The structure of bacteria is well studied using electron microscopy of whole cells and their ultrathin sections, as well as other methods. A bacterial cell is surrounded by a membrane consisting of a cell wall and a cytoplasmic membrane. Under the shell 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 under adverse conditions are able to form spores.

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 - drank (villi)

cell wall- a strong, elastic structure that gives the bacteria 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 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 cell wall of bacteria is made up of peptidoglycan. Peptidoglycan is a polymer. It is represented by parallel polysaccharide glycan chains, consisting of repeating residues of N-acetylglucosamine and N-acetylmuramic acid connected by a glycosidic bond. This bond is broken by lysozyme, which is 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 connected to L-lysine, and in gram-positive bacteria

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

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 found only in bacteria; The 4th amino acid is D-alanine (Fig. 2.4).

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 a multilayer peptidoglycan (murein, mucopeptide), which makes up 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 (pentaglycine) residues, which gives the peptidoglycan a rigid geometric structure (Fig. 2.4, b). Covalently bound 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 residues of glycerol and ribitol connected by phosphate bridges. The shape and strength of the bacteria is given by the rigid fibrous structure of the multilayer, with cross-linked peptide cross-links of peptidoglycan.

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 (blue-violet color of bacteria) during Gram staining is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, the subsequent treatment of a smear of bacteria with alcohol causes a narrowing of the pores in peptidoglycan and thereby retains the dye in the cell wall.

Gram-negative bacteria after exposure to alcohol lose the dye, which is due to a smaller amount of peptidoglycan (5-10% of the mass of the cell wall); 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 interconnected 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 of peptidoglycan is a layer lipoprotein, bound to peptidoglycan via DAP. It is followed by outer membrane cell wall.

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

Rice. 2.5. Structure of lipopolysaccharide

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

Lipid A - 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 Figure 2.5);

The core, or rod, of the cow 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 determines the toxicity of LPS 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. From lipid A, the core, or the core part of the LPS, departs. The most constant part of the core of LPS is ketodeoxyoctonic acid. O-specific polysaccharide chain extending from the core part of the LPS molecule,

consisting of repeating oligosaccharide units, determines the serogroup, serovar (a type of bacteria detected using immune serum) of a certain bacterial strain. Thus, the concept of LPS is associated with ideas about the O-antigen, according to which bacteria can be differentiated. Genetic changes can lead to defects, shortening of the LPS of bacteria and, as a result, the appearance of rough colonies of R-forms that lose their 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-antigenic specificity, observed in mutant rough strains. E. coli. The structure of the VOC resembles that of the human cytoplasmic membrane glycosphingolipid, so the VOC mimics the microbe, allowing it to evade the host's immune response.

The proteins of the matrix of the outer membrane permeate it in such a way that the protein molecules, called porins, they 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 membranes is periplasmic space, or periplasm containing enzymes (proteases, lipases, phosphatases, nucleases, β-lactamases), as well as components of transport systems.

In case of violation of the synthesis of the bacterial cell wall 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 devoid of a cell wall; spheroplasts Bacteria with a partially preserved cell wall. After removal of the cell wall inhibitor, such altered bacteria can reverse, i. acquire a full-fledged cell wall and restore its original shape.

Bacteria of the spheroid or protoplast type that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-shaped(from the name of the D. Lister Institute, where they first

you 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 the bacteria is removed, can reverse, returning to the original bacterial cell. L-forms can form many pathogens of infectious diseases.

cytoplasmic membrane under electron microscopy of ultrathin sections, it is a three-layer membrane (2 dark layers 2.5 nm thick each are separated by a light one - intermediate). In structure, it is similar to the plasmolemma of animal cells and consists of a double layer of lipids, mainly phospholipids, with embedded surface and integral proteins, as if penetrating through the membrane structure. Some of them are permeases involved in the transport of substances. Unlike eukaryotic cells, there are no sterols in the cytoplasmic membrane of a bacterial cell (with the exception of mycoplasmas).

