According to scientists, bacteria are over 3.5 billion years old. They existed on Earth long before the advent of highly organized organisms. Being at the origins of life, bacterial organisms received an elementary structure according to the prokaryotic type, characterized by the absence of a formed nucleus and nuclear membrane. One of the factors that influenced the formation of their biological properties is the shell of bacteria (cell wall).

The bacterial wall is designed to perform several fundamental functions:

• be the skeleton of a bacterium;
• give it a certain shape;
• communicate with the external environment;
• protect from the harmful effects of environmental factors;
• participate in the division of a bacterial cell that does not have a nucleus and a nuclear envelope;
• hold antigens and various kinds of receptors on its surface (typical for gram-negative bacteria).

Certain types of bacteria have an outer capsule, which is durable and serves to maintain the integrity of the microorganism for a long time. In this case, the shell in bacteria is an intermediate form between the cytoplasm and the capsule. Some bacteria (for example, leuconostoc) have the peculiarity of encapsulating several cells in one capsule. This is called a zoogel.

The chemical composition of the capsule is characterized by the presence of polysaccharides and a large amount of water. The capsule may also allow the bacterium to attach itself to a particular object.

How easily a substance penetrates through the shell depends on the degree of its absorption by the bacterium. Molecules with long chains that are resistant to biodegradation are more likely to penetrate.

What is a shell?

The bacterial membrane consists of lipopolysaccharides, proteins, lipoproteins, teichoic acids. The main component is murein (peptidoglycan).

The thickness of the cell wall can be different and reach 80 nm. The surface is not continuous, it has pores of various diameters through which the microbe receives nutrients and releases its waste products.

The significance of the outer wall is evidenced by its significant weight - it can vary from 10 to 50% of the dry mass of the entire bacterium. The cytoplasm can protrude, changing the external relief of the bacterium.

From above, the shell can be covered with cilia or flagella can be located on it, which consist of flagellin, a specific protein substance. For attachment to the bacterial membrane, flagella have special structures - flat discs. Bacteria with one flagellum are called monotrichous, those with two flagella are called amphitriches, those with a bunch are called lophotrichs, and those with many bunches are called peritrichs. Microorganisms that do not have flagella are called atrichia.

The cell wall has an inner part that begins to form after the completion of cell growth. Unlike the outer, it consists of a much smaller amount of water and has greater elasticity and strength.

The process of synthesis of the walls of microorganisms begins inside the bacterium. To do this, it has a network of polysaccharide complexes that alternate in a certain sequence (acetylglucosamine and acetylmuramic acid) and are linked by strong peptide bonds. The assembly of the wall is carried out outside, on the plasma membrane, where the shell is located.

Since the bacterium does not have a nucleus, it does not have a nuclear envelope.

The shell is an unstained thin structure, which cannot even be seen without special staining of the cells. For this, plasmolysis and a darkened field of view are used.

Gram stain

To study the detailed structure of the cell in 1884, Christian Gram proposed a special method for its coloring, which was later named after him. Gram stain divides all microorganisms into Gram-positive and Gram-negative. Each species has its own biochemical and biological properties. Different coloration is also due to the structure of the cell wall:

1. Gram positive Bacteria have a massive shell that includes polysaccharides, proteins and lipids. It is durable, the pores have a minimum size, the paint used for coloring penetrates deeply and is practically not washed out. Such microorganisms acquire a blue-violet color.
2. Gram negative bacterial cells have certain differences: their wall thickness is less, but the shell has two layers. The inner layer consists of peptidoglycan, which has a looser structure and wide pores. The Gram stain washes out easily with ethanol. The cell becomes discolored. In the future, the technique provides for the addition of a contrasting red dye, which stains the bacteria red or pink.

The proportion of gram-positive microbes that are harmless to humans is much higher than gram-negative ones. To date, three groups of gram-negative microorganisms that cause disease in humans have been classified:

• cocci (streptococci and staphylococci);
• non-spore-forming forms (corynebacteria and listeria);
• spore-forming forms (bacilli, clostridia).

Characteristics of the periplasmic space

Between the bacterial wall and the cytoplasmic membrane is the periplasmic space, which consists of enzymes. This component is an obligatory structure; it makes up 10-12% of the dry mass of the bacterium. If the membrane is destroyed for some reason, the cell dies. Genetic information is located directly in the cytoplasm, not separated from it by the nuclear envelope.

Regardless of whether the microbe is gram-positive or gram-negative, it is the osmotic barrier of the microorganism, the transporter of organic and inorganic molecules deep into the cell. A certain role of the periplasm in the growth of the microorganism has also been proven.

