When we talk about bacteria, we often think of something negative. And yet we know very little about them. The structure and vital activity of bacteria is rather primitive, but, according to the assumptions of some scientists, these are the most ancient inhabitants of the Earth, and for so many years they have not disappeared or died out. Many types of such microorganisms are used by humans for their own good, while others are the cause of serious diseases and even epidemics. But the harm of some bacteria is sometimes not commensurate with the benefits of others. Let's talk about these amazing microorganisms and get acquainted with their structure, physiology and classification.
These are non-nuclear, most often unicellular microorganisms. Their discovery in 1676 is the merit of the Dutch scientist A. Leeuwenhoek, who for the first time saw tiny bacteria under a microscope magnifying glass. But the French chemist and microbiologist Louis Pasteur first began to study their nature, physiology and role in human life in the 1850s. The structure of bacteria began to be actively investigated with the advent of electron microscopes. Its cell consists of a cytoplasmic membrane, a ribosome and a nucleotide. The bacterial DNA is concentrated in one place (nucleoplasm) and is a tangle of thin threads. The cytoplasm is separated from the cell wall by the cytoplasmic membrane; it contains a nucleotide, various membrane systems, and cellular inclusions. The ribosome of bacteria consists of 60% RNA, the rest is protein. The photo below shows the structure of Salmonella.
Bacteria have a cellular structure. The cell wall is about 20 nm thick and, unlike higher plants, has no fibrillar structure. Its strength is ensured by a special cover called a bag. It consists mainly of a polymeric substance - murein. Its components (subunits) are connected in a specific sequence into special polyglycan strands. Together with short peptides, they form a macromolecule that resembles a network. This is the murein bag.
These microorganisms are capable of active movement. It is carried out due to plasma flagella, which have a helical structure. Bacteria can move at speeds up to 200 microns per second and turn around their axis 13 times per second. The ability of flagella to move is provided by a special contractile protein - flagellin (an analogue of myosin in muscle cells).
They have the following dimensions: length - up to 20 microns, diameter - 10-20 nm. Each flagellum departs from the basal corpuscle, which is immersed in the bacterial cell membrane. The organs of locomotion can be single or arranged in whole bundles, as, for example, in spirilla. The number of flagella may depend on environmental conditions. For example, Proteus vulgaris, with a poor diet, has only two subpolar flagella, whereas under normal conditions of development, there can be from 2 to 50 of them in the bundles.
The structure of a bacterium (diagram below) is such that it can move around quite actively. Movement in most cases occurs by pushing and is carried out mainly in a liquid or wet environment. Depending on the acting factor, in other words - the type of external stimulus, it can be:
One of these methods, bacteria, the structural features of the cells of which allow them to move, can create clusters in places with optimal conditions for their life. In addition to flagella, some species have numerous thinner filaments - they are called "fimbria" or "pili", but their function has not yet been sufficiently understood. Bacteria that do not have special flagella are capable of sliding movement, however, it is characterized by a very low speed: about 250 microns per minute.
The second small group of bacteria is autotrophs. They are able to synthesize organic substances from inorganic substances, can partially assimilate atmospheric carbon dioxide and are chemotrophs. These bacteria occupy a very important place in the cycle of chemical elements in nature.
There are also two groups of true phototrophs. The structural features of bacteria in this category are that they contain a substance (pigment) bacteriochlorophyll, which is related in nature to plant chlorophyll, and since they lack photosystem II, photosynthesis proceeds without the release of oxygen.
The main mode of reproduction is the division of the original mother cell in two (amitosis). For forms that have an elongated shape, this always happens perpendicular to the longitudinal axis. In this case, the structure of the bacterium undergoes short-term changes: from the edge of the cell to the middle, a transverse partition is formed, along which the maternal organism is then divided. This explains the old name of the kingdom - Drobyanka. After division, cells can remain connected in unstable, loose chains.
These are the distinctive features of the structure of bacteria of some species, for example, streptococci.
The second way of reproduction is sporulation. It is directly associated with the desire to adapt to adverse conditions and is aimed at experiencing them. In some rod-shaped bacteria, spores form endogenously, that is, inside the cell. They are very resistant to heat and can be stored even with prolonged boiling. Spore formation begins with various chemical reactions in the mother cell, and about 75% of all its proteins are decomposed. Then division occurs. In this case, two daughter cells are formed. One of them (the smaller one) is covered with a thick shell, which by volume can occupy up to 50% - this is a dispute. It remains viable and ready for germination for 200-300 years.
