Elements of bacteria. Structure of bacterial cells

The structural components of a bacterial cell are divided into 2 types:

- basic structures(cell wall, cytoplasmic membrane with its derivatives, cytoplasm with ribosomes and various inclusions, nucleoid);

- temporary structures(capsule, mucous membrane, flagella, villi, endospores, formed only at certain stages of the bacterial life cycle).

Basic structures.

Cell wall located on the outside of the cytoplasmic membrane. The cytoplasmic membrane is not part of the cell wall. Functions of the cell wall:

Protection of bacteria from osmotic shock and other damaging factors;

Determination of the shape of bacteria;

Participation in bacterial metabolism.

The cell wall is permeated with pores through which bacterial exotoxins are transported. The thickness of the cell wall is 10–100 nm. The main component of the bacterial cell wall is peptidoglycan or murein, consisting of alternating N-acetyl-N-glucosamine and N-acetylmuramic acid residues connected by glycosidic bonds.

In 1884, H. Gram proposed a method for staining bacteria using gentian violet, iodine, ethyl alcohol and fuchsin. All bacteria, depending on their Gram stain, are divided into 2 groups: gram-positive and gram-negative bacteria. Cell wall of gram-positive bacteria fits tightly to the cytoplasmic membrane, its thickness is 20-100 nm. It contains teichoic acids (polymers of glycerol or ribitol), as well as small quantities of polysaccharides, proteins and lipids. Cell wall of gram-negative bacteria multilayer, its thickness is 14-17 nm. The inner layer (peptidoglycan) forms a thin continuous network. The outer layer consists of phospholipids, lipoprotein and proteins. The outer membrane proteins are tightly bound to the peptidoglycan layer.

Under certain conditions, bacteria lose the ability to fully or partially synthesize cell wall components, resulting in the formation of protoplasts, spheroplasts and L-forms of bacteria. Spheroplasts are bacteria with a partially destroyed cell wall. They are observed in gram-negative bacteria. Protoplasts- these are forms completely devoid of a cell wall. They are formed by gram-positive bacteria. L-shape bacteria are mutants of bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan (bacteria with a defective cell wall). They got their name from the name of the Lister Institute in England, where they were opened in 1935.

Cytoplasmic membrane (CPM) and its derivatives. The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of a bacterial cell that separates the cytoplasm from the cell wall. It makes up 8-15% of the dry mass of the cell. Its destruction leads to cell death. Electron microscopy revealed its three-layer structure. The cytoplasmic membrane is a complex of proteins (50-75%) and lipids (15-20%). The bulk of lipids is represented by phospholipids. In addition, a small amount of carbohydrates was found in the membrane.

The bacterial CPM performs the following functions:

Barrier function (molecular “sieve”);

Energy;

Selective transfer of various organic and inorganic molecules and ions using special carriers - translocases or permeases;

Replication and subsequent chromosome division.

During cell growth, the cytoplasmic membrane forms numerous invaginations (invaginates), called mesosomes.

Cytoplasm - This is the contents of the bacterial cell, bounded by the cytoplasmic membrane. It consists of cytosol and structural elements.

Cytosol- homogeneous fraction, including soluble RNA components, enzymes, and metabolic products.

Structural elements- these are ribosomes, intracytoplasmic membranes, inclusions and nucleoid.

Ribosomes- organelles that carry out protein biosynthesis. They consist of protein and RNA. They are granules with a diameter of 15-20 nm. One bacterial cell contains from 5,000 to 50,000 ribosomes. Ribosomes are the site of protein synthesis.

In the cytoplasm of prokaryotes, various inclusions are found, representing the cell's reserve substances. Of the polysaccharides, glycogen, starch and a starch-like substance - granulosa - are deposited in cells. Polyphosphates are contained in granules called volutinic, or metachromatic, grains.

Nucleoid is the nucleus of prokaryotes. It consists of one double-stranded DNA strand closed in a ring, which is considered as a bacterial chromosome. A nucleoid lacks a nuclear envelope.

In addition to the nucleoid, extrachromosomal genetic elements were found in the bacterial cell - plasmids, which are small circular DNA molecules capable of autonomous replication. The role of plasmids is that they encode additional characteristics that give the cell advantages in certain living conditions. The most common plasmids are those that determine the signs of antibiotic resistance of bacteria (R-plasmids), the synthesis of enterotoxins (Ent-plasmids) or hemolysins (Hly-plasmids).

TO temporary structures include capsule, flagella, pili, endospores of bacteria.