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Mandatory and optional structural components of a bacterial cell, their functions. The difference in the structure of the cell wall of gram-positive and gram-negative bacteria. L-forms and non-culturable forms of bacteria

Bacteria are prokaryotes and differ significantly from plant and animal cells (eukaryotes). They belong to unicellular organisms and consist of a cell wall, cytoplasmic membrane, cytoplasm, nucleoid (mandatory components of a bacterial cell). Some bacteria may have flagella, capsules, spores (optional components of a bacterial cell).

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

The cell wall is an important structural element of a bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria, this is the outer shell of the cell. Performs a number of functions: protects bacteria from osmotic shock and other damaging factors, determines their shape, participates in metabolism; in many species of pathogenic bacteria, it is toxic, contains surface antigens, and also carries specific receptors for phages on the surface. The bacterial cell wall has pores that are involved in the transport of exotoxins and other bacterial exoproteins.

The main component of the bacterial cell wall is peptidoglycan, or murein (lat. murus - wall), a support polymer that has a network structure and forms a rigid (hard) outer frame of the bacterial cell. Peptidoglycan has a main chain (backbone) consisting of alternating residues of N-acetyl-M-glucosamine and N-acetylmuramic acid connected by 1,4-glycosidic bonds, identical tetrapeptide side chains attached to N-acetylmuramic acid molecules, and short transverse peptide chains. bridges linking polysaccharide chains.

According to tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. Gram-positive bacteria firmly fix the complex of gentian violet and iodine, do not undergo discoloration with ethanol and therefore do not perceive the additional dye fuchsin, remaining stained purple. In gram-negative bacteria, this complex is easily washed out of the cell with ethanol, and they turn red upon additional application of fuchsin. In some bacteria, a positive Gram stain is observed only in the stage of active growth. The ability of prokaryotes to stain according to the Gram method or to decolorize with ethanol is determined by the specifics of the chemical composition and ultrastructure of their cell wall. bacterial chlamydia trachoma

L-forms of bacteria are phenotypic modifications, or mutants, of bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan. Thus, L-forms are bacteria that are defective in their cell wall. They are formed under the influence of L-transforming agents - antibiotics (penicillin, polymyxin, bacitracin, vencomycin, streptomycin), amino acids (glycine, methionine, leucine, etc.), lysozyme enzyme, ultraviolet and X-rays. Unlike protoplasts and spheroplasts, L-forms have a relatively high viability and a pronounced ability to reproduce. In terms of morphological and cultural properties, they differ sharply from the original bacteria, which is due to the loss of the cell wall and changes in metabolic activity. L-form cells have a well-developed system of intracytoplasmic membranes and myelin-like structures. Due to a defect in the cell wall, they are osmotically unstable and can only be cultivated on special media with high osmotic pressure; they pass through bacterial filters. There are stable and unstable L-forms of bacteria. The former are completely devoid of a rigid cell wall; they very rarely reverse to their original bacterial forms. The latter may have elements of the cell wall, in which they show similarities with spheroplasts; in the absence of the factor that caused their formation, they revert to the original cells.

The process of formation of L-forms is called L-transformation or L-induction. Almost all types of bacteria, including pathogens (causative agents of brucellosis, tuberculosis, listeria, etc.), have the ability to L-transformation.

L-forms are of great importance in the development of chronic recurrent infections, the carriage of pathogens, their long-term persistence in the body. The infectious process caused by L-forms of bacteria is characterized by atypicality, duration of the course, severity of the disease, and is difficult to respond to chemotherapy.

A capsule is a mucous layer located above the cell wall of a bacterium. The substance of the capsule is clearly delimited from the environment. The capsule is not an obligatory structure of a bacterial cell: its loss does not lead to the death of the bacterium.

The substance of the capsules consists of highly hydrophilic micelles, while their chemical composition is very diverse. The main components of most prokaryotic capsules are homo- or heteropolysaccharides (Entsrobacteria, etc.). In some species of bacilli, the capsules are built from a polypeptide.

Capsules ensure the survival of bacteria, protecting them from mechanical damage, drying out, infection by phages, toxic substances, and in pathogenic forms - from the action of the protective forces of the macroorganism: encapsulated cells are poorly phagocytosed. In some types of bacteria, including pathogenic ones, it promotes cell attachment to the substrate.

Flagella are organelles of bacterial movement, represented by thin, long, filamentous structures of a protein nature.

The flagellum consists of three parts: a spiral filament, a hook, and a basal body. Hook - a curved protein cylinder that acts as a flexible link between the basal body and the rigid filament of the flagellum. The basal body is a complex structure consisting of a central rod (axis) and rings.