The cytoplasmic membrane is a dynamic structure with mobile components, therefore it is presented 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 the 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 complex twisted structures are called intracytoplasmic membranes. The role of mesosomes and intracytoplasmic membranes has not been fully elucidated. It is even suggested that they are an artifact that occurs after the preparation (fixation) of the preparation 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, take part in the secretion of substances, spore formation, i.e. in processes with high energy consumption. The cytoplasm occupies the bulk of the bacterial

a nal 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 bind to bacterial ribosomes and inhibit bacterial protein synthesis without affecting protein synthesis in eukaryotic cells. Bacterial ribosomes can dissociate into two subunits: 50S and 30S. rRNA - conservative elements of bacteria ("molecular clock" of evolution). 16S rRNA is part of the small subunit of ribosomes, and 23S rRNA is part of the large subunit of ribosomes. The study of 16S rRNA is the basis of gene systematics, making it possible to assess the degree of relatedness of organisms.

In the cytoplasm there are various inclusions in the form of glycogen granules, polysaccharides, β-hydroxybutyric acid and polyphosphates (volutin). They accumulate with an excess of nutrients in the environment and serve 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. Toluidine blue or methylene blue stains volutin red-violet, and the bacterial cytoplasm blue. The characteristic arrangement of volutin granules is revealed in diphtheria bacillus in the form of intensely stained poles of the cell. Metachromatic staining of volutin is associated with a high content of polymerized inorganic polyphosphate. Under electron microscopy, they look like electron-dense granules 0.1–1 µm in size.

Nucleoid is the equivalent of 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 bacterial nucleoid, unlike eukaryotes, does not have a nuclear membrane, 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 detected under a light microscope after staining with specific DNA

methods: according to Felgen or according to Romanovsky-Giemsa. On electron diffraction patterns of ultrathin sections of bacteria, the nucleoid has the form of light zones with fibrillar, thread-like structures of DNA associated with certain areas with the cytoplasmic membrane or mesosome involved in chromosome replication.

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

Capsule, microcapsule, mucus.Capsule - a mucous structure more than 0.2 microns thick, firmly associated with the bacterial cell wall and having clearly defined outer boundaries. The capsule is distinguishable in smears-imprints from pathological material. In pure cultures of bacteria, the capsule is formed less frequently. It is detected by special methods of smear staining according to Burri-Gins, which create a negative contrast of the capsule substances: the ink creates a dark background around the capsule. The capsule consists of polysaccharides (exopolysaccharides), sometimes polypeptides, for example, in the anthrax bacillus, it consists of polymers of D-glutamic acid. The capsule is hydrophilic, contains a large amount of water. It prevents phagocytosis of bacteria. The capsule is antigenic: antibodies to the capsule cause its increase (capsule swelling reaction).

Many bacteria form microcapsule- mucous formation with a thickness of less than 0.2 microns, detected only with electron microscopy.

To be distinguished from a capsule slime - mucoid exopolysaccharides that do not have clear external boundaries. Slime 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 protective factors of the macroorganism and bacteriophages.

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

originating from the cytoplasmic membrane, are longer than the cell itself. The flagella are 12–20 nm thick and 3–15 µm long. 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 and two pairs in Gram-negative bacteria). The discs of the flagella are attached to the cytoplasmic membrane and cell wall. This creates the effect of an electric motor with a rod - a rotor that rotates the flagellum. The difference of proton potentials on the cytoplasmic membrane is used as an energy source. The rotation mechanism is provided by proton ATP synthetase. The speed of rotation of the flagellum can reach 100 rpm. If a bacterium has several flagella, they begin to rotate synchronously, intertwining into a single bundle, forming a kind of propeller.

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

The number of flagella in bacteria of different species varies from one (monotrich) in Vibrio cholerae to ten or hundreds extending along the perimeter of the bacterium (peritrich), in Escherichia coli, Proteus, etc. Lofotrichs have a bundle of flagella at one end of the cell. Amphitrichous have one flagellum or a bundle of flagella at opposite ends of the cell.

Flagella are detected using electron microscopy of preparations sprayed with heavy metals, or in a light microscope after processing by 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)- filamentous formations, thinner and shorter (3-10 nm * 0.3-10 microns) than flagella. Pili extend from the cell surface and are composed of the pilin protein. Several types of saws are known. Pili of a general type 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 twist, these pili are also called curls. Located-

they are located at the poles of the cell. These pili are found in pathogenic bacteria. They have antigenic properties, make contact between the bacterium and the host cell, and participate in the formation of a 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 form of a spindle, are called clostridia, such as bacteria of the genus Clostridium(from lat. Clostridium- spindle). The spores are acid-resistant, therefore they are stained red according to the Aujeszky method or according to the Ziehl-Nelsen method, and the vegetative cell is blue.

