List the inclusions that are found in a bacterial cell. Structure of bacteria: features

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 of 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. There is 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 in its appearance are observed. 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. Various chemical reactions also occur in the wall. 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.

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.

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 with ethanol is determined by the specific chemical composition and ultrastructure of their cell wall. 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 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-forms are given great importance in the development of chronic recurrent infections, carriage of pathogens, and 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.

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. The basal body is a 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.

The number of flagella and their locations in different species of bacteria are not the same, but are 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.

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 membrane structures. 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.

The structure of a typical bacterial cell is shown in Fig. 2.3. In Fig. Figure 2.4 shows an electron micrograph of a section of a rod-shaped bacterium. You can see how simple a bacterial cell is, especially if you compare it with eukaryotic cells (Figs. 7.5 and 7.6).

Rice. 2.3. Generalized diagram of the structure of a rod-shaped bacterium cell. On the right are structures that are found in every cell, on the left are structures that are not found in all cells. Flagellum there is one, as in Rhizobium, or several, as in Azotobacter; it is usually longer than the cell. Capsule may be mucoid, as in Azotobacter; if the capsule is loose, then it is called mucous layer. Tubular or bag-shaped photosynthetic membranes, containing pigments, are invaginations of the plasma membrane; in photosynthetic bacteria, such as Chromatium, such membranes are scattered throughout the cytoplasm. Number pili, or fimbriae, can reach from one to several hundred, as, for example, in Escherichia coli, Salmonella. Mesosoma is a multifold invagination of the plasma membrane, as, for example, in Bacillus subtilis. Cell wall tough and contains murein. Ribosomes, located throughout the cytoplasm, are smaller in size than in eukaryotes. From reserve nutrients Lipids, glycogen, and polyphosphates (volutin granules) can be found in bacterial cells. Cytoplasm does not contain any organelles; contains enzymes, etc.

Rice. 2.4. Electron micrograph of a section of a typical rod-shaped bacterium, Bacillus subtilis. The light areas contain DNA. × 50000

Capsules and mucous layers

Capsules and mucous layers are the mucous or sticky secretions of certain bacteria; such secretions are clearly visible after negative contrast (when it is not the preparation that is stained, but the background). Capsule is a relatively thick and compact formation, and slime layer much looser. In some cases, mucus serves to form colonies of individual cells. Both the capsule and the mucous layers provide additional protection for the cells. For example, encapsulated strains of pneumococci multiply freely in the human body and cause pneumonia, while non-capsulated strains are easily attacked and destroyed by phagocytes and are therefore completely harmless.

Cell wall

The cell wall gives the cell a certain shape and rigidity. It is clearly visible in the cut (Fig. 2.4). As in plants, the bacterial cell wall prevents osmotic swelling and cell rupture when, as often happens, they enter a hypotonic environment (Appendix Section P.1.5). Water, other small molecules, and various ions easily pass through tiny pores in the cell wall, but large molecules of proteins and nucleic acids do not pass through them. In addition, the cell wall has antigenic properties, which are given to it by the proteins and polysaccharides it contains.

Based on the structure of the cell wall, bacteria can be divided into two groups. Some are Gram stained, which is why they are called gram-positive, while others become discolored when the dye is washed off (Section 2.7), and therefore they are called gram negative. In the cell wall of both of them there is a special rigid lattice consisting of mureina. The murein molecule is a regular network of parallel polysaccharide chains linked to each other by short chains of peptides. Thus, each cell is surrounded by a network-like sac made up of just one molecule. (The polysaccharide portion of murein is described in Table 5.7).

In gram-positive bacteria, such as Lactobacillus, other substances, mainly polysaccharides and proteins, are built into the murein network. This creates a relatively thick and rigid package around the cell. In gram-negative bacteria, say Escherichia coli or Azotobacter, the cell wall is much thinner, but its structure is more complex. The murein layer of these bacteria is covered on the outside with a soft and smooth layer of lipids. This protects them from lysozyme. Lysozyme is found in saliva, tears and other biological fluids, as well as in the white of chicken eggs. It catalyzes the hydrolysis of certain bonds between carbohydrate residues and thus breaks down the polysaccharide backbone of murein. The cell wall ruptures, and if the cell is in a hypotonic solution, its lysis occurs (the cell osmotically swells and bursts). The lipid layer also gives the cell resistance to penicillin. This antibiotic prevents the formation of cross-links in the cell wall of gram-positive bacteria, which makes growing cells more sensitive to osmotic shock.

