Or a bacterial cell. Morphology of microorganisms

Bacteria are microscopic single-celled organisms. The structure of a 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. She is thin and soft.

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

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

Cytoplasm

The cytoplasm is the interior of the cell. 75% consists of water. In the cytoplasm are inclusions - drops of fat and glycogen. They are the reserve nutrients of the cell.

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

Nucleoid

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

DNA functions:

preserves hereditary information;
implements this information by controlling the synthesis of protein molecules characteristic of this type of bacteria.

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

Organelles

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

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

The only organelle found in bacteria is the ribosome. 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 medium.

Shapes of bacterial cells

The shape of bacterial cells is different. Bacteria in the form of a ball are called cocci. In the form of a comma - vibrios. Rod-shaped bacteria are bacilli. Spirilla look like a wavy line.

Rice. 2. Forms of bacterial cells.

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

sporulation

When unfavorable conditions occur, the bacterial cell enters a dormant state, which is called a spore. The reasons for the dispute 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 be in a state of spores for hundreds of years. When normal conditions are restored, the bacterium germinates from the spore in 4-5 hours and passes into the normal mode of life.

Rice. 3. The 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 in general terms, bacterial cells are similar to plant and animal cells, they have a membrane, cytoplasm, DNA. The main difference between bacterial cells is the absence of a formed nucleus. Therefore, bacteria are called pre-nuclear organisms (prokaryotes).

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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, advanced level oxygen;
- 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.

We cannot even imagine how many microorganisms constantly surround us. By holding the handrail on the bus, you have already planted about a hundred thousand bacteria on your hand, by going into a public toilet, you, again, have rewarded yourself with these microorganisms. Bacteria always and everywhere accompany a person. But there is no need to react negatively to this word, because bacteria are not only pathogenic, but also beneficial to the body.

Scientists were very surprised when they realized that some bacteria have retained their appearance for about a billion years. Such microorganisms were even compared with a Volkswagen car - the appearance of one of their models has not changed for 40 years, having an ideal shape.

Bacteria appeared on Earth among the first, so they deservedly can be called centenarians. Interesting is the fact that these cells do not have a formed nucleus, therefore, to this day, they attract a lot of attention to their structure.

What is bacteria?

Bacteria are microscopic organisms of plant origin. The structure of a bacterial cell (a table, diagrams exist for a clear understanding of the types of these cells) depends on its purpose.

These cells are ubiquitous because they can multiply rapidly. There is scientific evidence that in just six hours one cell can produce offspring of 250,000 bacteria. These single-celled organisms have many varieties that vary in shape.

Bacteria are very tenacious organisms, their spores can retain the ability to live for 30-40 years. These spores are carried with a breath of wind, a current of water, and in other ways. Viability is maintained up to a temperature of 100 degrees and with a slight frost. And yet, what is the structure of a bacterial cell? The table describes the main components of the bacterium, the functions of other organelles are outlined below.

Globular (cocci) bacteria

They are pathogenic in nature. Cocci are divided into groups depending on their location to each other:

Micrococci (small). The division occurs in one plane. Arrangement in a chaotic single order. Eat ready organic compounds, but do not depend on other organisms (saprophytes).
Diplococci (double). They divide in the same plane as micrococci, but form paired cells. Outwardly, they resemble beans or lancelets.
Streptococci (in the form of a chain). The division is the same, but the cells are interconnected and look like beads.
Staphylococci (grape bunch). This species divides in several planes, forming a cluster of cells similar to grapes.
Tetracocci (four). Cells divide in two perpendicular planes, forming tetrads.
Sarcins (ligament). Such cells divide in three planes that are mutually perpendicular to each other. At the same time, outwardly, they look like packages or bales, consisting of many individuals of an even number.

Cylindrical (rods) bacteria

The rods that form spores are divided into clostridia and bacilli. In size, these bacteria are short and very short. The end sections of the sticks are rounded, thickened or cut off. Depending on the location of the bacteria, several groups are distinguished: mono-, diplo- and streptobacteria.