Sporulation, the shape and location of spores in a cell (vegetative) are a species property of bacteria, which makes it possible to distinguish them from each other. The shape of the spores is oval and 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 pathogens of botulism, gas gangrene) and central (in anthrax bacilli).

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

The prospore protoplast contains a nucleoid, a protein-synthesizing system, and an energy-producing 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 that surrounds the inner membrane of the spore is called spore wall, it consists of peptidoglycan and is the main source of the cell wall during spore germination.

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

Outside of the outer cytoplasmic membrane is located spore shell, consisting of keratin-like proteins,

containing multiple intramolecular disulfide bonds. This shell provides resistance to chemical agents. Spores of some bacteria have an additional cover - exosporium lipoprotein nature. Thus, a multilayer poorly permeable shell is formed.

Sporulation is accompanied by intensive consumption by the prospore, and then by the emerging 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 multi-layered shell, calcium dipicolinate, low water content and sluggish metabolic processes. In the soil, for example, anthrax and tetanus pathogens can persist for decades.

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

Reading time: 6 min

Modern science has made fantastic progress in recent centuries. However, some mysteries still excite the minds of prominent scientists.

Today, the answer to the urgent question has not been found - how many varieties of bacteria exist on our vast planet?

Bacterium- an organism with a unique internal organization, which is characterized by all the processes characteristic of living organisms. The bacterial cell has many amazing features, one of which is the variety of shapes.

A bacterial cell can be spherical, rod-shaped, cubic, or star-shaped. In addition, the bacteria are slightly bent or form a variety of curls.

The shape of the cell plays an important role in the proper functioning of the microorganism, as it can affect the ability of the bacterium to attach to other surfaces, obtain the necessary substances and move.

The minimum cell size is usually 0.5 µm, however, in exceptional cases, the size of the bacterium can reach 5.0 µm.

The structure of the cell of any bacterium is strictly ordered. Its structure differs significantly from the structure of other cells, such as plants and animals. Cells of all types of bacteria do not have such elements as: a differentiated nucleus, intracellular membranes, mitochondria, lysosomes.

Bacteria have specific structural components - permanent and non-permanent.

Permanent components include: cytoplasmic membrane (plasmolemma), cell wall, nucleoid, cytoplasm. Non-permanent structures are: capsule, flagella, plasmids, pili, villi, fimbriae, spores.

cytoplasmic membrane

Any bacterium is enveloped by a cytoplasmic membrane (plasmolemma), which includes 3 layers. The membrane contains globulins responsible for the selective transport of various substances into the cell.

The plasma membrane also performs the following important functions:

• mechanical- ensures the autonomous functioning of the bacterium and all structural elements;
• receptor- proteins located in the plasmalemma act as receptors, that is, they help the cell to perceive various signals;
• energy Some proteins are responsible for the function of energy transfer.

Violation of the functioning of the plasma membrane leads to the fact that the bacterium collapses and dies.

cell wall

The structural component inherent only to bacterial cells is the cell wall. This is a rigid permeable membrane, which acts as an important component of the structural skeleton of the cell. It is located on the outside of the cytoplasmic membrane.

The cell wall performs the function of protection, and in addition gives the cell a permanent shape. Its surface is covered with numerous spores that let in the necessary substances and remove decay products from the microorganism.

Protection of internal components from osmotic and mechanical effects is another function of the wall. It plays an indispensable role in the control of cell division and the distribution of hereditary traits in it. It contains peptidoglycan, which gives the cell valuable immunobiological characteristics.

The thickness of the cell wall ranges from 0.01 to 0.04 µm. With age, bacteria grow and the amount of material from which it is built increases accordingly.

Nucleoid

Nucleoid is a prokaryote, which stores all the hereditary information of a bacterial cell. The nucleoid is located in the central part of the bacterium. Its properties are equivalent to the kernel.

A nucleoid is a single DNA molecule closed in a ring. The length of the molecule is 1 mm, and the amount of information is about 1000 features.

The nucleoid is the main carrier of material about the properties of the bacterium and the main factor in the transmission of these properties to offspring. The nucleoid in bacterial cells does not have a nucleolus, membrane, or basic proteins.

Cytoplasm

Cytoplasm- an aqueous solution containing the following components: mineral compounds, nutrients, proteins, carbohydrates and lipids. The ratio of these substances depends on the age and type of bacteria.