Flagella are not vital structures of a bacterial cell: there are phase variations of bacteria, when they are present in one phase of cell development and absent in another.

The number of flagella and the places of their localization in bacteria of different species are not the same, but they are stable for one species. Depending on this, the following groups of flagellated bacteria are distinguished: moiotrichous - bacteria with one polar flagellum; amphitrichous - bacteria with two polar flagella or having a bundle of flagella at both ends; lophotrichous - 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. Bacteria that do not have flagella are called atrichia.

Being organs of locomotion, flagella are typical of floating rod-shaped and tortuous forms of bacteria and are found only in isolated cases in cocci. They provide efficient movement in a liquid medium and slower movement on the surface of solid substrates.

Pili (fimbria, villi) - straight, thin, hollow protein cylinders extending from the surface of the bacterial cell. They are formed by a specific protein - pilin, originate from the cytoplasmic membrane, are found in mobile and immobile forms of bacteria and are visible only in an electron microscope. On the cell surface there can be from 1-2, 50-400 or more pili to several thousand.

There are two classes of pili: sexual (sekspili) and pili of a general type, which are more often called fimbriae. The same bacterium can have pili of different nature. Sex pili appear on the surface of bacteria in the process of conjugation and act as organelles through which the transfer of genetic material (DNA) from a donor to a recipient occurs.

Pili take part in the adhesion of bacteria into agglomerates, the attachment of microbes to various substrates, including cells (adhesive function), in the transport of metabolites, and also contribute to the formation of films on the surface of liquid media; cause agglutination of erythrocytes.

The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an essential polyfunctional component of the cell. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell.

The cytoplasmic membrane is chemically a protein-lipid complex consisting of proteins and lipids. The main part of membrane lipids is represented by phospholipids. It is built from two monomolecular protein layers, between which there is a lipid layer, consisting of two rows of correctly oriented lipid molecules.

The cytoplasmic membrane serves as an osmotic barrier of the cell, controls the entry of nutrients into the cell and the release of metabolic products to the outside, it contains substrate-specific permease enzymes that actively selectively transfer organic and inorganic molecules.

In the process of cell growth, the cytoplasmic membrane forms numerous invaginates that form the intracytoplasmic structures of the membrane. Local invaginates of the membrane are called mesosomes. These structures are well expressed in gram-positive bacteria, worse - in gram-negative ones and poorly - in rickettsiae and mycoplasmas.

Mesosomes, like the cytoplasmic membrane, are the centers of bacterial respiratory activity; therefore, they are sometimes called analogues of mitochondria. However, the significance of mesosomes has not yet been finally elucidated. They increase the working surface of the membranes, perhaps they perform only a structural function, dividing the bacterial cell into relatively separate compartments, which creates more favorable conditions for the enzymatic processes to occur. In pathogenic bacteria, they provide the transport of protein molecules of exotoxins.

Cytoplasm - the contents of a bacterial cell, delimited by the cytoplasmic membrane. It consists of cytosol - a homogeneous fraction, including soluble RNA components, substrate substances, enzymes, metabolic products, and structural elements - ribosomes, intracytoplasmic membranes, inclusions and a nucleoid.

Ribosomes are organelles that carry out protein synthesis. They consist of protein and RNA connected in a complex by hydrogen and hydrophobic bonds.

In the cytoplasm of bacteria, various types of inclusions are detected. They may be solid, liquid or gaseous, with or without a proteinaceous membrane, and are intermittently present. A significant part of them is reserve nutrients and products of cellular metabolism. Reserve nutrients include: polysaccharides, lipids, polyphosphates, sulfur deposits, etc. Of the inclusions of a polysaccharide nature, glycogen and a starch-like substance granulosa are more often found, which serve as a source of carbon and energy material. Lipids accumulate in cells in the form of fat granules and droplets. Mycobacteria accumulate waxes as reserve substances. The cells of some spirilla and others contain volutin granules formed by polyphosphates. They are characterized by metachromasia: toluidine blue and methylene blue stain them purple-red. Volutin granules play the role of phosphate depots. Inclusions surrounded by a membrane also include gas vacuoles, or aerosomes, they reduce the specific mass of cells and are found in aquatic prokaryotes.

Nucleoid is the nucleus of prokaryotes. It consists of one double-stranded DNA strand closed in a ring, which is considered as a single bacterial chromosome, or genophore.

The nucleoid in prokaryotes is not delimited from the rest of the cell by a membrane - it lacks a nuclear envelope.

The nucleoid structures include RNA polymerase, basic proteins and no histones; the chromosome is fixed on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosome. The nucleoid does not have a mitotic apparatus, and the divergence of the daughter nuclei is ensured by the growth of the cytoplasmic membrane.