Spiral (coiled) bacteria

These microscopic cells are of two types:

Vibrio (single curved or generally straight).
Spirilla (large in size, but few whorls).

Filamentous bacteria. There are two groups of such forms:

temporary threads.
permanent threads.

The structural features of a bacterial cell lie in the fact that in the course of its existence it is able to change forms, but polymorphism is not inherited. Various factors act on the cell in the process of metabolism in the body, as a result of which quantitative changes are observed in its appearance. But as soon as the action from the outside stops, the cell will take on its former image. What are the structural features of a bacterial cell can be revealed when it is examined with a microscope.

The structure of a bacterial cell, shell

The shell gives and maintains the shape of the cell, protects the internal components from damage. Due to incomplete permeability, not all substances can enter the cell, which contributes to the exchange of low- and high-molecular structures between the external environment and the cell itself. Also in the wall there are various chemical reactions. With the help of an electron microscope, it is not difficult to study what detailed structure a bacterial cell has.

The shell base contains the polymer murein. Gram-positive bacteria have a single-layered skeleton composed of murein. Here are polysaccharide and lipoprotein complexes, phosphates. In gram-negative cells, the murein skeleton has many layers. The outer layer adjacent to the cell wall is the cytoplasmic membrane. It also has certain layers containing proteins with lipids. The main function of the cytoplasmic membrane is to control the penetration of substances into the cell and their removal (osmotic barrier). This is a very important function for cells, as it is used to protect cells.

The composition of the cytoplasm

The living semi-liquid substance that fills the cell cavity is called the cytoplasm. A large amount of protein, a supply of nutrients (fats and fat-like substances) contains a bacterial cell. A photo taken during a study under a microscope shows well the constituent parts inside the cytoplasm. The main composition includes ribosomes, located in a chaotic manner and in large numbers. Also in the composition there are mesosomes containing enzymes of a redox nature. Due to them, the cell draws energy. The nucleus is presented in the form of a nuclear substance located in the bodies of chromatin.

Functions of ribosomes in cells

Ribosomes consist of subunits (2) and are nucleoproteins. Connecting with each other, these constituent elements form polysomes or polyribosomes. The main task of these inclusions is protein synthesis occurring on the basis of genetic information. Sedimentation rate 70S.

Features of the bacterial nucleus

The genetic material (DNA) is located in the unformed nucleus (nucleoid). This nucleus is located in several places in the cytoplasm, being a loose membrane. Bacteria that have such a nucleus are called prokaryotes. The nuclear apparatus is devoid of a membrane, nucleolus and a set of chromosomes. And deoxyribonucleic acid is located in it in fibril bundles. The diagram of the structure of a bacterial cell demonstrates in detail the structure of the nuclear apparatus.

Under certain conditions, bacteria can develop mucus membranes. As a result, the formation of a capsule occurs. If the mucus is very strong, then the bacteria turn into zooglea (total mucous mass).

bacterial cell capsule

The structure of a bacterial cell has a feature - it is the presence of a protective capsule consisting of polysaccharides or glycoproteins. Sometimes these capsules are composed of polypeptides or fiber. It is located on top of the cell membrane. The thickness of the capsule can be either thick or thin. Its formation occurs due to the conditions in which the cell enters. The main property of the capsule is the protection of bacteria from drying out.

In addition to the protective capsule, the structure of the bacterial cell provides for its motor ability.

Flagella on bacterial cells

Flagella are additional elements that carry out the movement of the cell. They are presented in the form of threads of different lengths, which consist of flagellin. It is a protein that has the ability to contract.

The composition of the flagellum is three-component (thread, hook, basal body). Depending on attachment and location, several groups of motile bacteria have been identified:

Monotrichous (these cells have 1 flagellum located polarly).
Lophotrichous (flagellums in the form of a bundle at one end of the cell).
Amphitrichous (beams at both ends).