Capsule - This is the mucous layer over the cell wall of the bacterium. The capsule substance consists of polysaccharide threads. The capsule is synthesized on the outer surface of the cytoplasmic membrane and is released onto the surface of the cell wall in specific areas.

Capsule functions:

The location of capsular antigens that determine the virulence, antigenic specificity and immunogenicity of bacteria;

Protection of cells from mechanical damage, drying out, toxic substances, infection by phages, the action of protective factors of the macroorganism;

The ability of cells to attach to a substrate.

Flagella – These are the organs of bacterial movement. Flagella are not vital structures, so they may or may not be present in bacteria depending on the growing conditions. The number of flagella and their locations vary in different bacteria. Depending on this, the following groups of flagellated bacteria are distinguished:

- monotrichs– bacteria with one polarly located flagellum;

- amphitrichs– bacteria with two polarly arranged flagella or having a bundle of flagella at both ends;

- lophotrichs– bacteria that have 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.

The chemical composition of flagella is represented by protein flagellin.

The surface structures of a bacterial cell also include villi And drank. These structures are involved in the adsorption of cells on the substrate (villi, general pili) and in the processes of transfer of genetic material (sexual pili). They are formed by a specific hydrophobic protein pilin.

In some bacteria, under certain conditions, dormant forms are formed, which ensure the survival of cells for a long time in unfavorable conditions - endospores. They are resistant to adverse environmental factors.

Location of spores in the cell:

Central (causative agent of anthrax);

Subterminal - closer to the end (causative agent of botulism);

Terminal - at the end of the stick (tetanus causative agent).

Form. There are several main forms of bacteria - coccoid, rod-shaped, convoluted and branching (Fig.).

Spherical (coccal) microbes are shaped like a ball, but they can be oval, flat, one-sided concave or slightly elongated. Spherical forms are formed as a result of cell division in one, two, three mutually perpendicular or different planes. When cells divide in one plane, cells can be arranged in pairs, and therefore such forms are called diplococci. If division occurs sequentially in one plane and the cells are connected in the form of a chain, this is streptococci (2). Division of the coccus in two mutually perpendicular planes leads to the formation of four cells, or tetracocca. Packet-shaped cocci, or sarcinas (3),- the result of division of cocci in three mutually perpendicular planes.

Rod-shaped, or cylindrical, forms are usually divided into bacteria and bacilli (Figure 3). Bacteria - rod-shaped, non-spore forming(written Bact., for example Bact. aceti). bacilli - rod-shaped, forming spores(they write Vas., for example Vas. subtilis). Bacteria and bacilli come in different shapes and sizes. The ends of the sticks are often rounded, but can be cut at right angles (the causative agent of anthrax), sometimes narrowed.

Figure - Basic forms of bacteria:

1- staphylococci; 2 - streptococci; 3 - sarcins; 4 - gonococci; 5 - pneumococci; 6 - pneumococcal capsule; 7 - Corynebacterium diphtheria; 8 - clostridia; 9 – bacilli; 10 - vibrios; 11 - spirilla; 12 - Treponema; 13 - borellia; 14 - Leptospira; 15 - actinomycetes; 16 - location of flagella: A - monotrich; b- lophotrichus; V - amphitrichus, G - peritrichus

Among the rod-shaped forms that form spores (bacilli), there are bacilli (9 ) And clostridia (8 ). Bacilli, excluding You. anthracis, mobile. Bacilli are aerobes. In bacilli, spores do not exceed the thickness of a vegetative cell. Clostridia are anaerobes. Spores are thicker than a vegetative cell. Such shapes resemble a spindle, a racket, a lemon, a drumstick. Clostridia take part in many processes in nature. They are causative agents of anaerobic infections. Cause ammonification of protein substances, urea. They decompose organophosphorus compounds. Fix molecular nitrogen, etc.

Rods, like cocci, can be arranged in pairs or in a chain. When bacteria combine in pairs, they form diplobacteria, with the same connection of bacilli - diplobacillus. Accordingly, streptobacteria And streptobacilli, if the cells are arranged in a chain. Tetrads and packets do not form rod-shaped forms, since they are divided in one plane, perpendicular to the longitudinal axis.