The bacterial nucleus is a differentiated structure. Depending on the stage of cell development, the nucleoid can be discrete (discontinuous) and consist of separate fragments. This is due to the fact that the division of a bacterial cell in time is carried out after the completion of the replication cycle of the DNA molecule and the formation of daughter chromosomes.

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

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

Some bacteria at the end of the period of active growth are able to form spores. This is preceded by depletion of the environment with nutrients, a change in its pH, and the accumulation of toxic metabolic products.

According to the chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. Spores contain less water and more lipids.

In the spore state, microorganisms are metabolically inactive, withstand high temperatures (140–150 °C), exposure to chemical disinfectants, and persist in the environment for a long time. High temperature resistance is associated with a very low water content and a high content of dipicolinic acid. Once in the body of humans and animals, spores germinate into vegetative cells. Spores are stained by a special method, which includes preheating the spores, as well as exposure to concentrated dye solutions at high temperatures.

Many types of Gram-negative bacteria, including pathogenic ones (Shigella, Salmonella, Vibrio cholerae, etc.), have a special adaptive, genetically regulated state, physiologically equivalent to cysts, into which they can pass under the influence of adverse conditions and remain viable for up to several years. The main feature of this condition is that such bacteria do not multiply and therefore do not form colonies on a dense nutrient medium. Such non-reproducing, but viable cells are called non-culturable forms of bacteria (NFB). NFB cells in an uncultivated state have active metabolic systems, including systems for electron transfer, protein and nucleic acid biosynthesis, and retain virulence. Their cell membrane is more viscous, the cells usually take the form of cocci, have a significantly reduced size. NFBs have a higher resistance in the environment and therefore can survive in it for a long time (for example, Vibrio cholerae in a dirty water body), maintaining the endemic state of a given region (water body).

To detect NFB, molecular genetic methods (DNA--DNA hybridization, CPR) are used, as well as a simpler method of direct counting of viable cells.

For these purposes, cytochemical methods (formation of formazan) or microautoradiography can also be used. The genetic mechanisms responsible for the transition of bacteria into NS and their reversion from it are not clear.

To study the structure of a bacterial cell, along with a light microscope, electron microscopic and microchemical studies are used to determine the ultrastructure of a bacterial cell.

A bacterial cell (Fig. 5) consists of the following parts: a three-layer membrane, cytoplasm with various inclusions, and a nuclear substance (nucleoid). Additional structural formations are capsules, spores, flagella, pili.

Rice. 5. Schematic representation of the structure of a bacterial cell. 1 - shell; 2 - mucous layer; 3 - cell wall; 4 - cytoplasmic membrane; 5 - cytoplasm; 6 - ribosome; 7 - polysome; 8 - inclusions; 9 - nucleoid; 10 - flagellum; 11 - drinking

shell The cell consists of an outer mucosal layer, a cell wall, and a cytoplasmic membrane.

The mucous capsular layer is outside the cell and performs a protective function.

The cell wall is one of the main structural elements of the cell, maintaining its shape and separating the cell from the environment. An important property of the cell wall is selective permeability, which ensures the penetration of essential nutrients (amino acids, carbohydrates, etc.) into the cell and the removal of metabolic products from the cell. The cell wall maintains a constant osmotic pressure inside the cell. The strength of the wall is provided by murein, a substance of a polysaccharide nature. Some substances destroy the cell wall, such as lysozyme.

Bacteria completely devoid of a cell wall are called protoplasts. They retain the ability to breathe, divide, synthesize enzymes; to the influence of external factors: mechanical damage, osmotic pressure, aeration, etc. Protoplasts can only be preserved in hypertonic solutions.

Bacteria with partially destroyed cell walls are called spheroplasts. If you suppress the process of cell wall synthesis with penicillin, then L-forms are formed, which in all types of bacteria are spherical large and small cells with vacuoles.

The cytoplasmic membrane adheres tightly to the cell wall from the inside. It is very thin (8-10 nm) and consists of proteins and phospholipids. This is a semi-permeable boundary layer through which the cell is nourished. The membrane contains permease enzymes that carry out active transport of substances, and respiratory enzymes. The cytoplasmic membrane forms mesosomes that take part in cell division. When a cell is placed in a hypertonic solution, the membrane can separate from the cell wall.

Cytoplasm- the interior of a bacterial cell. It is a colloidal system consisting of water, proteins, carbohydrates, lipids, various mineral salts. The chemical composition and consistency of the cytoplasm change depending on the age of the cell and environmental conditions. The cytoplasm contains the nuclear substance, ribosomes and various inclusions.