Flagella

Many bacteria are motile, and this mobility is due to the presence of one or more flagella. Flagella in bacteria are much simpler than in eukaryotes (Section 17.6.2, Table 2.1), and in their structure they resemble one of the microtubules of a eukaryotic flagellum. Flagella consist of identical spherical protein subunits flagellina(similar to muscle actin), which are arranged in a spiral and form a hollow cylinder with a diameter of about 10-20 nm. Despite the wavy shape of the flagella, they are quite rigid.

The flagella are driven by a unique mechanism. The base of the flagellum apparently rotates so that the flagellum seems to be screwed into the medium, without making random beats, and thus propels the cell forward. This is apparently the only structure known in nature that uses the wheel principle. Another interesting feature of flagella is the ability of individual flagellin subunits to spontaneously assemble into helical filaments in solution. Spontaneous self-assembly- a very important property of many complex biological structures. In this case, self-assembly is entirely determined by the amino acid sequence (primary structure) of flagellin.

Motile bacteria can move in response to certain stimuli, i.e. they are capable of taxis. For example, aerobic bacteria have positive aerotaxis (i.e., they swim to where the environment is richer in oxygen), and motile photosynthetic bacteria have positive phototaxis (i.e., they swim toward the light).

Flagella are most easily examined in an electron microscope (Fig. 2.5) using the metal sputtering technique (section A.2.5).

Rice. 2.5. Micrograph of a rod-shaped bacterium obtained using a transmission electron microscope. The cell wall, fimbriae and long wavy flagella are clearly visible, × 28000

Pili, or fimbriae

The cell walls of some Gram-negative bacteria have thin projections (rod-shaped protein projections) called drank or fimbriae(Fig. 2.5). They are shorter and thinner than flagella and serve to attach cells to each other or to some surface, imparting a specific “stickiness” to those strains that possess them. There are different types of drank. The most interesting are the so-called F-pili, which are encoded by a special plasmid (Section 2.2.4) and are associated with the sexual reproduction of bacteria.

Plasma membrane, mesosomes and photosynthetic membranes

Like all cells, the protoplasm of bacteria is surrounded by a semi-permeable membrane. In structure and function, the plasma membranes of bacteria do not differ from the membranes of eukaryotic cells (section 7.2.1). In some bacteria, the plasma membrane is invaginated into the cell and forms mesosomes and/or photosynthetic membranes.

Mesosomes- folded membrane structures (Fig. 2.3 and 2.4), on the surface of which there are enzymes involved in the respiration process. Therefore, mesosomes can be called primitive organelles. During cell division, mesosomes bind to DNA, which appears to facilitate separation of the two daughter DNA molecules after replication and promote the formation of a septum between the daughter cells.

In photosynthetic bacteria, photosynthetic pigments (including bacteriochlorophyll) are located in sac-shaped, tubular or lamellar invaginations of the plasma membrane. Similar membrane formations are also involved in nitrogen fixation.

Genetic material

Bacterial DNA is represented by single circular molecules about 1 mm long. Each such molecule consists of approximately 5·10 6 pairs of nucleotides. The total DNA content (genome) in a bacterial cell is much less than in a eukaryotic cell, and therefore the volume of information encoded in it is also less. On average, such DNA contains several thousand genes, which is approximately 500 times less than in a human cell (see also Table 2.1 and Fig. 2.3).

Ribosomes

See table. 2.1 (protein biosynthesis) and Fig. 2.3.

Controversy

Some bacteria (mostly belonging to the genus Clostridium or Bacillus) form endospores, i.e. spores found inside the cell. Endospores are thick-walled, long-lived formations that are extremely resistant to heat and short-wave radiation. They are located differently inside the cell, which serves as a very important feature for the identification and taxonomy of such bacteria (Fig. 2.6). If a resting, stable structure is formed from a whole cell, then it is called a cyst. Cysts are formed by some Azotobacter species.

Rice. 2.6. Different forms of bacteria, illustrated by several of the most common types of beneficial and pathogenic microbes.

A. Cocci (spherical)

An example is Staphylococcus aureus, living in the nasopharynx; different strains of staphylococci cause furunculosis, pneumonia, food poisoning and other diseases.

The cell of prokaryotic organisms has a complex, strictly ordered structure and has fundamental features of ultrastructural organization and chemical composition.

The structural components of a bacterial cell are divided into basic and temporary (Fig. 2). The main structures are: cell wall, cytoplasmic membrane with its derivatives, cytoplasm with ribosomes and various inclusions, nucleoid; temporary - capsule, mucous membrane, flagella, villi, endospores, formed only at certain stages of the bacterial life cycle; in some species they are completely absent.