The convoluted forms of microbes are determined not only by length and diameter, but also by the number of curls. Vibrios(10) resemble a comma in shape. Spirilla (11)- convoluted forms, forming up to 5 curls. Spirochetes- thin long crimped forms with many curls. They occupy an intermediate position between bacteria and protozoa. Mycobacteria- rods with lateral outgrowths (pathogens of tuberculosis, paratuberculosis). Corynebacteria resemble mycobacteria, but differ from them by thickenings formed at the ends and inclusions of grains in the cytoplasm (diphtheria bacillus). filamentous Bacteria are multicellular organisms that have the shape of a thread. Myxobacteria- sliding microbes, shaped like sticks or a spindle. Prostecobacteria may be triangular or other shape. Some of them have radial symmetry. Such organisms got their name from the presence of pointed outgrowths - prostek. They reproduce by division or budding.

Dimensions. The sizes of microorganisms are determined in micrometers (µm) (10 -6 m according to the SI system). The diameter of the spherical shapes is 0.7-1.2 microns; rod-shaped length 1.6-10 µm, width 0.3-1 µm. Viruses are even smaller creatures. Their sizes are determined in nanometers (1 nm = 10 -9 m). The filamentous forms of microbes reach a length of several tens of micrometers. In order to imagine the size of these creatures, it is enough to say that one drop of water can contain several million or billions of microorganisms.

Structure. A bacterial cell consists of a membrane, the outer layer of which is called the cell wall, and the inner layer is the cytoplasmic membrane, as well as cytoplasm with inclusions and a nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili, plasmids; Some bacteria are capable of forming spores under unfavorable conditions.

Cell wall - a strong, elastic structure that gives the bacterium a certain shape and, together with the underlying cytoplasmic membrane, “restrains” the high osmotic pressure in the bacterial cell. It protects the cell from the action of harmful environmental factors, participates in the process of cell division and transport of metabolites.

The thickest cell wall is in gram-positive bacteria (up to 50-60 nm); in gram-negative bacteria it is 15-20 nm.

The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, and proteins. The main component of the cell wall of gram-positive bacteria is multilayered peptidoglycan(murein, mucopeptide), constituting 40-90% of its mass. In gram-negative bacteria, the amount of peptidoglycan in the cell wall is 5-20%.

Cytoplasmic membrane adheres to the inner surface of the bacterial cell wall and surrounds the outer part of the cytoplasm. It consists of a double layer of lipids, as well as integral proteins that penetrate it through. The cytoplasmic membrane is involved in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell.

Cytoplasm bacterial cell is a semi-liquid, viscous, colloidal system . Cytoplasm occupies the bulk of the bacterial cell and consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes The cytoplasm contains various inclusions in the form of glycogen granules, polysaccharides, fatty acids and polyphosphates (volutin).

In some places, the cytoplasm is permeated with membrane structures - mesosomes , which originated from the cytoplasmic membrane and retained contact with it. Mesosomes perform various functions; they and the associated cytoplasmic membrane contain enzymes involved in energy processes - in supplying the cell with energy.

Ribosomes scattered in the cytoplasm in the form of small granules measuring 20-30 nm; Ribosomes are composed of approximately half RNA and protein. Ribosomes are responsible for the synthesis of cell proteins. There can be 5-50 thousand of them in a bacterial cell.

Nucleoid - equivalent to the nucleus in bacteria. It is located in cytoplasmobacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a coil. Unlike the nucleus of eukaryotes, the bacterial nucleoid does not have a nuclear envelope, nucleolus, or basic proteins (histones). Typically, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring.

In addition to the nucleoid, the bacterial cell may contain extrachromosomal factors of heredity - plasmids , representing covalently closed DNA rings and capable of replication regardless of the bacterial chromosome.

Capsule- a mucous structure firmly associated with the cell wall of bacteria and having clearly defined external boundaries. Usually the capsule consists of polysaccharides, sometimes of polypeptides, for example, in the anthrax bacillus. The capsule prevents phagocytosis of bacteria. Capsules are inherent in some types of bacteria or can be formed when a microbe enters a macroorganism.

Flagella bacteria determine cell motility. Flagella are thin filaments originating from the cytoplasmic membrane; they are attached to the cytoplasmic membrane and cell wall by special disks and are longer than the cell itself. They consist of a protein - flagellin, twisted in the form of a spiral.

Villi, or drank (fimbriae) , - thread-like formations, thinner and shorter than flagella. The pili extend from the cell surface and are composed of the protein pilin. They are responsible for attaching bacteria to the affected cell, for nutrition, and water-salt metabolism; sex drank (F-drank) characteristic of so-called “male” donor cells.