The cytoplasm contains various structural components: ribosomes, granules and mesosomes.

• Ribosomes are responsible for protein synthesis. Their chemical composition includes RNA molecules and protein.
• Mesosomes are involved in spore formation and cell reproduction. May be in the form of a bubble, loop, tubule.
• Granules serve as an additional energy resource for bacterial cells. These elements come in a variety of forms. They contain polysaccharides, starch, fat droplets.

Capsule

Capsule It is a mucous structure tightly bound to the cell wall. Examining it under a light microscope, one can see that the capsule envelops the cell and its outer boundaries have a clearly defined contour. In a bacterial cell, the capsule serves as a protective barrier against phages (viruses).

Bacteria form a capsule when environmental conditions become aggressive. The capsule includes in its composition mainly polysaccharides, and in certain cases it may contain fiber, glycoproteins, polypeptides.

The main functions of the capsule:

• • adhesion with cells in the human body. For example, streptococci stick together with tooth enamel and, in alliance with other microbes, provoke caries;
  • protection from negative environmental conditions: toxic substances, mechanical damage, elevated oxygen levels;
  • participation in water metabolism (protection of cells from drying out);
  • creation of an additional osmotic barrier.

The capsule forms 2 layers:

• internal - part of the cytoplasm layer;
• external - the result of the excretory function of the bacterium.

The classification was based on the structural features of the capsules. They are:

• normal;
• complex capsules;
• with cross-striped fibrils;
• discontinuous capsules.

Some bacteria also form a microcapsule, which is a mucous formation. The microcapsule can be detected only under an electron microscope, since the thickness of this element is only 0.2 microns or even less.

Flagella

Most bacteria have surface structures of the cell that provide its mobility and movement - flagella. These are long processes in the form of a left-handed spiral, built from flagellin (a contractile protein).

The main function of flagella is that they allow bacteria to move in a liquid environment in search of more favorable conditions. The number of flagella in one cell can vary: from one to several flagella, flagella on the entire surface of the cell or only on one of its poles.

There are several types of bacteria, depending on the number of flagella in them:

• Monotrichous- they have only one flagellum.
• lophotrichous- have a certain number of flagella at one end of the bacterium.
• amphitriches- characterized by the presence of flagella at polar opposite poles.
• Peritrichi- flagella are located over the entire surface of the bacterium, they are characterized by slow and smooth movement.
• Atrichi- flagella are absent.

Flagella perform motor activity, making rotational movements. If bacteria do not have flagella, it is still able to move, or rather, slide with the help of mucus on the surface of the cell.

Plasmids

Plasmids are small mobile DNA molecules separate from chromosomal heredity factors. These components usually contain genetic material that makes the bacterium more resistant to antibiotics.

They can transfer their properties from one microorganism to another. Despite all their features, plasmids do not act as important elements for the life of a bacterial cell.

Pili, villi, fimbriae

These structures are localized on the surfaces of bacteria. They count from two units to several thousand per cell. Both the bacterial mobile cell and the immobile cell have these structural elements, since they do not have any effect on the ability to move.

Quantitatively, pili reach several hundred per bacterium. There are pili that are responsible for nutrition, water-salt metabolism, as well as conjugation (sex) pili.

The villi are characterized by a hollow cylindrical shape. It is through these structures that viruses enter the bacterium.

Villi are not considered essential components of a bacterium, since even without them the process of division and growth can be successfully completed.

Fimbria are located, as a rule, at one end of the cell. These structures allow the microorganism to be fixed in the tissues of the body. Some fimbriae have special proteins that are in contact with the receptor endings of the cells.

Fimbria differ from flagella in that they are thicker and shorter, and also do not realize the function of movement.

controversy

Spores are formed in the event of negative physical or chemical manipulation of the bacterium (as a result of drying or lack of nutrients). They are diverse in spore size, since they can be completely different in different cells. The shape of the spores also differs - they are oval or spherical.

By location in the cell, spores are divided into:

• central - their position in the very center, as, for example, in anthrax;
• subterminal - located at the end of the stick, giving the shape of a club (in the causative agent of gas gangrene).

In a favorable environment, the spore life cycle includes the following stages:

• preparatory stage;
• activation stage;
• initiation stage;
• germination stage.