In a prokaryotic cell, the structures located outside the cytoplasmic membrane are called superficial (cell wall, capsule, flagella, villi).

The term "envelope" is currently used to refer to the cell wall and capsule of bacteria or just the cell wall; the cytoplasmic membrane is not part of the envelope and refers to the protoplast.

The cell wall is an important structural element of the bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria it is the outer cell membrane. It is obligatory for all prokaryotes, with the exception of mycoplasmas and L-form bacteria. 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 thickness of the cell wall is 10-100 nm, and it accounts for 5 to 50% of the dry matter of the cell.

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. The two types of bonds (glycosidic and peptide) that connect the peptidoglycan subunits give this heteropolymer a molecular network structure. The core of the peptidoglycan layer is the same in all bacterial species; Tetrapeptide protein chains and peptide (transverse) chains are different in different species.

Based on their tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. In 1884, H. Gram proposed a staining method that was used to differentiate bacteria. The essence of the method is that 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 with ethanol is determined by the specific chemical composition and ultrastructure of their cell wall. Peptidoglycan in gram-positive bacteria is the main component of the cell wall and makes up from 50 to 90%, in gram-negative bacteria it is 1-10%. The structural microfibrils of peptidoglycan of Gram-negative bacteria are cross-linked less compactly, therefore the pores in their peptidoglycan layer are much wider than in the molecular framework of Gram-positive bacteria. With such a structural organization of peptidoglycan, the violet complex of gentian violet and iodine in gram-negative bacteria will be washed out faster.

The cell wall of gram-positive bacteria is tightly adjacent to the cytoplasmic membrane, massive, and its thickness is in the range of 20-100 nm. It is characterized by the presence of teichoic acids; they are associated with peptidoglycan and are polymers of trihydric alcohol - glycerol or pentaatomic alcohol - ribitol, the residues of which are connected by phosphodiester bonds. Teichoic acids bind magnesium ions and participate in their transport into the cell. Polysaccharides, proteins and lipids are also found in small quantities in the cell wall of gram-positive prokaryotes.

Rice. 2. Scheme of the structure of a prokaryotic cell:

1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - nucleoid; 5 - cytoplasm; 6 - chromatophores; 7 - thylakoids; 8 - mesosoma; 9 - ribosomes; 10 - flagella; 11—basal body; 12 - drank; 13 - inclusion of sulfur; 14 — drops of fat; 15 — polyphosphate granules; 16 - plasmid

The cell wall of gram-negative bacteria is multilayered, its thickness is 14-17 nm. The inner layer is peptidoglycan, which forms a thin (2 nm) continuous network surrounding the cell. Peptidoglycan contains only mesodiaminopimelic acid and no lysine. The outer layer of the cell wall - the outer membrane - consists of phospholipids, lipopolysaccharide, lipoprotein and proteins. The outer membrane contains matrix proteins, which are tightly bound to the peptidoglycan layer. One of their functions is the formation of hydrophilic pores in the membrane, through which diffusion of molecules with a mass of up to 600, sometimes 900 occurs. Matrix proteins, in addition, also act as receptors for some phages. Lipopolysaccharide (LPS) in the cell walls of Gram-negative bacteria consists of lipid A and a polysaccharide. LPS, which is toxic to animals, is called endotoxin. Teichoic acids have not been found in gram-negative bacteria.

The structural components of the cell wall of Gram-negative bacteria are demarcated from the cytoplasmic membrane and separated by a space called the periplasm or periplasmic space.

Protoplasts and spheroplasts. Protoplasts are forms of prokaryotes completely devoid of a cell wall, usually formed in gram-positive bacteria. Spheroplasts are bacteria with a partially destroyed cell wall. They retain elements of the outer membrane. They are observed in gram-negative bacteria and much less frequently in gram-positive bacteria. They are formed as a result of the destruction of the peptidoglycan layer by lytic enzymes, for example lysozyme, or blocking the biosynthesis of peptidoglycan with the antibiotic penicillin, etc. in an environment with the appropriate osmotic pressure.

Protoplasts and spheroplasts have a spherical or hemispherical shape and are 3-10 times larger than the original cells. Under normal conditions, osmotic lysis occurs and they die. Under conditions of increased osmotic pressure, they are able to survive, grow and even divide for some time. When the factor that destroys peptidoglycan is removed, protoplasts, as a rule, die off, but can turn into L-forms; spheroplasts easily revert to the original bacteria, sometimes transform into L-forms or die.