Controversy - peculiar form of resting gram-positive bacteria formed in the external environment under unfavorable conditions for the existence of bacteria (drying, nutrient deficiency, etc.). The process of sporulation goes through several stages, during which part of the cytoplasm and the chromosome are separated and surrounded by a cytoplasmic membrane; A prospore is formed, then a multilayer, poorly permeable shell is formed, which gives the spore resistance to temperature and other unfavorable factors. In this case, one spore is formed inside one bacterium. Sporulation contributes to the preservation of the species and is not a method of reproduction, like in mushrooms. Bacterial spores can persist in the soil for a long time (the causative agents of anthrax and tetanus - for decades). Under favorable conditions, spores germinate, and one bacterium is formed from one spore.

Mobility. Spherical bacteria are usually nonmotile. Rod-shaped bacteria are either motile or immobile. Curved and spiral-shaped bacteria are motile. The movement of bacteria is carried out using flagella. Flagella can perform rotational movements. The presence of flagella and their location are a constant feature for the species and have diagnostic value. The speed of movement is high: in a second, a cell with flagella can cover a distance 20-50 times greater than the length of its body.

Flagella are located on the surface of the bacterial body singly - monotrichial flagellation, a bunch at one end of the cell - lophotrichial, in a bundle at both ends of the cell - amphitrichyal; they can be located on the entire surface of the cell - peritrichial flagellation. Under unfavorable living conditions, with cell aging, and mechanical stress, mobility may be lost.

Related information.

A bacterial cell, despite its apparent simplicity of structure, is a very complex organism, characterized by processes characteristic of all living beings. The bacterial cell is covered with a dense membrane, consisting of a cell wall, a cytoplasmic membrane, and in some species, a capsule.

Cell wall– one of the main elements of the structure of a bacterial cell is a surface layer located outside the cytoplasmic membrane. The wall performs protective and supporting functions, and also gives the cell a permanent, characteristic shape (for example, the shape of a rod or coccus), because has a certain rigidity (stiffness), and represents the external skeleton of the cell. Inside the bacterial cell, the osmotic pressure is several times, and sometimes tens of times, higher than in the external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as the cell wall. The main structural component of the walls, the basis of their rigid structure in almost all bacteria studied to date, is murein. The surface of the cell wall of some rod-shaped bacteria is covered with projections, spines, or bumps. Using a staining method first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-positive and gram-negative. The cell wall is responsible for the Gram staining of bacteria. The ability or inability to stain by Gram is associated with differences in the chemical composition of the bacterial cell walls. The cell wall is permeable: through it, nutrients freely pass into the cell, and metabolic products exit into the environment. Large molecules with high molecular weight do not pass through the shell.

The outer layer of cytoplasm is closely adjacent to the cell wall of the bacterial cell - cytoplasmic membrane, usually consisting of a bilayer of lipids, each of the surfaces of which is covered with a monomolecular layer of protein. The membrane makes up about 8-15% of the cell's lipids. The total thickness of the membrane is approximately 9 nm. The cytoplasmic membrane plays the role of an osmotic barrier that controls the transport of substances into and out of the bacterial cell.

The cell wall of many bacteria is surrounded on top by a layer of mucous material - capsule. The thickness of the capsule can be many times greater than the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule. The capsule is not an essential part of the cell; it is formed depending on the conditions in which the bacteria find themselves. It serves as a protective cover for the cell and participates in water metabolism, protecting the cell from drying out.

Under the cytoplasmic membrane in bacteria there is a itoplasma, representing the entire contents of the cell, with the exception of the nucleus and cell wall. The cytoplasm of bacteria is a dispersed mixture of colloids consisting of water, proteins, carbohydrates, lipids, mineral compounds and other substances. The liquid structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, plastids and other structures, as well as reserve nutrients.

Bacteria do not have such a nucleus as higher organisms, but have its analogue “nuclear equivalent” - nucleoid, which is an evolutionarily more primitive form of organization of nuclear matter. The nucleoid of a bacterial cell is located in its central part.

A resting bacterial cell usually contains one nucleoid; cells in the pre-division phase have two nucleoids; in the phase of logarithmic growth - reproduction - up to four or more nucleoids. In addition to the nucleoid, the cytoplasm of a bacterial cell can contain hundreds of times shorter DNA strands - the so-called extrachromosomal factors of heredity, called plasmid. As it turned out, plasmids are not necessarily present in bacteria, but they give the body additional properties that are beneficial to it, in particular those related to reproduction, drug resistance, pathogenicity, etc.