There are many interesting facts about bacteria. Thus, it has long been proven that mobile phone contains a huge number of these cells, even on the toilet seat there are fewer of them. Other bacteria allow us to live qualitatively - to eat, to perform certain activities, to free our body from the decay products of nutrients without any problems. Bacteria are truly diverse, their functions are multifaceted, but we should not forget about their pathological effect on the body, so it is important to monitor our own hygiene and cleanliness around us.

The cell of prokaryotic organisms has a complex, strictly ordered structure and has the fundamental features of ultra structural organization And chemical composition.

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

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

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

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

The main component of the bacterial cell wall is peptidoglycan, or murein (lat. murus - wall), a supporting 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 N-acetyl-M-glucosamine and N-acetylmuramic acid residues connected by 1,4-glycosidic bonds, identical tetrapeptide side chains attached to N-acetylmuramic acid molecules, and short transverse peptide chains. bridges linking polysaccharide chains. The two types of bonds (glycosidic and peptide) that connect the peptidoglycan subunits give this heteropolymer the structure of a molecular network. The backbone of the peptidoglycan layer is the same in all types of bacteria; tetrapeptide protein chains and peptide (transverse) chains are different in different species.

According to tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. In 1884, X. Gram proposed a staining method that was used to differentiate bacteria. The essence of the method lies in the fact that 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 colored 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. Peptidoglycan in gram-positive bacteria is the main component of the cell wall and ranges from 50 to 90%, in gram-negative bacteria - 1-10%. Structural microfibrils of peptidoglycan of Gram-negative bacteria are cross-linked less compactly; therefore, the pores in their peptidoglycan layer are much wider than in the molecular framework of Gram-positive bacteria. With this structural organization of peptidoglycan, the violet complex of gentian violet and iodine in gram-negative bacteria will be washed out faster.

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

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

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

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

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

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

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

L-Forms of bacteria. These are phenotypic modifications, or mutants, of bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan. Thus, L-forms are bacteria that are defective in the cell wall. They got their name due to the fact that they were isolated and described at the Lister Institute in England in 1935. They are formed when exposed to L-transforming agents - antibiotics (penicillin, polymyxin, bacitracin, vencomycin, streptomycin), amino acids (glycine, methionine, leucine, etc.), the enzyme lysozyme, ultraviolet and x-rays. Unlike protoplasts and spheroplasts, L-forms have 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.

The L-forms of bacteria are polymorphic. There are elementary bodies 0.2-1 microns in size (minimal reproductive elements), balls - 1-5, large bodies - 5-50, threads - up to 4 microns and more. L-form cells have a well-developed system of intracytoplasmic membranes and myelin-like structures. Due to a defect in the cell wall, they are osmotically unstable and can only be 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, which brings them closer to protoplasts; 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-shapes attached great importance in the development of chronic recurrent infections, the carriage of pathogens, their long-term persistence in the body. The transplacental invasiveness of elementary bodies of L-forms of bacteria has been proven.

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. Depending on the thickness of the layer and the strength of the connection with the bacterial cell, a macrocapsule is distinguished with a thickness of more than 0.2 μm, which is clearly visible in a light microscope, and a microcapsule with a thickness of less than 0.2 μm, which can be detected only with an electron microscope or detected by chemical and immunological methods. The macrocapsule (true capsule) is formed by B. anlhracis, C1. perfringens, microcapsule - Escherichia coJi. The capsule is not an obligatory structure of a bacterial cell: its loss does not lead to the death of the bacterium. Capsular mutants of bacteria are known, for example, the anthrax vaccine strain STI-1.

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

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

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

The capsule is a multifunctional organoid that plays an important biological role. It is the site of localization of capsular antigens that determine the virulence, antigenic specificity, and immunogenicity of bacteria. The loss of the capsule in pathogenic bacteria sharply reduces their virulence, for example, in acapsular strains of the anthrax bacillus. Capsules ensure the survival of bacteria, protecting them from mechanical damage, drying out, infection with phages, toxic substances, and in pathogenic forms - from the action of defensive forces macroorganism: encapsulated cells are poorly phagocytosed. In some types of bacteria, including pathogenic ones, it promotes cell attachment to the substrate.