L-Forms of bacteria. These 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 that are defective in the cell wall. They received their name due to the fact that they were isolated and described at the Lister Institute in England in 1935. 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 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-forms of bacteria are polymorphic. There are elementary bodies measuring 0.2-1 microns (minimal reproductive elements), spheres - 1-5, large bodies - 5-50, threads - up to 4 microns or more. 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, which makes them similar to protoplasts; 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.

L-forms are given great importance in the development of chronic recurrent infections, carriage of pathogens, and their long-term persistence in the body. The transplacental invasiveness of elementary bodies of L-form bacteria has been proven.

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 the mucous layer located above the cell wall of the bacterium. The substance of the capsule is clearly demarcated from the environment. Depending on the thickness of the layer and the strength of the connection with the bacterial cell, a macrocapsule with a thickness of more than 0.2 microns, clearly visible in a light microscope, and a microcapsule with a thickness of less than 0.2 microns, detectable only with an electron microscope or detected by chemical and immunological methods, are distinguished. The macrocapsule (true capsule) is formed by B. anlhracis, C1. perfringens, microcapsule - Escherichia coJi. The capsule is not an essential structure of the bacterial cell: its loss does not lead to the death of the bacterium. Capsuleless mutants of bacteria are known, for example the anthrax vaccine strain STI-1.

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. Thus, the composition of the capsule of B. anthracis includes the D-glutamic acid polypeptide (dextrorotatory isomer). The composition of the microcapsule of mammalian Mycobacterium tuberculosis includes glycopeptides represented by an ester of trehalose and mycolic acid (cord factor).

Capsule synthesis is a complex process and has its own characteristics in different prokaryotes; It is believed that capsule biopolymers are synthesized on the outer surface of the cytoplasmic membrane and are released onto the surface of the cell wall in certain specific areas.

There are bacteria that synthesize mucus, which is deposited on the surface of the cell wall in the form of a structureless layer of polysaccharide nature. The mucous substance surrounding the cell is often thicker than the diameter of the cell. In the saprophytic bacterium Leuconostoca, the formation of one capsule for many individuals is observed. Such accumulations of bacteria enclosed in a common capsule are called zooglea.

The capsule is a multifunctional organelle that plays an important biological role. It is the site of localization of capsular antigens that determine the virulence, antigenic specificity and immunogenicity of bacteria. The loss of the capsule in pathogenic bacteria sharply reduces their virulence, for example, in noncapsular strains of the anthrax bacillus. 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.

In veterinary microbiology, detection of the capsule is used as a differential morphological sign of the pathogen when testing for anthrax.

For coloring capsules, special methods are used - Romanovsky - Giemsa, Gins - Burri, Olt, Mikhin, etc.

The microcapsule and mucous layer are determined by serological reactions (RA), the antigenic components of the capsule are identified using the immunofluorescence method (RIF) and RDD.

Flagella are organelles of bacterial movement, represented by thin, long, thread-like structures of a protein nature. Their length exceeds the bacterial cell several times and is 10-20 microns, and in some spirilla it reaches 80-90 microns. The flagellum filament (fibril) is a complete spiral cylinder with a diameter of 12-20 nm. In Vibrios and Proteus, the filament is surrounded by a sheath 35 nm thick.

Rice. 3. Flagella:

a - monotrichs; b - amphitrichs; c - lophotrichs; d - peritrichous

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. Thus, in the causative agent of tetanus in old cultures, cells without flagella predominate.

The number of flagella (from I to 50 or more) and the places of their localization in bacteria of different species are not the same, but are 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; peritrichs are bacteria with many flagella located on the sides of the cell or on its entire surface (Fig. 3). 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. The speed of movement of monotrichs and lophotrichs reaches 50 μm/s, amphitrichy and peritrichs move more slowly and usually cover a distance equal to the size of their cell in 1 s.

Bacteria move randomly, but they are capable of directed forms of movement - taxis, which are determined by external stimuli. Reacting to various environmental factors, bacteria are localized in an optimal habitat zone in a short time. Taxis can be positive and negative. It is customary to distinguish between: chemotaxis, aerotaxis, phototaxis, magnotaxis. Chemotaxis is caused by differences in the concentration of chemicals in the environment, aerotaxis by oxygen, phototaxis by light intensity, magnetotaxis is determined by the ability of microorganisms to navigate in a magnetic field.

Identification of motile flagellar forms of bacteria is important for their identification in the laboratory diagnosis of infectious diseases.

Pili (fimbriae, villi) are straight, thin, hollow protein cylinders 3-25 nm thick and up to 12 µm long, 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 (Fig. 4). On the surface of the cell there can be from 1-2, 50-400 or more pili to several thousand.