Some bacteria have appendage structures on the surface; the most widespread of them are flagella – organs of movement of bacteria. Bacteria may have one, two or more flagella. Their location is different: at one end of the cell, at two, across the entire surface, etc.

A bacterium with one flagellum is called monotrichoma; a bacterium with a bundle of flagella at one end of the cell - lophotrichoma; on both ends - amphitrichous; a bacterium with flagella located over the entire surface of the cell is called peritrichome. The number of flagella varies in different types of bacteria and can reach up to 100. The thickness of the flagella ranges from 10 to 20 nm, length - from 3 to 15 µm, and for the same bacterial cell the length can vary depending on the state of the culture and environmental factors .

Biologists of the 19th and early 20th centuries regarded bacteria as primitive organisms from the point of view of cellular organization and were considered the extreme limit of life. The authoritative German scientist Cohn wrote that bacteria are the “smallest” and “simplest” of all living forms, forming the boundary line of life; life does not exist beyond these forms.

However, as science developed, more advanced microscopic equipment and new research methods were created. The use of modern research methods in the study of bacterial cells - electrolyte and phase-contrast microscopy, differentiated centrifugation, the use of isotopes - made it possible to identify individual cellular structures and clarify their biological role.

A bacterial cell has a complex, strictly ordered structure. From an anatomical point of view, the bacterium is morphologically differentiated. It distinguishes between basic and temporary structures. The main components of a cell include the cell wall, cytoplasmic membrane, cytoplasm with ribosomes, various inclusions, and nucleoid. These structures are found only at certain stages of bacterial development.

The cell wall is a strong, elastic structure located between the cytoplasmic membrane and the capsule, and in non-capsular bacterial species it is the outer shell of the cell. The cell wall is a thin, colorless structure; it is not visible in an ordinary microscope without special treatment. The cell wall gives bacteria a permanent shape and represents the skeleton of the cell. It can be observed by light microscopy only in large forms of bacteria. For example, in the sulfur bacterium Beggiatoa mirabilis the wall is clearly visible and has a double-circuit structure. The bacterial cell wall can be viewed during plasmolysis in a darkened field of view of a microscope. Mycoplasma and L-form bacteria do not have a cell wall; for all other prokaryotes it is an obligatory structure. The cell wall makes up on average 20% of the dry weight of bacteria; its thickness can reach up to 50 nm or more. The cell membrane performs vital functions: it protects the bacterium from damaging environmental factors, osmotic shock, participates in metabolism and in the process of cell division, contains surface antigens and specific receptors for phages, and transports metabolites. The bacterial shell is semi-permeable, which ensures selective penetration of nutrients into the cell from the external environment. The supporting polymer of the cell wall is called peptidoglycan (synonyms: mucopeptide, murein - from the Latin murus - wall) forms a network structure covalently bound by teichoic acids (from the Greek teichos - wall). When studying ultrathin sections of the cell wall, it was found that it is evenly adjacent to the underlying structures, permeated with pores, thanks to which various substances enter the cell and vice versa. The obtained photograms showed that the cell wall is not characterized by the same electron-optical density, i.e., it has layering. The wall frames the bacterium; its thickness and density are the same along the entire perimeter of the microbial cell. The cell wall accounts for 5 to 50% of cell dry matter.

When studying the anatomy of microorganisms using a light microscope, the need arose to stain them. This need was realized by H. Gram, who in 1884 proposed a staining method named after him and widely used to differentiate bacteria in our time. In relation to Gram staining, all microorganisms are divided into two groups: gram-positive (gram-positive) and gram-negative (gram-negative). The essence of the method is that gram-positive bacteria firmly bind the complex of gentian violet and iodine, which is not discolored by ethanol and does not accept the additional dye fuchsin, remaining blue-violet. In gram-negative bacteria, the mentioned complex is washed out of the bacterial body with ethanol and when treated with magenta, they turn red (fuchsin color).

This Gram staining of prokaryotes is explained by the specific chemical composition and structure of their cell wall. The cell wall of gram-positive bacteria is massive, thick (20-100 nm), tightly adjacent to the cytoplasmic membrane, most of its chemical composition is represented by peptidoglycan (40-90%), which is associated with teichoic acids. The wall of gram-positive microorganisms contains small amounts of polysaccharides, lipids, and proteins. The structural microfibrils of peptidoglycan are cross-linked tightly, compactly, the pores in it are narrow and therefore the violet complex is not washed out, the bacteria are painted blue-violet.