Nucleoid, the nuclear substance of a cell, its hereditary apparatus. The nuclear substance of prokaryotes, unlike eukaryotes, does not have its own membrane. The nucleoid of a mature cell is a double strand of DNA coiled into a ring. The DNA molecule encodes the genetic information of the cell. According to genetic terminology, the nuclear substance is called genophore or genome.

Ribosomes are located in the cytoplasm of the cell and perform the function of protein synthesis. The ribosome contains 60% RNA and 40% protein. The number of ribosomes in a cell reaches 10,000. Combining together, ribosomes form polysomes.

Inclusions - granules containing various reserve nutrients: starch, glycogen, fat, volutin. They are located in the cytoplasm.

Bacterial cells in the process of life form protective organelles - capsules and spores.

Capsule- the outer compacted mucous layer adjacent to the cell wall. This is a protective organ that appears in some bacteria when they enter the body of humans and animals. The capsule protects the microorganism from the protective factors of the body (causative agents of pneumonia and anthrax). Some microorganisms have a permanent capsule (Klebsiella).

controversy found only in rod-shaped bacteria. They are formed when a microorganism enters unfavorable environmental conditions (high temperatures, drying, pH changes, a decrease in the amount of nutrients in the environment, etc.). Spores are located inside the bacterial cell and represent a compacted area of ​​the cytoplasm with a nucleoid, dressed in its own dense shell. In chemical composition, they differ from vegetative cells in a small amount of water, an increased content of lipids and calcium salts, which contributes to the high resistance of spores. Sporulation occurs within 18-20 hours; when a microorganism enters favorable conditions, the spore germinates into a vegetative form within 4-5 hours. Only one spore is formed in a bacterial cell, therefore, spores are not reproductive organs, but serve to survive adverse conditions.

The spore-forming aerobic bacteria are called bacilli, and the anaerobic bacteria are called clostridia.

Spores differ in shape, size and location in the cell. They can be located centrally, subterminally and terminally (Fig. 6). In the causative agent of anthrax, the spore is located centrally, its size does not exceed the diameter of the cell. The spore of the causative agent of botulism is located closer to the end of the cell - subterminally and exceeds the width of the cell. In the causative agent of tetanus, a rounded spore is located at the end of the cell - terminally and significantly exceeds the width of the cell.

Flagella- organs of movement, characteristic of rod-shaped bacteria. These are thin filamentous fibrils, consisting of a protein - flagellin. Their length significantly exceeds the length of a bacterial cell. Flagella extend from the basal body located in the cytoplasm and exit to the cell surface. Their presence can be detected by determining the mobility of cells under a microscope, in a semi-liquid nutrient medium, or by staining with special methods. The ultrastructure of the flagella was studied using an electron microscope. According to the location of the flagella, bacteria are divided into groups (see Fig. 6): monotrichous - with one flagellum (the causative agent of cholera); amphitrichous - with bundles or single flagella at both ends of the cell (spirilla); lophotrichous - with a bundle of flagella at one end of the cell (fecal alkaline former); peritrichous - flagella are located over the entire surface of the cell (intestinal bacteria). The speed of movement of bacteria depends on the number and location of flagella (monotrichous are the most active), on the age of bacteria and the influence of environmental factors.

Rice. 6. Variants of the location of spores and flagella in bacteria. I - disputes: 1 - central; 2 - subterminal; 3 - terminal; II - flagella: 1 - monotrichous; 2 - amphitriches; 3 - lophotrichous; 4 - peritrichous

Pili or fimbriae- villi located on the surface of bacterial cells. They are shorter and thinner than flagella and also have a spiral structure. Consist of drinking from protein - pilin. Some pili (there are several hundred of them) serve to attach bacteria to animal and human cells, while others (single ones) are associated with the transfer of genetic material from cell to cell.

Mycoplasmas

Mycoplasmas are cells that do not have a cell wall, but are surrounded by a three-layer lipoprotein cytoplasmic membrane. Mycoplasmas can be spherical, oval, in the form of threads and stars. Mycoplasmas according to Bergi's classification are separated into a separate group. Currently, these microorganisms are receiving increasing attention as causative agents of inflammatory diseases. Their sizes are different: from a few micrometers to 125-150 nm. Small mycoplasmas pass through bacterial filters and are called filterable forms.

Spirochetes

Spirochetes (see Fig. 52) (from Latin speira - bend, chaite - hair) - thin, convoluted, mobile unicellular organisms, measuring from 5 to 500 microns in length and 0.3-0.75 microns in width. With the simplest, they are related by the method of movement by shortening the internal axial thread, consisting of a bundle of fibrils. The nature of the movement of spirochetes is different: translational, rotational, flexion, wavy. The rest of the cell structure is typical of bacteria. Some spirochetes stain weakly with aniline dyes. Spirochetes are divided into genera according to the number and shape of thread curls and its end. In addition to saprophytic forms, common in nature and the human body, among spirochetes there are pathogens - the causative agents of syphilis and other diseases.