Rice. 4. Drank

Pili of the general type are located peritrichially (Escherichia coli) or at the poles (pseudomonas); one bacterium can contain hundreds of them. They take part in the adhesion 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.

Cytoplasmic membrane and its derivatives. 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 and makes up 8-15% of its dry mass. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell. Ultrathin sections in an electron microscope reveal its three-layer structure - two limiting osmiophilic layers, each 2-3 nm thick, and one osmiophobic central layer 4-5 nm thick.

Chemically, the cytoplasmic membrane is a protein-lipid complex consisting of 50-75% proteins and 15-50% lipids. The main part of membrane lipids (70-90%) 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.

Cytoplasmic membrane enzymes catalyze the final steps in the synthesis of membrane lipids, cell wall components, capsule and exoenzymes; Oxidative phosphorylation enzymes and electron transport enzymes responsible for energy synthesis are localized on the membrane.

A connection between mesosomes and the bacterial chromosome has been established; such structures are called nucleoidosomes. Mesosomes integrated with the nucleoid take part in karyokinesis and cytokinesis of microbial cells, ensuring the distribution of the genome after the end of DNA replication and the subsequent divergence of daughter chromosomes. 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.

Ribosomes are organelles that carry out protein biosynthesis. They consist of protein and RNA, connected into a complex by hydrogen and hydrophobic bonds. Bacterial ribosomes are granules with a diameter of 15-20 nm, have a sedimentation constant of 70S and are formed from two ribonucleoprotein subunits: 30S and 50S. One bacterial cell can contain from 5000-50,000 ribosomes; through mRNA they are combined into polysome aggregates consisting of 50-55 ribosomes with high protein-synthesizing activity.

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; these include membrane-surrounded granules of poly-/3-hydroxybutyric acid, which sharply refract light and are clearly visible in a light microscope. Anthrax bacilli and aerobic spore-forming saprophytic bacteria are also detected. Mycobacteria accumulate waxes as reserve substances. The cells of some measles nonbacteria, 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, 1.1-1.6 nm long, 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 mesosoms. The bacterial chromosome replicates in a polyconservative manner: the parent DNA double helix unwinds and a new complementary chain is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the separation of daughter nuclei is ensured by the growth of the cytoplasmic membrane.

The nucleoid contains the bulk of the genetic information of the bacterial cell.

In addition to the nucleoid, extrachromosomal genetic elements are found in the cells of many bacteria - plasmids, which are small circular DNA molecules capable of autonomous replication.

The structure of bacteria has been well studied using electron microscopy of whole cells and their ultrathin sections, as well as other methods. The bacterial cell is surrounded by a membrane consisting of a cell wall and a cytoplasmic membrane. Under the shell there is protoplasm, consisting of cytoplasm with inclusions and a hereditary apparatus - an analogue of the nucleus, called the nucleoid (Fig. 2.2). There are additional structures: capsule, microcapsule, mucus, flagella, pili. Some bacteria are capable of forming spores under unfavorable conditions.

Rice. 2.2. Structure of a bacterial cell: 1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - mesosomes; 5 - nucleoid; 6 - plasmid; 7 - ribosomes; 8 - inclusions; 9 - flagellum; 10 - pili (villi)

Cell wall- a strong, elastic structure that gives the bacterium a certain shape and, together with the underlying cytoplasmic membrane, restrains high osmotic pressure in the bacterial cell. It is involved in the process of cell division and transport of metabolites, has receptors for bacteriophages, bacteriocins and various substances. The thickest cell wall is found in gram-positive bacteria (Fig. 2.3). So, if the thickness of the cell wall of gram-negative bacteria is about 15-20 nm, then in gram-positive bacteria it can reach 50 nm or more.

The basis of the bacterial cell wall is peptidoglycan. Peptidoglycan is a polymer. It is represented by parallel polysaccharide glycan chains consisting of repeating N-acetylglucosamine and N-acetylmuramic acid residues connected by a glycosidic bond. This bond is broken by lysozyme, which is an acetylmuramidase.

A tetrapeptide is attached to N-acetylmuramic acid by covalent bonds. The tetrapeptide consists of L-alanine, which is linked to N-acetylmuramic acid; D-glutamine, which in gram-positive bacteria is combined with L-lysine, and in gram-tri-

Rice. 2.3. Scheme of the architecture of the bacterial cell wall

beneficial bacteria - with diaminopimelic acid (DAP), which is a precursor of lysine in the process of bacterial biosynthesis of amino acids and is a unique compound present only in bacteria; The 4th amino acid is D-alanine (Fig. 2.4).