Spores are distinguished by their special vitality, which is achieved due to their shell. It is multilayered and consists mainly of protein. The increased resistance of spores to negative conditions and external influences is ensured precisely due to proteins.

Structural components of a cell are the shell of bacteria, consisting of a cell wall, a cytoplasmic membrane, and sometimes a capsule; cytoplasm; ribosomes; various cytoplasmic inclusions; nucleoid (nucleus). Some types of bacteria also have spores, flagella, cilia (pili, fimbriae) (Fig. 2).

cell wall obligatory formation of bacteria of most species. Its structure depends on the type and affiliation
bacteria to groups differentiated by Gram staining. The mass of the cell wall is about 20% of the dry mass of the entire cell, the thickness is from 15 to 80 nm.

Rice. 3. Scheme of the structure of a bacterial cell

1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - cytoplasm; 5 - mesosomes; 6 - ribosomes; 7 - nucleoid; 8 - intracytoplasmic membrane formations; 9 - fat drops; 10 - polysaccharide granules; 11 - polyphosphate granules; 12 - sulfur inclusions; 13 - flagella; 14 - basal body

The cell wall has pores up to 1 nm in diameter, so it is a semi-permeable membrane through which nutrients penetrate and metabolic products are released.

These substances can penetrate into the microbial cell only after preliminary hydrolytic cleavage by specific enzymes secreted by bacteria into the external environment.

The chemical composition of the cell wall is heterogeneous, but it is constant for a certain type of bacteria, which is used for identification. Nitrogenous compounds, lipids, cellulose, polysaccharides, pectin substances were found in the composition of the cell wall.

The most important chemical component of the cell wall is a complex polysaccharide peptide. It is also called peptidoglycan, glycopeptide, murein (from lat. Murus - wall).

Murein is a structural polymer composed of glycan molecules formed by acetylglucosamine and acetylmuramic acid. Its synthesis is carried out in the cytoplasm at the level of the cytoplasmic membrane.

The cell wall peptidoglycan of various types has a specific amino acid composition and, depending on this, a certain chemotype, which is taken into account when identifying lactic acid and other bacteria.

In the cell wall of Gram-negative bacteria, peptidoglycan is represented by a single layer, while in the wall of Gram-positive bacteria it forms several layers.

In 1884, Gram proposed a tissue staining method that was used to stain prokaryotic cells. If, during Gram staining, fixed cells are treated with an alcoholic solution of crystal violet dye, and then with an iodine solution, then these substances form a stable colored complex with murein.

In homopositive microorganisms, the stained violet complex does not dissolve under the influence of ethanol and, accordingly, does not discolor; when stained with fuchsin (red dye), the cells remain dark purple.

In gram-negative species of microorganisms, gentian violet is dissolved in ethanol and washed out with water, and when stained with fuchsin, the cell turns red.

The ability of microorganisms to stain with analine dyes and according to the Gram method is called tinctorial properties . They must be studied in young (18-24 hour) cultures, as some Gram-positive bacteria in old cultures lose their ability to stain positive by the Gram method.

The significance of peptidoglycan lies in the fact that, thanks to it, the cell wall has rigidity, i.e. elasticity, and is the protective frame of the bacterial cell.

When peptidoglycan is destroyed, for example, under the action of lysozyme, the cell wall loses its rigidity and collapses. The contents of the cell (cytoplasm), together with the cytoplasmic membrane, acquires a spherical shape, that is, it becomes a protoplast (spheroplast).

Many synthesizing and degrading enzymes are associated with the cell wall. Cell wall components are synthesized in the cytoplasmic membrane and then transported into the cell wall.

cytoplasmic membrane located under the cell wall and tightly adjacent to its inner surface. It is a semi-permeable membrane surrounding the cytoplasm and the inner contents of the cell - the protoplast. The cytoplasmic membrane is the thickened outer layer of the cytoplasm.

The cytoplasmic membrane is the main barrier between the cytoplasm and the environment, the violation of its integrity leads to cell death. It consists of proteins (50-75%), lipids (15-45%), in many species - carbohydrates (1-19%).

The main lipid component of the membrane are phospho- and glycolipids.

The cytoplasmic membrane, with the help of enzymes localized in it, performs various functions: it synthesizes membrane lipids - components of the cell wall; membrane enzymes - selectively transport various organic and inorganic molecules and ions through the membrane, the membrane is involved in the transformation of cellular energy, as well as in chromosome replication, in the transfer of electrochemical energy and electrons.