Some species are capable of sexual reproduction. This process was first discovered in 1946, when the cell structure of the bacterium Escherichia coli was studied. It turned out that partial transfer of genetic material is possible. That is, DNA fragments are transferred from one cell (donor) to another (recipient) during conjugation. This is done with the help of bacteriophages or by transformation.
The structure of the bacterium and the features of its physiology are such that, under ideal conditions, the division process occurs constantly and very quickly (every 20-30 minutes). But in the natural environment, it is limited by various factors (sunlight, nutrient medium, temperature, etc.).
The classification of these microorganisms is based on the different structure of the bacterial cell wall, which determines the preservation of the aniline dye in the cell or its leaching. This was revealed by H.K. Gram, and subsequently, in accordance with his name, two large divisions of microorganisms were distinguished, which we will discuss below.
These microorganisms have a multilayer murein cover (30-70% of the total dry mass of the cell wall), due to which the aniline dye is not washed out of the cells (the photo above schematically shows the structure of a gram-positive bacterium on the left, and a gram-negative one on the right). Their feature is that diaminopimelic acid is often replaced by lysine. The protein content is much less, and the polysaccharides are absent or linked by covalent bonds. All bacteria in this department are divided into several groups:
In such microorganisms, the murein network is very thin, its share of the dry mass of the entire cell wall is only 10%, the rest is lipoproteins, lipopolysaccharides, etc. Substances received during Gram staining are easily washed out. By the type of nutrition, gram-negative bacteria are phototrophs or chemotrophs, some species are capable of photosynthesis. The classification within the department is in the process of formation, various families are combined into 12 groups, based on the characteristics of morphology, metabolism and other factors.
Despite its seemingly invisibility, bacteria are of great importance to humans, both positive and negative. The production of many food products is impossible without the participation of individual representatives of this kingdom. The structure and vital functions of bacteria allow us to receive many dairy products (cheeses, yoghurts, kefir and much more). These microorganisms are involved in the processes of fermentation and fermentation.
Numerous types of bacteria are causative agents of diseases in animals and humans, such as anthrax, tetanus, diphtheria, tuberculosis, plague, etc. But at the same time, microorganisms are involved in various industrial industries: genetic engineering, obtaining antibiotics, enzymes and others proteins, artificial decomposition of waste (for example, methane digestion of waste water), enrichment of metals. Some bacteria grow on substrates rich in petroleum products, and this serves as an indicator in the search and development of new deposits.
The main differences between a prokaryotic (bacterial) cell and a eukaryotic one are: the absence of a formed nucleus (i.e., nuclear membrane), the absence of intracellular membranes, nucleoli, Golgi complex, lysosomes, mitochondria.
The main structures of a bacterial cell are:
Nucleoid - is a hereditary (genetic) material of a bacterial cell, represented by 1 DNA molecule, closed in a ring and supercoiled (twisted into a loose ball). DNA length is about 1mm. The amount of information is about 1000 genes (traits). The nucleoid is not separated from the cytoplasm by the membrane.
Cytoplasm is a colloid, i.e. an aqueous solution of proteins, carbohydrates. Lipids, minerals, which are ribosomes, inclusions, plasmids.
Protein biosynthesis occurs on ribosomes. Ribosomes of prokaryotes differ from eukaryotic ones in smaller sizes (70 S).
Inclusions are reserve nutrients of the bacterial cell, as well as accumulations of pigments. Reserve nutrients include: granules of volutin (inorganic polyphosphate), glycogen, granulosis, starch, drops of fat, accumulations of pigment, sulfur, calcium. Inclusions, as a rule, are formed when bacteria are grown on rich nutrient media and disappear during fasting.
Cell membrane - limits the cytoplasm. It is composed of a double layer of phospholipids and embedded membrane proteins. In addition to the barrier and transport functions, CMs play the role of a center of metabolic activity (in contrast to a eukaryotic cell). Membrane proteins responsible for the transfer of essential substances into the cell are called permeases. On the inner surface of the CM there are enzyme assemblies, i.e. ordered clusters of enzyme molecules responsible for the synthesis of energy carriers - ATP molecules. BM can form invaginations into the cytoplasm, which are called mesosomes. There are two types of mesosomes:
Septal - form transverse septa during cell division.