The cell wall of gram-positive bacteria contains small amounts 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. Tetrapeptides of different layers of peptidoglycan in gram-positive bacteria are connected to each other by polypeptide chains of 5 glycine residues (pentaglycine), which gives the peptidoglycan a rigid geometric structure (Fig. 2.4, b). Covalently linked to the peptidoglycan of the cell wall of gram-positive bacteria teichoic acids(from Greek tekhos- wall), the molecules of which are chains of 8-50 glycerol and ribitol residues connected by phosphate bridges. The shape and strength of bacteria is given by the rigid fibrous structure of the multilayer peptidoglycan, with cross-links of peptides.

Rice. 2.4. Structure of peptidoglycan: a - gram-negative bacteria; b - gram-positive bacteria

The ability of Gram-positive bacteria to retain gentian violet in combination with iodine when stained using Gram stain (blue-violet color of bacteria) is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, subsequent treatment of a bacterial smear with alcohol causes a narrowing of the pores in the peptidoglycan and thereby retains the dye in the cell wall.

Gram-negative bacteria lose the dye after exposure to alcohol, which is due to a smaller amount of peptidoglycan (5-10% of the cell wall mass); they are discolored with alcohol, and when treated with fuchsin or safranin they become red. This is due to the structural features of the cell wall. Peptidoglycan in the cell wall of gram-negative bacteria is represented by 1-2 layers. The tetrapeptides of the layers are connected to each other by a direct peptide bond between the amino group of DAP of one tetrapeptide and the carboxyl group of D-alanine of the tetrapeptide of another layer (Fig. 2.4, a). Outside the peptidoglycan there is a layer lipoprotein, connected to peptidoglycan through DAP. Followed by outer membrane cell wall.

Outer membrane is a mosaic structure composed of lipopolysaccharides (LPS), phospholipids and proteins. Its inner layer is represented by phospholipids, and the outer layer contains LPS (Fig. 2.5). Thus, the outer mem-

Rice. 2.5. Lipopolysaccharide structure

the brane is asymmetric. The outer membrane LPS consists of three fragments:

Lipid A has a conservative structure, almost the same in gram-negative bacteria. Lipid A consists of phosphorylated glucosamine disaccharide units to which long chains of fatty acids are attached (see Fig. 2.5);

Core, or core, crustal part (from lat. core- core), relatively conservative oligosaccharide structure;

A highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences.

LPS is anchored in the outer membrane by lipid A, which causes LPS toxicity and is therefore identified with endotoxin. The destruction of bacteria by antibiotics leads to the release of large amounts of endotoxin, which can cause endotoxic shock in the patient. The core, or core part, of LPS extends from lipid A. The most constant part of the LPS core is ketodeoxyoctonic acid. O-specific polysaccharide chain extending from the core of the LPS molecule,

consisting of repeating oligosaccharide units, determines the serogroup, serovar (a type of bacteria detected using immune serum) of a particular strain of bacteria. Thus, the concept of LPS is associated with the concept of O-antigen, by which bacteria can be differentiated. Genetic changes can lead to defects, shortening of bacterial LPS and, as a result, the appearance of rough colonies of R-forms that lose O-antigen specificity.

Not all gram-negative bacteria have a complete O-specific polysaccharide chain, consisting of repeating oligosaccharide units. In particular, bacteria of the genus Neisseria have a short glycolipid called lipooligosaccharide (LOS). It is comparable to the R form, which has lost O-antigen specificity, observed in mutant rough strains E. coli. The structure of VOC resembles the structure of the glycosphingolipid of the human cytoplasmic membrane, so VOC mimics the microbe, allowing it to evade the host's immune response.

The matrix proteins of the outer membrane permeate it in such a way that protein molecules called porinami, border hydrophilic pores through which water and small hydrophilic molecules with a relative mass of up to 700 D pass.

Between the outer and cytoplasmic membrane is periplasmic space, or periplasm containing enzymes (proteases, lipases, phosphatases, nucleases, β-lactamases), as well as components of transport systems.

When the synthesis of the bacterial cell wall is disrupted under the influence of lysozyme, penicillin, protective factors of the body and other compounds, cells with an altered (often spherical) shape are formed: protoplasts- bacteria completely lacking a cell wall; spheroplasts- bacteria with a partially preserved cell wall. After removal of the cell wall inhibitor, such altered bacteria can reverse, i.e. acquire a full cell wall and restore its original shape.