Thus, the cytoplasmic membrane provides selective entry into the cell and removal from it of various substances and ions.

Derivatives of the cytoplasmic membrane are mesosomes . These are spherical structures formed when the membrane is twisted into a curl. They are located on both sides - at the site of the formation of the cell septum or near the zone of localization of nuclear DNA.

Mesosomes are functionally equivalent to mitochondria in cells of higher organisms. They participate in the redox reactions of bacteria, play an important role in the synthesis of organic substances, in the formation of the cell wall.

Capsule is a derivative of the outer layer of the cell coat and is a mucous membrane surrounding one or more microbial cells. Its thickness can reach 10 microns, which is many times greater than the thickness of the bacterium itself.

The capsule performs a protective function. The chemical composition of the bacterial capsule is different. In most cases, it consists of complex polysaccharides, mucopolysaccharides, sometimes polypeptides.

Capsulation is usually a species trait. However, the appearance of a microcapsule often depends on the conditions of bacterial cultivation.

Cytoplasm- a complex colloidal system with a large amount of water (80-85%), in which proteins, carbohydrates, lipids, as well as mineral compounds and other substances are dispersed.

The cytoplasm is the contents of a cell surrounded by a cytoplasmic membrane. It is divided into two functional parts.

One part of the cytoplasm is in the state of a sol (solution), has a homogeneous structure and contains a set of soluble ribonucleic acids, enzyme proteins and metabolic products.

The other part is represented by ribosomes, inclusions of various chemical nature, genetic apparatus, and other intracytoplasmic structures.

Ribosomes- these are submicroscopic granules, which are spherical nucleoprotein particles with a diameter of 10 to 20 nm, a molecular weight of about 2-4 million.

Ribosomes of prokaryotes consist of 60% RNA (ribonucleic acid), located in the center, and 40 % protein that coats the nucleic acid on the outside.

Cytoplasmic inclusions are metabolic products, as well as reserve products, due to which the cell lives in conditions of nutrient deficiency.

The genetic material of prokaryotes consists of a double strand of deoxyribonucleic acid (DNA) of a compact structure located in the central part of the cytoplasm and not separated from it by a membrane. Bacterial DNA does not differ in structure from eukaryotic DNA, but since it is not separated from the cytoplasm by a membrane, the genetic material is called nucleoid or genophore. Nuclear structures are spherical or horseshoe shaped.

controversy bacteria are a dormant, non-reproducing form. They form inside the cell, are round or oval formations. Spores form predominantly gram-positive bacteria, rod-shaped with aerobic and anaerobic type of respiration in old cultures, as well as in adverse environmental conditions (lack of nutrients and moisture, accumulation of metabolic products in the medium, changes in pH and cultivation temperature, the presence or absence of atmospheric oxygen and etc.) can switch to an alternative development program, resulting in disputes. In this case, one spore is formed in the cell. This indicates that sporulation in bacteria is an adaptation for the preservation of the species (individual) and is not a way of their reproduction. The process of sporulation occurs, as a rule, in the external environment within 18-24 hours.

A mature spore is approximately 0.1 of the volume of the mother cell. Spores in different bacteria differ in shape, size, location in the cell.

Microorganisms whose spore diameter does not exceed the width of the vegetative cell are called bacilli, bacteria that have spores, the diameter of which is 1.5-2 times larger than the diameter of the cell, are called clostridia.

Inside the microbial cell, the spore can be located in the middle - the central position, at the end - the terminal and between the center and the end of the cell - the subterminal position.

Flagella bacteria are locomotor organs (organs of movement), with the help of which bacteria can move at a speed of up to 50-60 microns / s. At the same time, for 1 s, the bacteria cover the length of their body by 50-100 times. The length of the flagella exceeds the length of bacteria by 5-6 times. The thickness of the flagella is on average 12-30 nm.

The number of flagella, their size and location are constant for certain types of prokaryotes and therefore are taken into account when identifying them.

Depending on the number and location of the flagella, bacteria are divided into monotrichous (monopolar monotrichous) - cells with one flagellum at one end, lophotrichous (monopolar polytrichous) - a bundle of flagella is located at one of the ends, amphitrichous (bipolar polytrichous) - flagella are located at each of poles, peritrichous - flagella are located over the entire surface of the cell (Fig. 4) and atrichous - bacteria devoid of flagella.