The structure and composition of gram-negative microorganisms is characterized by some features. The cell wall of gram-negative bacteria is thinner than that of gram-positive bacteria and is 14-17 nm. It consists of two layers: external and internal. The inner layer is represented by peptidoglycan, which encircles the cell in the form of a thin (2 nm) continuous network. Peptidoglycan in gram-negative bacteria is 1-10%, its microfibrils are cross-linked less tightly than those of gram-positive bacteria, the pores are wider and therefore the complex of gentian violet and iodine is washed out of the wall with ethanol, the microorganisms are painted red (the color of an additional dye - fuchsin). The outer layer contains phospholipids, monopolysaccharides, lipoprotein and proteins. Lipopolysaccharide (LPS) from the cell walls of gram-negative bacteria, toxic to animals, is called endotoxin. Teichoic acids have not been found in gram-negative bacteria. The gap between the cell wall and the cytoplasmic membrane is called the periplasmic space, which contains enzymes.

Under the influence of lysozyme, penicillin and other compounds, cell wall synthesis is disrupted and cells with an altered shape are formed: protoplasts - bacteria completely devoid of a cell wall and spheroplasts - bacteria with a partially destroyed cell wall. Protoplasts and spheroplasts are spherical in shape and 3-10 times larger than the original cells. Under conditions of increased osmotic pressure, they can grow and even multiply, but under normal conditions they lyse and die. When the inhibitory factor is removed, protoplasts and spheroplasts can reverse to their original form, sometimes transforming into L-forms of bacteria. L-forms of bacteria were isolated in 1935 at the Lister Institute. They are formed as a result of the action of various kinds of L-transforming agents (antibiotics, amino acids, ultraviolet rays, x-rays, etc.) on bacteria. These are bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan. Compared to protoplasts and spheroplasts, they are more stable and have the ability to reproduce. The causative agents of many infectious diseases can form L-forms.

The cytoplasmic membrane (plasmolemma) is a semi-permeable, three-layer multiprotein cell structure that separates the cytoplasm from the cell wall. This is an essential component of the cell, constituting 8-15% of its dry matter. When the cytoplasmic membrane is destroyed, the cell dies. Chemically, the membrane is a protein-lipid complex consisting of proteins (50-70%) and lipids (15-50%). The cytoplasmic membrane performs important functions in the life of the cell. It is the osmotic barrier of the cell, participates in the processes of metabolism, cell growth, carries out selective transfer of molecules of organic and inorganic substances, etc. During cell growth, the cytoplasmic membrane forms invaginates - protrusions, which are called mesosomes. Mesosomes are well expressed in gram-positive bacteria, worse in gram-negative bacteria, and very poorly in rickettsia and mycoplasmas. Mesosomes associated with the bacterial nucleoid are called nucleosomes. They take part in karyopinesis and karyokenesis of microbial cells. The significance of mesosomes has not been fully elucidated. It is assumed that they take an active part in the process of bacterial respiration, so they are compared by analogy with mitochondria. Perhaps mesosomes perform a structural function and, dividing the cell into separate sections, contribute to the orderliness of metabolic processes.

The cytoplasm of the cell is a semi-liquid mass that occupies the main volume of the bacterium, containing up to 90% water. It consists of a homogeneous fraction called cytosol, which includes structural elements - ribosomes, intracytoplasmic membranes, various types of formation, nucleoid. In addition, the cytoplasm contains soluble RNA components, substrate substances, enzymes, and metabolic products.

Cytoplasm forms the internal environment of the cell, which unites all intracellular structures and ensures their interaction with each other.

The most important structural component of a proplasmotic cell is the nucleoid, which is an analogue of the nucleus in eukaryotes. It is freely located in the cytoplasm, in the central zone of the cell, and is a double-stranded DNA closed in a ring and tightly packed like a ball. The nucleoid, unlike the clearly defined nucleus of eukaryotes, does not have a nuclear membrane, nucleoli, or basic proteins (histones). Despite this, it is believed that the nucleoid is a differentiated structure. Depending on the functional state of the cell, the nucleoid can be discrete and consist of individual fragments. Its discreteness is explained by cell division and replication of the DNA molecule. Nucleoid DNA is the carrier of the genetic information of the bacterial cell. With light microscopy, the nucleoid can be detected by staining bacteria using special methods (Feulgen, Romanosky-Giemsa). In addition to the nucleoid, extrachromosomal heredity factors have been found in the cells of many types of prokaryotes - plasmids, which are DNA molecules capable of autonomous replication.