Rickettsia

Viruses

Among viruses, a group of phages is distinguished (from Latin phagos - devouring), causing lysis (destruction) of microorganism cells. While retaining the properties and composition inherent in viruses, phages differ in the structure of the virion (see Chapter 8). They do not cause disease in humans and animals.

test questions

1. Tell us about the classification of microorganisms.

2. What are the main properties of representatives of the kingdom of prokaryotes.

3. List and characterize the main forms of bacteria.

4. Name the main organelles of the cell and their purpose.

5. Give a brief description of the main groups of bacteria and viruses.

Microbiology: lecture notes Tkachenko Ksenia Viktorovna

1. Structural features of a bacterial cell. Main organelles and their functions

Differences between bacteria and other cells

1. Bacteria are prokaryotes, that is, they do not have a separate nucleus.

2. The cell wall of bacteria contains a special peptidoglycan - murein.

3. There is no Golgi apparatus, endoplasmic reticulum, mitochondria in a bacterial cell.

4. The role of mitochondria is performed by mesosomes - invaginations of the cytoplasmic membrane.

5. There are many ribosomes in a bacterial cell.

6. Bacteria may have special movement organelles - flagella.

7. The sizes of bacteria range from 0.3-0.5 to 5-10 microns.

According to the shape of the cells, bacteria are divided into cocci, rods and convoluted.

In a bacterial cell, there are:

1) main organelles:

a) nucleoid;

b) cytoplasm;

c) ribosomes;

d) cytoplasmic membrane;

e) cell wall;

2) additional organelles:

b) capsules;

c) villi;

d) flagella.

The cytoplasm is a complex colloidal system consisting of water (75%), mineral compounds, proteins, RNA and DNA, which are part of the nucleoid organelles, ribosomes, mesosomes, and inclusions.

Nucleoid is a nuclear substance dispersed in the cytoplasm of a cell. It does not have a nuclear membrane or nucleoli. It contains DNA, represented by a double-stranded helix. Usually closed in a ring and attached to the cytoplasmic membrane. Contains about 60 million base pairs. It is pure DNA, it contains no histone proteins. Their protective function is performed by methylated nitrogenous bases. The nucleoid encodes the basic genetic information, i.e. the cell genome.

Along with the nucleoid, the cytoplasm can contain autonomous circular DNA molecules with a lower molecular weight - plasmids. They also encode hereditary information, but it is not vital for a bacterial cell.

Ribosomes are ribonucleoprotein particles 20 nm in size, consisting of two subunits - 30 S and 50 S. Ribosomes are responsible for protein synthesis. Before protein synthesis begins, these subunits combine into one - 70 S. Unlike eukaryotic cells, bacterial ribosomes are not united in the endoplasmic reticulum.

Mesosomes are derivatives of the cytoplasmic membrane. Mesosomes can be in the form of concentric membranes, vesicles, tubules, in the form of a loop. The mesosomes are associated with the nucleoid. They are involved in cell division and spore formation.

Inclusions are metabolic products of microorganisms that are located in their cytoplasm and are used as reserve nutrients. These include inclusions of glycogen, starch, sulfur, polyphosphate (volutin), etc.

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The general structure of a bacterial cell is shown in Figure 2. The internal organization of a bacterial cell is complex. Each systematic group of microorganisms has its own specific structural features.

Cell wall. The bacterial cell is covered with a dense membrane. This surface layer, located outside the cytoplasmic membrane, is called the cell wall (Fig. 2, 14). The wall performs protective and supporting functions, and also gives the cell a permanent, characteristic shape (for example, the shape of a rod or coccus) and is the outer skeleton of the cell. This dense shell makes bacteria related to plant cells, which distinguishes them from animal cells that have soft shells. Inside the bacterial cell, the osmotic pressure is several times, and sometimes tens of times higher than in the external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as the cell wall.

The thickness of the cell wall is 0.01-0.04 µm. It is from 10 to 50% of the dry mass of bacteria. The amount of material from which the cell wall is built changes during bacterial growth and usually increases with age.

Murein (glycopeptide, mucopeptide) is the main structural component of the walls, the basis of their rigid structure in almost all bacteria studied so far. This is an organic compound of a complex structure, which includes sugars that carry nitrogen - amino sugars and 4-5 amino acids. Moreover, the amino acids of cell walls have an unusual shape (D-stereoisomers), which is rarely found in nature.