The nature of the movement of bacteria depends on the number of flagella, age, characteristics of the culture, temperature, the presence of various chemicals and other factors. Monotrichous have the highest mobility.

Flagella are more often found in rod-shaped bacteria; they are not vital cell structures, since there are non-flagellated variants of motile bacteria.

The bacterial cell as a whole is arranged 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 synthesizing the protein 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 - a small one (30S), which includes 16S rRNA, and a large one - 50S, including 23S and 5S rRNA molecules.

The photograph obtained with the help of transmission microscopy (Fig. 1) clearly shows a bright zone in which the genetic apparatus is located and the processes of transcription and translation take place. 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 more than one DNA molecule. For example, Deinococus radiodurans, a bacterium known for its phenomenal resistance to radiation and the ability to safely withstand a dose of radiation 2,000 times the lethal dose for humans, has two copies of its genomic DNA. Bacteria are known to have three or four copies. Some species of DNA may not be closed in a circle, and some Agrobacterium contain one circular and one linear DNA.

In addition to the nucleoid, the genetic material can be present in the cell in the form of additional small circular DNA molecules - plasmids. Plasmids replicate independently of the nucleoid and often contain genes that are useful for the cell, giving the cell, for example, resistance to antibiotics, the ability to absorb new substrates, the ability to conjugate, and much more. Plasmids can be transferred both from the mother cell to the 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 molecules of N-acetylglucosamine and N-acetylmuramic acid, cross-linked into long chains by β-1-4-glycosidic bonds, adjacent chains, in turn, are connected by transverse peptide bridges (Fig. 2) . So it turns out one big network surrounding the cell.

Gram-positive bacteria have a thick cell wall that sits on top of a 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 under transverse compression and tension, acting as springs. Because the murein layer is thick, it stains easily with the Gram method: the cells look bright purple because the dye (gentian or methyl violet) gets stuck in the cell wall layer.

In Gram-negative bacteria, the murein layer is very thin (with the exception of cyanobacteria), therefore, when stained by Gram, the violet dye is washed out, and the cells are stained in the color of the second dye (Fig. 3).

The cell wall of gram-negative bacteria is covered on top with another, outer, membrane attached to the peptidoglycan 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. The 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 Gram-positive and Gram-negative cells differ in the structure of the integument (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, make the filament spin. In gram-negative bacteria, in addition to this design, there are two more rings that additionally fix the flagellum in the outer membrane and cell wall.

The bacterial flagellum itself consists of the flagellin protein, 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 on the surface of bacterial cells - pili. These are protein villi that allow bacteria to adhere 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 kinds of inclusions (reserve lipids, sulfur) and gas bubbles surrounded by a protein membrane. Without a membrane, a cell can store polysaccharide molecules, cyanophycin (as a nitrogen depot), and can also contain carboxysomes - vesicles containing the enzyme RuBisCO, which is necessary for fixing carbon dioxide in the Calvin cycle.

In microbiology, this term refers to a nutrient that can be taken up by a microorganism.

This name of the groups comes from the name of the doctor G.K. Gram, who developed a method for staining bacterial cell walls, which makes it possible to distinguish between cells with different types of cell wall structure.

Ribulose bisphosphate carboxylase/oxygenase

The main differences between a prokaryotic (bacterial) cell and a eukaryotic one are: the absence of a formalized nucleus (i.e., nuclear membrane), the absence of intracellular membranes, nucleoli, the Golgi complex, lysosomes, and mitochondria.

The main structures of a bacterial cell are:

Nucleoid - is a hereditary (genetic) material of a bacterial cell, represented by 1 DNA molecule, closed in a ring and supercoiled (twisted into a loose ball). The length of the DNA is about 1mm. The amount of information is about 1000 genes (features). The nucleoid is not separated from the cytoplasm by a membrane.

The cytoplasm is a colloid, i.e. aqueous solution of proteins, carbohydrates. Lipids, minerals, in which there are ribosomes, inclusions, plasmids.

Protein synthesis takes place on ribosomes. Ribosomes of prokaryotes differ from eukaryotic ones in smaller sizes (70 S).