Cell organelles include ribosomes - spherical ribonucleic particles with a diameter of 15-20 nm. A prokaryotic cell can contain from 5 to 20 thousand ribosomes. The ribosome consists of small and large subunits, which have Sverberg sedimentation constants of 30 and 50 S, respectively. One molecule of messenger RNA usually combines several ribosomes like beads strung on a thread. Such associations of ribosomes are called polysomes. Ribosomes have high synthesizing activity; they synthesize proteins necessary for the life of a microbial cell.

In the cytoplasm of bacteria, various types of inclusions have been identified, which can be solid, liquid and gaseous. They are reserve nutrients (polysaccharides, lipids, sulfur deposits, etc.) and metabolic products.

Capsule is a mucous structure, more than 0.2 microns thick, associated with the cell wall and clearly demarcated from the environment. It is detected by light microscopy in the case of staining bacteria using special methods (according to Olt, Mikhin, Burri-Gins). Many bacteria form a microcapsule - a mucous formation less than 0.2 microns, identified only by electron microscopy or by chemical and immunochemical methods. The capsule is not an essential structure of the cell; its loss does not cause the death of the bacterium. It is necessary to distinguish mucus from the capsule - mucoid exopolysaccharides. Mucous substances are deposited on the surface of the cell, often exceeding its diameter and have no clear boundaries.

The substance of prokaryotic capsules consists mainly of homo- or heteropolysaccharides. Some bacteria (for example, Leuconostoc) have several microbial cells enclosed in a capsule. Bacteria enclosed in one capsule form clusters called zoogels.

The capsule performs important biological functions. It contains capsular antigens that determine the virulence, specificity and immunogenicity of bacteria. The capsule protects the microbial cell from mechanical stress, drying out, infection by phages, toxic substances, and phagocytosis. In some types of bacteria, including pathogenic ones, it promotes the attachment of cells to the substrate.

Flagella are the organelles of bacterial movement. They are thin, long, thread-like structures consisting of the protein flagellin (from the Latin flagellum - flagellum). This protein has antigen specificity. The length of the flagella exceeds the length of the bacterial cell several times and is 3-12 µm, and the thickness is 12-20 nm. Flagella are attached to the cytoplasmic membrane and cell wall by special discs. Flagella are detected using electron microscopy or in a light microscope, but after processing the preparations with special methods. Flagella are not vital cell structures. The number of flagella varies among different types of bacteria (from 1 to 50) and the places of their localization are also different, but stable for each species. Depending on the location of the flagella, they are distinguished: monotrichous - bacteria with one polarly located flagellum; amphitirichs - bacteria with two polarly arranged flagella, or a bundle of flagella at each end; lophotrichs - bacteria with a bundle of flagella at one end of the cell; peritrichs are bacteria with many flagella located around the entire perimeter of the cell. Bacteria without flagella are called atrichia. Flagella are typical of floating rod-shaped and convoluted forms and, as an exception, are found in cocci. Monotrichs and lophotrichs move at a speed of 50 microns per second. Bacteria usually move randomly. Under the influence of environmental factors, bacteria are capable of directed forms of movement - taxis. Taxis can be positive and negative. There are chemotaxis - caused by the difference in the concentration of chemicals in the environment, aerotaxis - oxygen, phototaxis - light intensity, magnetotaxis - characterized by the ability of microorganisms to navigate in a magnetic field.

Pili (villi) are thread-like formations shorter than flagella. Their length reaches from 0.3 to 10 microns, thickness 3-10 nm. Pili originate from the cytoplasmic membrane and are found in motile and non-motile forms of microorganisms. They can only be identified using electron microscopy. On the surface of a bacterial cell there can be from 1-2 to several tens, hundreds and even thousands of pili. Pili are composed of the protein pilin and have antigenic activity.

There are general and sexual types of pili. The former are responsible for adhesion, i.e., attachment of bacteria to the affected cell, nutrition, water-salt metabolism, clumping of bacteria into agglomerates, the latter are the transfer of hereditary material (DNA) from donor to recipient. The same species of bacteria can have both types of pili.

Spores (endospores) are a special form of resting cells, characterized by a sharp decrease in the level of metabolism and high resistance. Spores are formed under unfavorable conditions for the existence of bacteria. One spore is formed inside one cell. Sporulation is observed during nutrient deficiency, pH changes, lack of C, N, P, desiccation, accumulation of metabolic products in the environment surrounding the cell, etc. Spores are characterized by genome repression, anabolism, low water content in the cytoplasm, increased concentration of calcium cations, the appearance dipicolinic acid.