In veterinary microbiology, capsule detection is used as a differential morphological trait pathogen in anthrax testing.

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

The microcapsule and the mucous layer are determined by serological tests (RA), the antigenic components of the capsule are identified using the immunofluorescent method (IF) and RDD.

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

The flagellum consists of three parts: a spiral filament, a hook, and a basal body. The hook is a curved protein cylinder that acts as a flexible connecting 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.

Rice. 3. Flagella:

a - monotrichous; b - amphitriches; c - lophotrichous; g - peritrichous

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

The number of flagella (from I to 50 or more) and their location in bacteria different types are not the same, but 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 (Fig. 3). 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. The speed of movement of monotrichs and lophotrichs reaches 50 μm/s, amphitrichs and peritrichs move more slowly and usually cover a distance equal to the size of their cell in 1 s.

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

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

Pili (fimbriae, villi) are straight, thin, hollow protein cylinders 3–25 nm thick and up to 12 µm long, extending from the surface of the bacterial cell. They are formed by a specific protein - pilin, originate from the cytoplasmic membrane, are found in mobile and immobile forms of bacteria and are visible only in an electron microscope (Fig. 4). On the cell surface there can be from 1-2, 50-400 or more pili to several thousand.

Rice. 4. Drank

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 of a general type are located peritrichially (E. coli) or at the poles (pseudomonas); one bacterium can contain hundreds of them. They take part in the adhesion of bacteria into agglomerates, the attachment of microbes to various substrates, including cells (adhesive function), in the transport of metabolites, and also contribute to the formation of films on the surface of liquid media; cause agglutination of erythrocytes.

Cytoplasmic membrane and its derivatives. The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an obligatory polyfunctional component of the cell and makes up 8-15% of its dry mass. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell. On ultrathin sections in an electron microscope, its three-layer structure is revealed - two limiting osmiophilic layers, each 2-3 nm thick, and one osmiophobic central layer 4-5 nm thick.

The cytoplasmic membrane is chemically a protein-lipid complex consisting of 50-75% proteins and 15-50% lipids. The main part of membrane lipids (70-90%) is represented by phospholipids. It is built from two monomolecular protein layers, between which there is a lipid layer, consisting of two rows of correctly oriented lipid molecules.

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

Cytoplasmic membrane enzymes catalyze the final steps in the synthesis of membrane lipids, cell wall components, capsules, and exoenzymes; the membrane contains enzymes of oxidative phosphorylation and electron transport enzymes responsible for energy synthesis.

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

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

The cytoplasm is 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 nucleoid.

Ribosomes are organelles that carry out protein synthesis. They consist of protein and RNA connected in a complex by hydrogen and hydrophobic bonds. Bacterial ribosomes are granules 15–20 nm in diameter, have a sedimentation constant of 70S, and are formed from two ribonucleoprotein subunits: 30S and 50S. One bacterial cell can contain from 5,000-50,000 ribosomes; by means of mRNA, they are combined into polysomes-aggregates consisting of 50-55 ribosomes with high protein-synthesizing activity.

found in the cytoplasm of bacteria various types inclusions. 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, these include poly-/3-hydroxybutyric acid granules surrounded by a membrane, which sharply refract light and are clearly visible in a light microscope. Anthrax bacilli and aerobic spore-forming saprophytic bacteria are also detected. Mycobacteria accumulate waxes as reserve substances. The cells of some coryne bacteria, 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 1.1-1.6 nm long, 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 mesosomes. The bacterial chromosome replicates in a polyconservative way: the parent DNA double helix unwinds and a new complementary chain is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the 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, plasmids, were found in the cells of many bacteria, represented by small circular DNA molecules capable of autonomous replication.

Sizes - from 1 to 15 microns. Basic forms:

Forms of bacteria:

mesosomes

mureina gram-positive(stained by Gram) and gram negative

nucleoid. Plasmids episome.