Lateral - serve to increase the surface of BM and increase the rate of metabolic processes.
Nucleoid, CP and CM form a protoplast.
One of the distinctive properties of bacteria is a very high intracellular osmotic pressure (from 5 to 20 atm), which is the result of intensive metabolism. Therefore, to protect against osmotic shock, the bacterial cell is surrounded by a strong cell wall.
According to the structure of the cell wall, all bacteria are divided into 2 groups: Having a single-layer cell wall - Gram-positive. Having a two-layer cell wall - Gram-negative. The names Gram + and Gram- have their own prehistory. In 1884, the Danish microbiologist Hans Christian Gram developed an original method for staining microbes, as a result of which some bacteria were stained blue (gram +), and others red (gram-). The chemical basis of various coloration of bacteria by the Gram method was discovered relatively recently - about 35 years ago. It turned out that G- and G + bacteria have a different structure of the cell wall. The structure of the cell wall of G + bacteria. The basis of the cell wall of G + bacteria are 2 polymers: peptidoglycan and teichoic acids. Peptidoglycan is a linear polymer in which the residues of muramic acid and acetylglucosamine alternate. A tetrapeptide (protein) is covalently bound to muramic acid. The peptidoglycan strands are interconnected through peptides and form a strong framework - the basis of the cell wall. Between the peptidoglycan strands is another polymer - teichoic acids (glycerol TC and ribitol TC) - a polymer of polyphosphates. Teichoic acids appear on the surface of the cell wall and are the main AGs of G + bacteria. In addition, the composition of the cell wall of G + bacteria includes Mg ribonucleate. The wall of G-bacteria consists of 2 layers: the inner layer is represented by a mono- or bilayer of peptidoglycan (thin layer). The outer layer consists of lipopolysaccharides, lipoprotein, proteins, phospholipids. The LPSs of all G-bacteria have toxic and threshold properties and are called endotoxins.
When exposed to certain substances, such as penicillin, the synthesis of the peptidoglycan layer is disrupted. In this case, a protoplast is formed from G + bacteria, and a spheroplast from G + (since the outer layer of the cell wall is preserved).
Under certain conditions of cultivation, cells devoid of a cell wall retain the ability to grow and divide, and such forms are called L-forms (after the name of the Lister Institute, where this phenomenon was discovered). In some cases, after the elimination of the factor that inhibits the synthesis of the cell wall, the L-forms can be converted to their original forms.
Many bacteria synthesize a mucous substance consisting of mucopolysaccharides, which is deposited on the outside of the cell wall, surrounding the bacterial cell with a mucous membrane. This is a capsule. The function of the capsule is to protect bacteria from phase cytosis.
Surface structures of a bacterial cell.
Organs of attachment to the substrate (adhesion) - saw (fimbria) or cilia. They start from the cell membrane. Composed of pilin protein. The number of saws can be up to 400 per square.
Organs of transmission of hereditary information - F-drank or sex-drank. F-pili are formed only if the cell is odd with the plasmid, because F-pili proteins encodes plasmid DNA. They are a thin, long tube that attaches to another bacterial cell. Through the formed channel, the plasmid passes into the neighboring bacterial cell.
The organs of movement - flagella - are spiral threads. Their length can exceed their diameter by 10 or more times. Flagella are composed of the protein flagellin. The base of the flagellum is connected to the cell membrane through the basal corpuscle. The basal body consists of a system of rings, which, while rotating, transmit the rotational movement of the flagellum. According to the location of the flagellum, bacteria are divided into mono-, lopho-, amphi-, peritrichous.
Bacteria ("stick" from ancient Greek) represent the kingdom (group) of non-nuclear (prokaryotic) microorganisms, unicellular, as a rule. Today, about ten thousand of their species are known and described. Scientists estimate that there are more than a million of them.
It can be round, twisted, rod-shaped. In rare cases, there are cubic, tetrahedral, star-shaped, as well as O- or C-shaped forms. determines the abilities that a bacterial cell possesses. For example, depending on the form, microorganisms have one or another degree of mobility, the ability to attach to a surface, one way or another to absorb nutrient compounds.
A bacterial cell includes three obligatory structures: a cytoplasmic membrane, ribosomes, and a nucleoid.