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The constituent parts of the cell wall, its components, form a complex strong structure (Fig. 3, 4 and 5).

Using the method of staining, first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-positive and gram negative. Gram-positive organisms are able to bind certain aniline dyes, such as crystal violet, and after treatment with iodine and then with alcohol (or acetone), retain the iodine-dye complex. The same bacteria in which this complex is destroyed under the influence of ethyl alcohol (cells become discolored) are gram-negative.

The chemical composition of the cell walls of Gram-positive and Gram-negative bacteria is different.

In gram-positive bacteria, the cell walls include, in addition to mucopeptides, polysaccharides (complex, high-molecular sugars), teichoic acids (complex in composition and structure, compounds consisting of sugars, alcohols, amino acids and phosphoric acid). Polysaccharides and teichoic acids are associated with the framework of the walls - murein. We do not yet know what structure these constituent parts of the cell wall of gram-positive bacteria form. With the help of electronic photographs, thin sections (layering) were not found in the walls of gram-positive bacteria. Probably, all these substances are very closely related to each other.

The walls of gram-negative bacteria are more complex in chemical composition, they contain a significant amount of lipids (fats) associated with proteins and sugars in complex complexes - lipoproteins and lipopolysaccharides. In general, there is less murein in the cell walls of gram-negative bacteria than in gram-positive bacteria. The wall structure of Gram-negative bacteria is also more complex. Using an electron microscope, it was found that the walls of these bacteria are multilayered (Fig. 6).

The inner layer is murein. Above it is a wider layer of loosely packed protein molecules. This layer is in turn covered by a layer of lipopolysaccharide. The top layer is made up of lipoproteins.

The cell wall is permeable: through it, nutrients freely pass into the cell, and metabolic products are released into the environment. Large molecules with high molecular weight do not pass through the shell.

Capsule. The cell wall of many bacteria is surrounded from above by a layer of mucous material - a capsule (Fig. 7). The thickness of the capsule can be many times greater than the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule.

The capsule is not an obligatory part of the cell, it is formed depending on the conditions in which the bacteria enter. It serves as a protective cover of the cell and participates in water exchange, protecting the cell from drying out.

By chemical composition, capsules are most often polysaccharides. Sometimes they consist of glycoproteins (complex complexes of sugars and proteins) and polypeptides (genus Bacillus), in rare cases - of fiber (genus Acetobacter).

Mucous substances secreted into the substrate by some bacteria determine, for example, the mucous-viscous consistency of spoiled milk and beer.

Cytoplasm. The entire contents of a cell, with the exception of the nucleus and cell wall, is called the cytoplasm. The liquid, structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, mitochondria, plastids and other structures, as well as reserve nutrients. The cytoplasm has an extremely complex, fine structure (layered, granular). With the help of an electron microscope, many interesting details of the structure of the cell have been revealed.

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The outer lipoprotein layer of the bacterial protoplast, which has special physical and chemical properties, is called the cytoplasmic membrane (Fig. 2, 15).

Inside the cytoplasm are all vital structures and organelles.

The cytoplasmic membrane plays a very important role - it regulates the flow of substances into the cell and the release of metabolic products to the outside.

Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes. In addition, the membrane is the synthesis of some of the components of the cell, mainly the components of the cell wall and capsule. Finally, the most important enzymes (biological catalysts) are located in the cytoplasmic membrane. The orderly arrangement of enzymes on membranes makes it possible to regulate their activity and prevent the destruction of some enzymes by others. Ribosomes are attached to the membrane - structural particles on which protein is synthesized. The membrane is made up of lipoproteins. It is strong enough and can provide the temporary existence of a cell without a shell. The cytoplasmic membrane makes up to 20% of the dry mass of the cell.

In electron photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75 Å thick, consisting of a light layer (lipids) enclosed between two darker ones (proteins). Each layer has a width of 20-30A. Such a membrane is called elementary (Table 30, Fig. 8).

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Between the plasma membrane and the cell wall there is a connection in the form of desmoses - bridges. The cytoplasmic membrane often gives invaginations - invaginations into the cell. These invaginations form special membrane structures in the cytoplasm, called mesosomes. Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside such membranous sacs (Fig. 2). These structures perform a variety of functions in bacteria. Some of these structures are analogues of mitochondria. Others perform the functions of the endoplasmic reticulum or the Golgi apparatus. By invagination of the cytoplasmic membrane, the photosynthetic apparatus of bacteria is also formed. After invagination of the cytoplasm, the membrane continues to grow and forms stacks (Table 30), which, by analogy with plant chloroplast granules, are called thylakoid stacks. These membranes, which often fill most of the cytoplasm of a bacterial cell, contain pigments (bacteriochlorophyll, carotenoids) and enzymes (cytochromes) that carry out the process of photosynthesis.