Many bacteria have flagella(10) and pili (fimbriae)

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sporulation

Reproduction.

Conjugation

Transformation

transduction

Viruses

Virus sizes are 10–300 nm. Virus shape:

capsid Supercapsid

virion

The structure of bacterial cells

The first bacteria appeared probably more than 3.5 billion years ago and for nearly a billion years were the only living beings on our planet. They are now ubiquitous and various processes occurring in nature.

The shape and size of bacteria

Bacteria are single-celled microscopic organisms. They have the form of sticks, balls, spirals. Some species form clusters but several thousand cells. The length of rod-shaped bacteria is 0.002-0.003 mm. Therefore, even with a microscope, individual bacteria are very difficult to see. However, they are easy to spot with the naked eye when they develop in large numbers and form colonies. Under laboratory conditions, colonies of bacteria are grown on special media containing the necessary nutrients.

The bacterial cell, like the cells of plants, fungi and animals, is covered with a plasma membrane. But unlike them, a dense cell membrane is located on the outside of the membrane. It consists of a durable substance and performs both protective and supporting functions, giving the cell a permanent shape. Through the cell membrane, nutrients freely pass into the cell, and unnecessary substances go into environment. Often, an additional protective layer of mucus is produced on top of the cell membrane in bacteria - a capsule.

On the surface of the cell membrane of some bacteria there are outgrowths - long flagella (one, two or more) or short thin villi. They help bacteria move around. In the cytoplasm of a bacterial cell there is a nuclear substance - a nucleoid, which carries hereditary information.

What is the structure of bacterial cells, or is everything as simple as it seems

The nuclear substance, unlike the nucleus, is not separated from the cytoplasm. Due to the absence of a formed nucleus and other structural features of the cell, all bacteria are combined into a separate kingdom of living nature - the kingdom of Bacteria.

Distribution of bacteria and their role in nature

Bacteria are the most common living things on earth. They live everywhere: in water, air, soil. Bacteria are able to live even where other organisms cannot survive: in hot springs, in the ice of Antarctica, in underground oil fields, and even inside nuclear reactors. Each bacterial cell is very small, but the total number of bacteria on earth is enormous. This
associated with a high rate of bacterial growth. Bacteria perform a variety of functions in nature.

The role of bacteria in the formation of fuel minerals is great. For millions of years, they decomposed the remains of marine organisms and land plants. As a result of the vital activity of bacteria, deposits of oil, natural gas, and coal were formed.

The structure of a bacterial cell

Sizes - from 1 to 15 microns. Basic forms: 1) cocci (spherical), 2) bacilli (rod-shaped), 3) vibrios (curved in the form of a comma), 4) spirilla and spirochetes (spiral twisted).

Forms of bacteria:
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.

The structure of a bacterial cell:
1 - cytoplasmic membrane wound; 2 - cell wall; 3 - slime capsule; 4 - cytoplasm; 5 - chromosomal DNA; 6 - ribosomes; 7 - meso-soma; 8 - photo-synthetic membrane wounds; 9 - inclusion; 10 - burn-tiki; 11 - drinking.

The bacterial cell is surrounded by a membrane. The inner layer of the membrane is represented by a cytoplasmic membrane (1), over which there is a cell wall (2); above the cell wall in many bacteria there is a mucous capsule (3). The structure and functions of the cytoplasmic membrane of eukaryotic and prokaryotic cells do not differ. The membrane may form folds called mesosomes(7). They may have different shape(saccular, tubular, lamellar, etc.).

Enzymes are located on the surface of mesosomes. The cell wall is thick, dense, rigid, composed of mureina(main component) and others organic matter. Murein is a regular network of parallel polysaccharide chains linked together by short protein chains. Bacteria are classified according to their cell wall structure. gram-positive(stained by Gram) and gram negative(not dyed). In gram-negative bacteria, the wall is thinner, more complex, and there is a layer of lipids above the murein layer on the outside. The inner space is filled with cytoplasm (4).