Inclusions are reserve nutrients of a bacterial cell, as well as accumulations of pigments. Reserve nutrients include: granules of volutin (inorganic polyphosphate), glycogen, granulosa, starch, fat drops, accumulations of pigment, sulfur, calcium. Inclusions, as a rule, are formed when bacteria are grown on rich nutrient media and disappear during starvation.

Cell membrane - limits the cytoplasm. Consists of a double layer of phospholipids and embedded membrane proteins. CMs, in addition to the barrier and transport functions, play the role of a center of metabolic activity (in contrast to the eukaryotic cell). Membrane proteins responsible for transporting essential substances into the cell are called permeases. On the inner surface of the CM there are enzyme ensembles, i.e. ordered accumulations of enzyme molecules responsible for the synthesis of energy carriers - ATP molecules. CM can form invaginations into the cytoplasm, which are called mesosomes. There are two types of mesosomes:

Septal - form transverse partitions in the process of cell division.

Lateral - serve to increase the surface of the CM and increase the rate of metabolic processes.

Nucleoid, CP and CM form a protoplast.

One of the distinctive properties of bacteria is a very high intracellular osmotic pressure (from 5 to 20 atm), which is the result of intensive metabolism. Therefore, to protect against osmotic shock, the bacterial cell is surrounded by a strong cell wall.

According to the structure of the cell wall, all bacteria are divided into 2 groups: Having a single-layer cell wall - Gram-positive. Having a two-layer cell wall - Gram-negative. The names Gram+ and Gram- have their own history. In 1884, the Danish microbiologist Hans Christian Gram developed an original method for staining microbes, as a result of which some bacteria were stained blue (gram+) and others red (gram-). The chemical basis of the different coloration of bacteria according to the Gram method was elucidated relatively recently - about 35 years ago. It turned out that G- and G+ bacteria have different cell wall structures. The structure of the cell wall of G+ bacteria. The basis of the cell wall of G+ bacteria is made up of 2 polymers: peptidoglycan and teichoic acids. Peptidoglycan is a linear polymer with alternating muramic acid and acetylglucosamine residues. A tetrapeptide (protein) is covalently bound to muramic acid. The strands of peptidoglycan are interconnected through peptides and form a strong framework - the basis of the cell wall. Between the strands of peptidoglycan is another polymer - teichoic acids (glycerol TK and ribitol TK) - a polymer of polyphosphates. Teichoic acids act on the surface of the cell wall and are the main antigens of G+ bacteria. In addition, Mg ribonucleate is included in the cell wall of G+ bacteria. The wall of G-bacteria consists of 2 layers: the inner layer is represented by a mono- or bilayer of peptidoglycan (thin layer). The outer layer consists of lipopolysaccharides, lipoprotein, proteins, phospholipids. LPS of all G-bacteria have toxic and threshold properties and are called endotoxins.

When exposed to certain substances, such as penicillin, the synthesis of the peptidoglycan layer is disrupted. At the same time, a protoplast is formed from G+ bacteria, and a spheroplast from G-bacteria (because the outer layer of the cell wall is preserved).

Under certain conditions of cultivation, cells lacking a cell wall retain the ability to grow and divide, and such forms are called L-forms (after the name of the Lister Institute, where this phenomenon was discovered). In some cases, after the elimination of the factor that inhibits the synthesis of the cell wall, the L-forms can turn into their original forms.

Many bacteria synthesize a mucous substance, consisting of mucopolysaccharides, which is deposited on the outside of the cell wall, surrounding the bacterial cell with a mucous sheath. This is a capsule. The function of the capsule is to protect bacteria from phasocytosis.

Surface structures of a bacterial cell.

The organs of attachment to the substrate (adhesion) are pili (fimbriae) or cilia. They start from the cell membrane. Composed of pilin protein. The number of pili can reach 400 per 1 cell.

The organs of transmission of hereditary information are F-drank or sex-drank. F-pills are formed only if the cell is odd to the plasmid, because F-pili proteins encode plasmid DNA. They are a thin long tube that attaches to another bacterial cell. Through the formed channel, the plasmid passes into the neighboring bacterial cell.

The organs of movement - flagella - are spiral threads. Their length can exceed their diameter by 10 or more times. The flagella are made up of the protein flagellin. The base of the flagellum is connected to the cell membrane through the basal body. The basal body consists of a system of rings that, while rotating, transmit rotational motion to the flagellum. According to the location of the flagellum, bacteria are divided into mono-, lopho-, amphi-, peritrichous.