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The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cage. Ribosomes are made up of RNA and protein. In bacteria, many ribosomes are located freely in the cytoplasm, some of them can be associated with membranes.

Ribosomes are the centers of protein synthesis in the cell. At the same time, they often combine with each other, forming aggregates called polyribosomes or polysomes.

The cytoplasm of bacterial cells often contains granules of various shapes and sizes. However, their presence cannot be considered as some kind of permanent feature of the microorganism, usually it is largely associated with the physical and chemical conditions of the environment. Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These reserve substances are formed when the body is supplied with a sufficient amount of nutrients, and, conversely, are used when the body enters conditions that are less favorable in terms of nutrition.

In many bacteria, the granules are composed of starch or other polysaccharides - glycogen and granulosa. Some bacteria, when grown on a sugar-rich medium, have droplets of fat inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules are composed of polymetaphosphate (reserve substance, including phosphoric acid residues). Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria accumulate volutin more often under unusual nutritional conditions, such as on a medium that does not contain sulfur. Sulfur droplets are found in the cytoplasm of some sulfur bacteria.

In addition to various structural components, the cytoplasm consists of a liquid part - a soluble fraction. It contains proteins, various enzymes, t-RNA, some pigments and low molecular weight compounds - sugars, amino acids.

As a result of the presence of low molecular weight compounds in the cytoplasm, a difference arises in the osmotic pressure of the cellular contents and the external environment, and this pressure may be different for different microorganisms. The highest osmotic pressure was noted in gram-positive bacteria - 30 atm, in gram-negative bacteria it is much lower - 4-8 atm.

Nuclear device. In the central part of the cell, the nuclear substance is localized - deoxyribonucleic acid a (DNA).

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Bacteria do not have such a nucleus as in higher organisms (eukaryotes), but there is its analogue - the "nuclear equivalent" - nucleoid(see Fig. 2, 8), which is an evolutionarily more primitive form of organization of nuclear matter. Microorganisms that do not have a real nucleus, but have its analogue, belong to prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, most of the DNA is concentrated in one or more places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The nucleus is surrounded by a shell membrane.

In bacteria, DNA is less densely packed than in true nuclei; A nucleoid does not have a membrane, a nucleolus, or a set of chromosomes. Bacterial DNA is not associated with the main proteins - histones - and is located in the nucleoid in the form of a bundle of fibrils.

Flagella. Some bacteria have adnexal structures on their surface; the most widespread of them are flagella - the organs of movement of bacteria.

The flagellum is anchored under the cytoplasmic membrane by two pairs of discs. Bacteria can have one, two, or many flagella. Their location is different: at one end of the cell, at two, over the entire surface, etc. (Fig. 9). Bacterial flagella have a diameter of 0.01-0.03 microns, their length can be many times greater than the length of the cell. Bacterial flagella Consist of a protein - flagellin - and are twisted helical filaments.

On the surface of some bacterial cells there are thin villi - fimbriae.

Plant life: in 6 volumes. - M.: Enlightenment. Under the editorship of A. L. Takhtadzhyan, editor-in-chief corr. USSR Academy of Sciences, prof. A.A. Fedorov. 1974 .

- (Greek bakterion bacillus) a large group (type) of microscopic, predominantly unicellular organisms with a cell wall, containing a lot of deoxyribonucleic acid (DNA), having a primitive nucleus devoid of visible ... ...

- (from Bacteria and Greek phagos - eater; literally eaters of bacteria) phages, bacterial viruses that cause the destruction (lysis) of bacteria and other microorganisms. B. multiply in cells, lyse them and pass into others, as a rule, ... ... Great Soviet Encyclopedia

I Medicine Medicine is a system of scientific knowledge and practice aimed at strengthening and maintaining health, prolonging people's lives, and preventing and treating human diseases. To accomplish these tasks, M. studies the structure and ... ... Medical Encyclopedia Big Medical Encyclopedia

A section of genetics (See. Genetics) and molecular biology (See. Molecular biology), which aims to understand the material foundations of heredity (See. Heredity) and variability (See. Variability) of living beings through research ... ... Great Soviet Encyclopedia

Term bacteriophage English term bacteriophage Synonyms phages, bacterial viruses Abbreviations Associated terms biological nanoobjects, DNA, capsid, nanopharmacology, nanomaterial-based vectors Definition (from bacterium and Greek ??????… … Encyclopedic Dictionary of Nanotechnology