The genetic material is represented by circular DNA molecules. These DNAs can be conditionally divided into "chromosomal" and plasmid. “Chromosomal” DNA (5) is one, attached to the membrane, contains several thousand genes, unlike eukaryotic chromosomal DNA, it is not linear, not associated with proteins. The area in which this DNA is located is called nucleoid. Plasmids- extrachromosomal genetic elements. They are small circular DNA, not associated with proteins, not attached to the membrane, contain a small number of genes. The number of plasmids can be different. The most studied plasmids carrying information about resistance to medicines(R-factor) involved in the sexual process (F-factor). A plasmid that can combine with a chromosome is called episome.

In a bacterial cell, all membrane organelles characteristic of a eukaryotic cell (mitochondria, plastids, ER, Golgi apparatus, lysosomes) are absent.

In the cytoplasm of bacteria there are 70S-type ribosomes (6) and inclusions (9). Typically, ribosomes are assembled into polysomes. Each ribosome consists of a small (30S) and a large subunit (50S). The function of ribosomes is to assemble a polypeptide chain. Inclusions can be represented by lumps of starch, glycogen, volutin, lipid drops.

Many bacteria have flagella(10) and pili (fimbriae)(eleven). Flagella are not limited by a membrane, have a wavy shape and consist of spherical flagellin protein subunits. These subunits are arranged in a spiral and form a hollow cylinder 10–20 nm in diameter. The prokaryotic flagellum in its structure resembles one of the microtubules of the eukaryotic flagellum. The number and arrangement of flagella may vary. Pili are straight filamentous structures on the surface of bacteria. They are thinner and shorter than flagella. They are short hollow cylinders of pilin protein. Pili serve to attach bacteria to the substrate and to each other. During conjugation, special F-pili are formed, through which genetic material is transferred from one bacterial cell to another.

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sporulation bacteria have a way of experiencing adverse conditions. Spores are usually formed one at a time inside the "mother cell" and are called endospores. Spores are highly resistant to radiation, extreme temperatures, desiccation, and other factors that cause vegetative cell death.

Reproduction. Bacteria reproduce asexually by dividing the "mother cell" in two. Before division, DNA replication occurs.

Rarely, bacteria have a sexual process in which recombination of genetic material occurs. It should be emphasized that bacteria never form gametes, do not merge the contents of the cells, but transfer of DNA from the donor cell to the recipient cell takes place. There are three ways of DNA transfer: conjugation, transformation, transduction.

Conjugation- unidirectional transfer of the F-plasmid from the donor cell to the recipient cell in contact with each other. In this case, the bacteria are connected to each other by special F-pilae (F-fimbria), through the channels of which DNA fragments are transferred. Conjugation can be divided into the following stages: 1) F-plasmid unwinding, 2) penetration of one of the F-plasmid chains into the recipient cell through the F-pill, 3) synthesis of a complementary chain on a single-stranded DNA template (occurs as in a donor cell (F + ) and in the recipient cell (F-)).

Transformation- unidirectional transfer of DNA fragments from the donor cell to the recipient cell, not in contact with each other. In this case, the donor cell either “seeds” a small fragment of DNA from itself, or the DNA enters the environment after the death of this cell.

Bacteria cell. Structure

In any case, the DNA is actively absorbed by the recipient cell and integrated into its own "chromosome".

transduction- transfer of a DNA fragment from a donor cell to a recipient cell using bacteriophages.

Viruses

Viruses are made up of nucleic acid(DNA or RNA) and proteins that form a shell around this nucleic acid, i.e. are a nucleoprotein complex. Some viruses contain lipids and carbohydrates. Viruses always contain one type of nucleic acid - either DNA or RNA. Moreover, each of the nucleic acids can be both single-stranded and double-stranded, both linear and circular.