Bacteria of the spheroid 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-shapes(from the name of the D. Lister Institute, where they first

have been studied). L-forms can also arise as a result of mutations. 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. L-forms can be produced by many pathogens of infectious diseases.

Cytoplasmic membrane in electron microscopy of ultrathin sections, it is a three-layer membrane (2 dark layers, each 2.5 nm thick, separated by a light intermediate layer). In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of lipids, mainly phospholipids, with embedded surface and integral proteins that seem to penetrate through the structure of the membrane. Some of them are permeases involved in the transport of substances. Unlike eukaryotic cells, the cytoplasmic membrane of a bacterial cell lacks sterols (with the exception of mycoplasmas).

The cytoplasmic membrane is a dynamic structure with mobile components, so it is thought of as a mobile fluid structure. It surrounds the outer part of the cytoplasm of bacteria and is involved in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell (due to enzymes of the electron transport chain, adenosine triphosphatase - ATPase, etc.). With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complexly twisted membrane structures, called mesosomes. Less complexly twisted structures are called intracytoplasmic membranes. The role of mesosomes and intracytoplasmic membranes is not fully understood. It is even suggested that they are an artifact that occurs after preparing (fixing) a specimen for electron microscopy. Nevertheless, it is believed that derivatives of the cytoplasmic membrane participate in cell division, providing energy for the synthesis of the cell wall, and take part in the secretion of substances, sporulation, i.e. in processes with high energy consumption. Cytoplasm occupies the main volume of bacteria

cell and consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes, responsible for the synthesis (translation) of proteins.

Ribosomes bacteria have a size of about 20 nm and a sedimentation coefficient of 70S, in contrast to the 80S ribosomes characteristic of eukaryotic cells. Therefore, some antibiotics, by binding to bacterial ribosomes, inhibit bacterial protein synthesis without affecting protein synthesis in eukaryotic cells. Bacterial ribosomes can dissociate into two subunits: 50S and 30S. rRNA is a conserved element of bacteria (“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, β-hydroxybutyric acid and polyphosphates (volutin). They accumulate when there is an excess of nutrients in the environment and act as reserve substances for nutrition and energy needs.

Volyutin has an affinity for basic dyes and is easily detected using special staining methods (for example, according to Neisser) in the form of metachromatic granules. With toluidine blue or methylene blue, volutin is stained red-violet, and the cytoplasm of the bacterium is stained blue. The characteristic arrangement of volutin granules is revealed in the diphtheria bacillus in the form of intensely stained cell poles. The metachromatic coloration of volutin is associated with a high content of polymerized inorganic polyphosphate. Under electron microscopy, they look like electron-dense granules 0.1-1 microns in size.

Nucleoid- equivalent to the nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA, tightly packed like a ball. The nucleoid of bacteria, unlike eukaryotes, does not have a nuclear envelope, nucleolus and basic proteins (histones). Most bacteria contain one chromosome, represented by a DNA molecule closed in a ring. But some bacteria have two ring-shaped chromosomes (V. cholerae) and linear chromosomes (see section 5.1.1). The nucleoid is revealed in a light microscope after staining with DNA-specific stains

methods: according to Feulgen or according to Romanovsky-Giemsa. In electron diffraction patterns of ultrathin sections of bacteria, the nucleoid appears as light zones with fibrillar, thread-like structures of DNA bound in certain areas to the cytoplasmic membrane or mesosome involved in chromosome replication.

In addition to the nucleoid, the bacterial cell contains extrachromosomal heredity factors - plasmids (see section 5.1.2), which are covalently closed rings of DNA.

Capsule, microcapsule, mucus.Capsule - a mucous structure more than 0.2 microns 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 using special methods of staining a smear according to Burri-Gins, which creates 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 contains a large amount of water. It prevents the phagocytosis of bacteria. The capsule is antigenic: antibodies to the capsule cause its enlargement (capsule swelling reaction).

Many bacteria form microcapsule- mucous formation less than 0.2 microns thick, detectable only by electron microscopy.

It should be distinguished from a capsule slime - mucoid exopolysaccharides that do not have clear external boundaries. Mucus is soluble in water.

Mucoid exopolysaccharides are characteristic of mucoid strains of Pseudomonas aeruginosa, often found in the sputum of patients with cystic fibrosis. Bacterial exopolysaccharides are involved in adhesion (sticking to substrates); they are also called glycocalyx.

The capsule and mucus protect bacteria from damage and drying out, since, being hydrophilic, they bind water well and prevent the action of the protective factors of the macroorganism and bacteriophages.