There are several layers from the outside of the membrane. In particular, there is a mucous membrane, a capsule, a cell wall. In addition, various surface structures develop from the outside: villi, flagella. Cytoplasm and membrane are combined into the concept of "protoplast".
The bacterial cell with all its contents is limited from the external environment by means of a membrane. Inside, in a homogeneous fraction of the cytoplasm, there are proteins, soluble RNA, substrates of metabolic reactions, and various compounds. The rest contains various structural elements.
Does not contain nuclear membranes and any other intracytoplasmic membranes that are not derived from the cytoplasmic membrane. At the same time, some prokaryotes are characterized by local "protrusions" of the main shell. These "protrusions" - mesosomes - perform different functions and divide the bacterial cell into functionally different parts.
All the data necessary for life is contained in one DNA. The chromosome, which the bacterial cell includes, usually has the form of a ring, covalently closed. At one point, DNA is attached to the membrane and placed in a separate, but not separated from the cytoplasm, structure. This structure is called "nucleoid". When unfolded, the bacterial chromosome is more than a millimeter long. As a rule, it is presented in one copy. In other words, virtually all prokaryotes are haploid. However, under certain specific conditions, a bacterial cell can contain copies of its chromosome.
At the same time, this structural element is not required. In laboratory conditions, some forms of prokaryotes were obtained in which the wall was completely or partially absent. These bacteria could exist under normal conditions, but in some cases they lost the ability to divide. In nature, there is a group of prokaryotes that do not contain walls in their structure.
An amorphous layer - a capsule - can be located on the outer surface of the wall. The mucous layers are separated from the microorganism quite easily, they have no connection with the cell. The covers also have a thin structure, they are not amorphous.
Reproduction of bacteria of some forms is carried out by means of equal, binary cross-division or budding. Different groups have different variants of division. So, for example, in cyanobacteria, reproduction occurs in multiple ways - several successive binary divisions. As a result, from four to a thousand new microorganisms are formed. They have special mechanisms through which the plasticity of the genotype is provided, which is necessary for adaptation to a changeable external environment and evolution.
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.
Most bacteria have three shells:
Directly with the contents of the cell - the cytoplasm, the cell membrane comes into contact. She is thin and soft.
The cell wall is a dense, thicker membrane. Its function is to protect and support the cage. The cell wall and membrane have pores through which the substances it needs enter the cell.
Many bacteria have a mucous capsule that has a protective function and adheres to different surfaces.
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It is thanks to the mucous membrane that streptococci (one of the types of bacteria) adhere to the teeth and cause caries.
The cytoplasm is the inner content of the cell. It consists of 75% water. In the cytoplasm there are inclusions - drops of fat and glycogen. They are the storage nutrients of the cell.
Rice. 1. Diagram of the structure of a bacterial cell.
Nucleoid means "like a nucleus". Bacteria do not have a real, or, as they say, a formed nucleus. This means that they do not have a nuclear envelope and nuclear space, like the cells of fungi, plants and animals. DNA is found right in the cytoplasm.
DNA functions:
The absence of a true nucleus is the most important characteristic of a bacterial cell.
Unlike plant and animal cells, bacteria do not have membrane organelles.
But the cell membrane of bacteria in some places penetrates the cytoplasm, forming folds called the mesosome. The mesosome is involved in cell reproduction and energy exchange and, as it were, replaces membrane organelles.
The only organoid found in bacteria is the ribosome. These are small bodies that are located in the cytoplasm and synthesize proteins.
Many bacteria have a flagellum that helps them move around in a liquid environment.
The cell shape of bacteria is different. Ball-shaped bacteria are called cocci. In the form of a comma - vibrios. Rod-shaped bacteria are bacilli. Spirillae have the appearance of 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 in length. (1 μm = 0.001 mm).
When unfavorable conditions occur, the bacterial cell goes into a dormant state, which is called a spore. The causes of sporulation can be:
The transition occurs quickly, within 18-20 hours, and the cell can be in a state of spore for hundreds of years. When normal conditions are restored, the bacterium grows out of the spore in 4-5 hours and goes into normal life.
Rice. 3. Scheme of controversy formation.
Bacteria multiply by fission. The period from the birth of a cell to its division is 20-30 minutes. Therefore, bacteria are widespread on Earth.
We learned that, in general terms, bacterial cells are similar to those of plants and animals, they have a membrane, cytoplasm, and DNA. The main difference between bacterial cells is the absence of a formed nucleus. Therefore, bacteria are called prenuclear organisms (prokaryotes).