Virus sizes are 10–300 nm. Virus shape: spherical, rod-shaped, filiform, cylindrical, etc.

capsid- the shell of the virus, formed by protein subunits, stacked in a certain way. The capsid protects the nucleic acid of the virus from various influences, ensures the deposition of the virus on the surface of the host cell. Supercapsid characteristic of complex viruses (HIV, influenza viruses, herpes). Occurs during the exit of the virus from the host cell and is a modified section of the nuclear or outer cytoplasmic membrane of the host cell.

If the virus is inside the host cell, then it exists in the form of a nucleic acid. If the virus is outside the host cell, then it is a nucleoprotein complex, and this free form of existence is called virion. Viruses are highly specific; they can use a strictly defined circle of hosts for their life activity.

The structure of a bacterial cell

Sizes - from 1 to 15 microns. Basic forms: 1) cocci (spherical), 2) bacilli (rod-shaped), 3) vibrios (curved in the form of a comma), 4) spirilla and spirochetes (spiral twisted).

Forms of bacteria:
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.

The bacterial cell is surrounded by a membrane. The inner layer of the membrane is represented by a cytoplasmic membrane (1), over which there is a cell wall (2); above the cell wall in many bacteria there is a mucous capsule (3). The structure and functions of the cytoplasmic membrane of eukaryotic and prokaryotic cells do not differ. The membrane may form folds called mesosomes(7). They can have a different shape (bag-shaped, tubular, lamellar, etc.).

Enzymes are located on the surface of mesosomes. The cell wall is thick, dense, rigid, composed of mureina(main component) and other organic substances. Murein is a regular network of parallel polysaccharide chains linked together by short protein chains. Bacteria are classified according to their cell wall structure. gram-positive(stained by Gram) and gram negative(not dyed). In gram-negative bacteria, the wall is thinner, more complex, and there is a layer of lipids above the murein layer on the outside. The inner space is filled with cytoplasm (4).

The genetic material is represented by circular DNA molecules. These DNAs can be conditionally divided into "chromosomal" and plasmid. “Chromosomal” DNA (5) is one, attached to the membrane, contains several thousand genes, unlike eukaryotic chromosomal DNA, it is not linear, not associated with proteins. The area in which this DNA is located is called nucleoid. Plasmids- extrachromosomal genetic elements. They are small circular DNA, not associated with proteins, not attached to the membrane, contain a small number of genes. The number of plasmids can be different. The most studied plasmids are those that carry information about drug resistance (R-factor) and are involved in the sexual process (F-factor). A plasmid that can combine with a chromosome is called episome.

In a bacterial cell, all membrane organelles characteristic of a eukaryotic cell (mitochondria, plastids, ER, Golgi apparatus, lysosomes) are absent.

Many bacteria have flagella(10) and pili (fimbriae)(eleven). Flagella are not limited by a membrane, have a wavy shape and consist of spherical flagellin protein subunits.

The structure of a bacterial cell: features. What is the structure of a bacterial cell?

These subunits are arranged in a spiral and form a hollow cylinder 10–20 nm in diameter. The prokaryotic flagellum in its structure resembles one of the microtubules of the eukaryotic flagellum. The number and arrangement of flagella may vary. Pili are straight filamentous structures on the surface of bacteria. They are thinner and shorter than flagella. They are short hollow cylinders of pilin protein. Pili serve to attach bacteria to the substrate and to each other. During conjugation, special F-pili are formed, through which genetic material is transferred from one bacterial cell to another.

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Reproduction. Bacteria reproduce asexually by dividing the "mother cell" in two. Before division, DNA replication occurs.

transduction- transfer of a DNA fragment from a donor cell to a recipient cell using bacteriophages.

Viruses

Viruses consist of a nucleic acid (DNA or RNA) and proteins that form a shell around this nucleic acid, i.e. are a nucleoprotein complex. Some viruses contain lipids and carbohydrates. Viruses always contain one type of nucleic acid - either DNA or RNA. Moreover, each of the nucleic acids can be both single-stranded and double-stranded, both linear and circular.

Virus sizes are 10–300 nm. Virus shape: spherical, rod-shaped, filiform, cylindrical, etc.