Flagella bacteria determine the mobility of the bacterial cell. Flagella are thin filaments that take on

They originate 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 three parts: a spiral filament, a hook and a basal body containing a rod with special discs (one pair of discs in gram-positive bacteria and two pairs 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 rod - a rotor - rotating the flagellum. The proton potential difference on the cytoplasmic membrane is used as an energy source. The rotation mechanism is provided by proton ATP synthetase. The rotation speed of the flagellum can reach 100 rps. If a bacterium has several flagella, they begin to rotate synchronously, intertwining into a single bundle, forming a kind of propeller.

Flagella are made of a protein called flagellin. (flagellum- flagellum), which is an antigen - the so-called H-antigen. Flagellin subunits are twisted in a spiral.

The number of flagella in different species of bacteria varies from one (monotrichus) in Vibrio cholerae to tens and hundreds extending along the perimeter of the bacterium (peritrichus), 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.

Flagella are detected using electron microscopy of preparations coated with heavy metals, or in a light microscope after treatment with special methods based on etching and adsorption of various substances leading to an increase in the thickness of the flagella (for example, after silvering).

Villi, or pili (fimbriae)- thread-like formations, thinner and shorter (3-10 nm * 0.3-10 µm) than flagella. The pili extend from the cell surface and are composed of the protein pilin. Several types of pili are known. General type pili are responsible for attachment to the substrate, nutrition, and water-salt metabolism. They are numerous - several hundred per cell. Sex pili (1-3 per cell) create contact between cells, transferring genetic information between them by conjugation (see Chapter 5). Of particular interest are type IV pili, in which the ends are hydrophobic, as a result of which they curl; these pili are also called curls. Location

They are located at the poles of the cell. These pili are found in pathogenic bacteria. They have antigenic properties, bring bacteria into contact with the host cell, and participate in the formation of biofilm (see Chapter 3). Many pili are receptors for bacteriophages.

Disputes - a peculiar form of resting bacteria with a gram-positive type of cell wall structure. Spore-forming bacteria of the genus Bacillus, in which the size of the spore does not exceed the diameter of the cell are called bacilli. Spore-forming bacteria in which the size of the spore exceeds the diameter of the cell, which is why they take the shape of a spindle, are called clostridia, for example bacteria of the genus Clostridium(from lat. Clostridium- spindle). The spores are acid-resistant, therefore they are stained red using the Aujeszky method or the Ziehl-Neelsen method, and the vegetative cell is stained blue.

Sporulation, the shape and location of spores in a cell (vegetative) are a species property of bacteria, which allows them to be distinguished from each other. The shape of the spores can be oval or spherical, the 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 botulism, gas gangrene) and central (in the anthrax bacillus).

The process of sporulation (sporulation) goes through a number of stages, during which part of the cytoplasm and chromosome of the bacterial vegetative cell are separated, surrounded by an ingrowing cytoplasmic membrane - a prospore is formed.

The prospore protoplast contains a nucleoid, a protein synthesizing system, and an energy production system based on glycolysis. Cytochromes are absent even in aerobes. Does not contain ATP, energy for germination is stored in the form of 3-glycerol phosphate.

The prospore is surrounded by two cytoplasmic membranes. The layer surrounding the inner membrane of the spore is called wall of spores, it consists of peptidoglycan and is the main source of cell wall during spore germination.

Between the outer membrane and the spore wall, a thick layer is formed consisting of peptidoglycan, which has many cross-links - cortex.

Located outside the outer cytoplasmic membrane spore shell, consisting of keratin-like proteins, co-

holding multiple intramolecular disulfide bonds. This shell provides resistance to chemical agents. The spores of some bacteria have an additional covering - exosporium lipoprotein nature. In this way, a multilayer, poorly permeable shell is formed.

Sporulation is accompanied by intensive consumption by the prospore and then by the developing spore shell of dipicolinic acid and calcium ions. The spore acquires heat resistance, which is associated with the presence of calcium dipicolinate in it.

The spore can persist for a long time due to the presence of a multilayer shell, calcium dipicolinate, low water content and sluggish metabolic processes. In soil, for example, the pathogens of anthrax and tetanus can persist for decades.

Under favorable conditions, spores germinate, going through three successive stages: activation, initiation, growth. In this case, one bacterium is formed from one spore. Activation is readiness for germination. At a temperature of 60-80 °C, the spore is activated for germination. Germination initiation lasts several minutes. The outgrowth stage is characterized by rapid growth, accompanied by the destruction of the shell and the emergence of a seedling.

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