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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 uncultivated 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 (essential components of a bacterial cell). Some bacteria can 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 the bacterial cell, located between the cytoplasmic membrane and the capsule; in capsule-free bacteria, it is the outer membrane 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 types of pathogenic bacteria, it is toxic, contains surface antigens, and also carries specific receptors for phages on the surface. In the cell wall of bacteria there are pores that are involved in the transport of exotoxins and other exoproteins of bacteria.
The main component of the bacterial cell wall is peptidoglycan, or murein (Latin murus - wall), a support polymer that has a reticular structure and forms a rigid (rigid) outer skeleton of a bacterial cell. Peptidoglycan has a backbone (backbone) consisting of alternating N-acetyl-M-glucosamine and N-acetylmuramic acid residues linked by 1,4-glycosidic bonds, identical tetrapeptide side chains attached to N-acetylmuramic acid molecules, and short transverse peptidic bridges connecting 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 ethanol bleaching 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 with additional application of fuchsin they turn red. 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 bleach with ethanol is determined by the specifics of the chemical composition and ultrastructure of their cell walls. bacterial chlamydia trachoma
L-forms of bacteria are phenotypic modifications, or mutants, of bacteria that have partially or completely lost the ability to synthesize peptidoglycan of the cell wall. Thus, L-forms are bacteria defective along the cell wall. 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. In contrast to 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 bacteria filters. There are stable and unstable L-forms of bacteria. The former are completely devoid of a rigid cell wall; they are very rarely reversed 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 are reversed into the original cells.
The process of formation of L-forms is called L-transformation or L-induction. Almost all types of bacteria, including pathogenic ones (causative agents of brucellosis, tuberculosis, listeria, etc.), have the ability to L-transformation.
L-Forms are of great importance in the development of chronic recurrent infections, the carriage of pathogens, and their long-term persistence in the body. The infectious process caused by L-forms of bacteria is characterized by atypicality, duration of the course, severity of the disease, it is difficult to chemotherapy.
A capsule is a mucous layer located above the bacterial cell wall. The substance of the capsule is clearly delimited from the environment. The capsule is not an obligatory structure of the bacterial cell: its loss does not lead to the death of the bacterium.
The substance of the capsules consists of highly hydrophilic micelles, but their chemical composition is very diverse. The main components of most prokaryotic capsules are homo- or hetsropolysaccharides (entrobacteria, etc.). In some species of bacilli, the capsule is constructed from a polypeptide.
Capsules ensure the survival of bacteria, protecting them from mechanical damage, drying, infection by phages, toxic substances, and in pathogenic forms - from the action of the protective forces of the macroorganism: encapsulated cells are poorly phagocytized. In some types of bacteria, including pathogenic ones, it promotes the attachment of cells to the substrate.
Flagella are organelles of the movement of bacteria, represented by thin, long, filamentous structures of a protein nature.
The flagellum consists of three parts: the spiral filament, the hook, and the basal corpuscle. The hook is a curved proteinaceous cylinder that acts as a flexible link between the basal corpuscle and the rigid filament of the flagellum. The basal body is a complex structure consisting of a central rod (axis) and rings.
Flagella are not vital structures of a bacterial cell: there are phase variations in bacteria, when in one phase of cell development they are present, in the other they are absent.
The number of flagella and the place 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 flagellar bacteria are distinguished: myotriches - bacteria with one polarized flagellum; amphitrichs - bacteria with two polarized flagella or having a bundle of flagella at both ends; lofotrichi - 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.
As organs of movement, flagella are typical of floating rod-shaped and convoluted forms of bacteria and are found only in isolated cases in cocci. They provide efficient movement in liquid media and slower movement over the surface of solid substrates.
Drank (fimbriae, villi) - straight, thin, hollow protein cylinders extending from the surface of the bacterial cell. 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 surface of the cell there can be from 1-2, 50-400 and more pili up to several thousand.
There are two classes of pili: sexual (sexpili) and a general type of pili, which are more often called fimbria. The same bacteria can have a drink of a different nature. Sex pili arise on the surface of bacteria during conjugation and act as organelles through which genetic material (DNA) is transferred from donor to recipient.
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 red blood cells.