Spores in the field of view of a light microscope have the appearance of oval, highly refracting light formations with a size of 0.8-1.5 microns. Bacteria in which the size of the spore does not exceed the diameter of the cell are called bacilli, and those in which it exceeds are called clostridia. The spore in the cell can be located centrally, closer to the end - subterminal, at the end of the bacterium - terminal. The structure of the spore is complex, but the same type in different types of bacteria. The central part of the spore is called sporoplasm, it contains nucleic acids, proteins and dipicolinic acid. The sporoplasm contains a nucleoid, ribosomes and vaguely defined membrane structures. The sporoplasm is framed by a cytoplasmic membrane, followed by a rudimentary peptidoglycan layer, then a massive layer of cortex, or otherwise, a cortex. On the surface of the cortex there is an outer membrane. The outside of the spore is covered with a multilayer shell, which, together with the specific elements of the spore and calcium dipicolinate, determines its stability. The main purpose of spores is to preserve bacteria in unfavorable environmental conditions. Spores are resistant to high temperatures and chemicals, and can exist for a long time in a dormant state for tens or even hundreds of years.

Video: Cell nucleus of a houseplant leaf undermicroscope

Bacteria are microscopic single-celled organisms. The structure of the bacterial cell has features that are the reason for the separation of bacteria into a separate kingdom of the living world.

Cell membranes

Most bacteria have three shells:

• cell membrane;
• cell wall;
• mucous capsule.

The cell membrane is in direct contact with the contents of the cell - the cytoplasm. It is thin and soft.

The cell wall is a dense, thicker membrane. Its function is to protect and support the cell. The cell wall and membrane have pores through which the substances it needs enter the cell.

Many bacteria have a mucous capsule that performs a protective function and ensures adhesion to different surfaces.

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It is thanks to the mucous membrane that streptococci (a type of bacteria) stick to the teeth and cause caries.

Cytoplasm

Cytoplasm is the internal contents of a cell. 75% consists of water. In the cytoplasm there are inclusions - drops of fat and glycogen. They are the cell's reserve nutrients.

Rice. 1. Diagram of the structure of a bacterial cell.

Nucleoid

Nucleoid means “like a nucleus.” Bacteria do not have a real, or, as they also say, formed nucleus. This means that they do not have a nuclear envelope and nuclear space, like the cells of fungi, plants and animals. DNA is found directly in the cytoplasm.

Functions of DNA:

• stores hereditary information;
• implements this information by controlling the synthesis of protein molecules characteristic of a given type of bacteria.

The absence of a true nucleus is the most important feature of a bacterial cell.

Organoids

Unlike plant and animal cells, bacteria do not have organelles built from membranes.

But the bacterial cell membrane in some places penetrates the cytoplasm, forming folds called mesosomes. The mesosome is involved in cell reproduction and energy exchange and, as it were, replaces membrane organelles.

The only organelles present in bacteria are ribosomes. These are small bodies that are located in the cytoplasm and synthesize proteins.

Many bacteria have a flagellum, with which they move in a liquid environment.

Bacterial cell shapes

The shape of bacterial cells is different. Ball-shaped bacteria are called cocci. In the form of a comma - vibrios. Rod-shaped bacteria are bacilli. Spirilla have the appearance of a wavy line.

Rice. 2. Shapes of bacterial cells.

Bacteria can only be seen under a microscope. The average cell size is 1-10 microns. Bacteria up to 100 microns in length are found. (1 µm = 0.001 mm).

Sporulation

When unfavorable conditions occur, the bacterial cell enters a dormant state called a spore. The causes of sporulation may be:

• low and high temperatures;
• drought;
• lack of nutrition;
• life-threatening substances.

The transition occurs quickly, within 18-20 hours, and the cell can remain in a state of spores for hundreds of years. When normal conditions are restored, the bacterium germinates from the spore within 4-5 hours and returns to its normal mode of life.

Rice. 3. Scheme of spore formation.

Reproduction

Bacteria reproduce by division. The period from the birth of a cell to its division is 20-30 minutes. Therefore, bacteria are widespread on Earth.

What have we learned?

We learned that, in general terms, bacterial cells are similar to plant and animal cells, they have a membrane, cytoplasm, and DNA. The main difference between bacterial cells is the absence of a formed nucleus. Therefore, bacteria are called prenuclear organisms (prokaryotes).

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