplasmic membrane. It also has certain layers containing proteins with lipids. The main function of the cytoplasmic membrane is to control the penetration of substances into the cell and their removal (osmotic barrier). This is a very important function for cells, as it helps protect cells.
Composition of the cytoplasm
The living semi-liquid substance that fills the cell cavity is called cytoplasm. The bacterial cell contains a large amount of protein and a supply of nutrients (fats and fat-like substances). A photo taken during a microscope examination clearly shows the constituent parts inside the cytoplasm. The main composition includes ribosomes, arranged in a chaotic order and in large numbers. It also contains mesosomes containing redox enzymes. Due to them, the cell draws energy. The nucleus is presented in the form of a nuclear substance located in chromatin bodies.
Functions of ribosomes in cells
Ribosomes consist of subunits (2) and are nucleoproteins. By connecting with each other, these constituent elements form polysomes or polyribosomes. The main task of these inclusions is protein synthesis, which occurs on the basis of genetic information. Sedimentation rate 70S.
Features of the bacterial nucleus
The genetic material (DNA) is located in the unformed nucleus (nucleoid). This nucleus is located in several places in the cytoplasm, being a loose shell. Bacteria that have such a nucleus are called prokaryotes. The nuclear apparatus lacks a membrane, a nucleolus, and a set of chromosomes. And deoxyribonucleic acid is located in it in fibril bundles. The diagram of the structure of a bacterial cell demonstrates in detail the structure of the nuclear apparatus.
Under certain conditions, bacteria may develop mucilaginous membranes. As a result, a capsule is formed. If mucus is very strong, then the bacteria turn into zooglea (general mucous mass).
Bacterial cell capsule
The structure of the bacterial cell has a peculiarity - the presence of a protective capsule consisting of polysaccharides or glycoproteins. Sometimes these capsules are composed of polypeptides or fiber. It is located on top of the cell membrane. The thickness of the capsule can be either thick or thin. Its formation occurs due to the conditions in which the cell finds itself. The main property of the capsule is to protect the bacteria from drying out.
In addition to the protective capsule, the structure of the bacterial cell provides for its motor ability.
Flagella on bacterial cells
Flagella are additional elements that carry out cell movement. They are presented in the form of threads of different lengths, which consist of flagellin. This is a protein that has the ability to contract.
The composition of the flagellum is three-component (filament, hook, basal body). Depending on their attachment and location, several groups of motile bacteria have been identified:
- Monotrichs (these cells have 1 flagellum located polarly).
- Lophotrichs (flagella in the form of a bundle at one end of the cell).
- Amphitrichy (tufts at both ends).
There are many interesting facts about bacteria. So, it has long been proven that a mobile phone contains a huge number of these cells, even on a toilet seat there are fewer of them. Other bacteria allow us to live a quality life - eat, perform certain activities, and free our body from nutrient breakdown products without problems. Bacteria are truly diverse, their functions are multifaceted, but we should not forget about their pathological effect on the body, so it is important to monitor our own hygiene and the cleanliness around us.