The cytoplasmic membrane (plasmolemma) is a semipermeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an obligatory multifunctional component of the cell. The 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. Most of the membrane lipids are phospholipids. It is built of 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 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, which form the intracytoplasmic membrane structures. Local invaginates of the membrane are called mesosomes. These structures are well pronounced in gram-positive bacteria, worse in gram-negative bacteria, and poorly in rickettsia and mycoplasma.
Mesosomes, like the cytoplasmic membrane, are the centers of the respiratory activity of bacteria, therefore they are sometimes called mitochondrial analogs. However, the significance of mesosomes has not yet been finally clarified. They increase the working surface of the membranes, possibly performing only a structural function, separating the bacterial cell into relatively isolated compartments, which creates more favorable conditions for enzymatic processes. In pathogenic bacteria, they provide transport of protein molecules of exotoxins.
Cytoplasm - the contents of a bacterial cell, delimited by the cytoplasmic membrane. Consists of cytosol - a homogeneous fraction that includes 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 biosynthesis. They consist of protein and RNA, linked into a complex by hydrogen and hydrophobic bonds.
In the cytoplasm of bacteria, various types of inclusions are detected. They can be solid, liquid and gaseous, with or without a protein membrane, and are not permanently present. A significant part of them are reserve nutrients and products of cellular metabolism. Reserve nutrients include: polysaccharides, lipids, polyphosphates, sulfur deposits, etc. Of the inclusions of a polysaccharide nature, glycogen and a starch-like substance of granulosis are more often found, which serve as a source of carbon and an energy material. Lipids accumulate in cells in the form of granules and fat droplets. Mycobacteria accumulate waxes as reserve substances. The cells of some spirilla and others contain volutin granules formed by polyphosphates. They are characterized by metachromasia: toluidine blue and methylene blue color them purple-red. Volutin granules act as phosphate depots. The inclusions surrounded by a membrane also include gas vacuoles, or aerosomes, they reduce the specific gravity of cells, and are found in aquatic prokaryotes.
Nucleoid is the nucleus of prokaryotes. It consists of one double-stranded DNA strand closed in a ring, which is considered as a single bacterial chromosome, or genophore.
The nucleoid in prokaryotes is not delimited from the rest of the cell by a membrane - it does not have a nuclear envelope.
The structure of the nucleoid includes 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 daughter nuclei is provided by the growth of the cytoplasmic membrane.
The bacterial nucleus is a differentiated structure. Depending on the stage of development of the cell, 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 occurs after the completion of the replication cycle of the DNA molecule and the formation of daughter chromosomes.
The nucleoid contains the bulk of the genetic information of the bacterial cell.
In addition to the nucleoid, extrachromosomal genetic elements were found in the cells of many bacteria - plasmids, represented by small circular DNA molecules capable of autonomous replication
Some bacteria can form spores at the end of their active growth period. This is preceded by the depletion of nutrients in the medium, a change in its pH, and the accumulation of toxic metabolic products.
In terms of chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. Spores contain less water and more lipids.
In a 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. The high temperature resistance is due to the very low water content and high dipicolinic acid content. Once in the body of humans and animals, spores germinate into vegetative cells. Spores are colored by a special method, which includes preliminary heating of the spore, as well as exposure to concentrated paint solutions at high temperatures.
Many species of gram-negative bacteria, including pathogenic ones (Shigella, Salmonella, Vibrio cholerae, etc.), have a special adaptive, genetically regulated state, physiologically equivalent to cysts, into which they can pass under the influence of unfavorable conditions and remain viable for up to several years. The main feature of this condition is that such bacteria do not multiply and therefore do not form colonies on a dense nutrient medium. Such non-multiplying, but viable cells are called uncultivated forms of bacteria (NPB). NPB cells in an uncultivated state have active metabolic systems, including electron transfer systems, protein and nucleic acid biosynthesis, and retain virulence. Their cell membrane is more viscous, the cells usually take the form of cocci and are significantly reduced in size. NPB have a higher stability in the external environment and therefore can survive in it for a long time (for example, Vibrio cholerae in a dirty water body), maintaining the endemic state of this region (water body).
Molecular genetic methods (DNA - DNA hybridization, CPR), as well as a simpler method of direct counting of viable cells, are used to detect NPP.
For these purposes, you can also use the methods of cytochemical (formation of formazan) or microautoradiography. The genetic mechanisms responsible for the transition of bacteria to the NS and their reversal from it are not clear.
