The structure of microbes. Microbiology with microbiological research techniques - the structure of bacteria. Morphological forms of bacteria
Microorganisms (microbes) are single-celled organisms smaller than 0.1 mm in size that cannot be seen with the naked eye. These include bacteria, microalgae, some lower filamentous fungi, yeast, and protozoa (Fig. 1). Microbiology studies them.
Rice. 1. Microbiology objects.
In Fig. 2. You can see some representatives of single-celled protozoa. Sometimes the objects of this science include the most primitive organisms on Earth - viruses that do not have a cellular structure and are complexes of nucleic acids(genetic material) and protein. More often they are isolated into a completely separate field of study (Virology), since microbiology is rather aimed at the study of microscopic single-celled organisms.
Rice. 2. Individual representatives of unicellular eukaryotes (protozoa).
The sciences of algology and mycology, which study algae and fungi, respectively, are separate disciplines that overlap with microbiology in the study of microscopic living objects. Bacteriology is a true branch of microbiology. This science deals exclusively with the study of prokaryotic microorganisms (Fig. 3).
Rice. 3. Scheme of a prokaryotic cell.
Unlike eukaryotes, which include all multicellular organisms, as well as protozoa, microscopic algae and fungi, prokaryotes do not have a formed nucleus containing genetic material and real organelles (permanent specialized structures of the cell).
Prokaryotes include true bacteria and archaea, which according to modern classification are designated as domains (superkingdoms) Archaea and Eubacteria (Fig. 4).
Rice. 4. Domains of modern biological classification.
Structural features of bacteria
Bacteria are an important link in the cycle of substances in nature; they decompose plant and animal residues, clean bodies of water contaminated with organic matter, and modify inorganic compounds. Without them, life on earth could not exist. These microorganisms are distributed everywhere, in soil, water, air, animal and plant organisms.
Bacteria differ in the following morphological features:
- Cell shape (round, rod-shaped, filamentous, convoluted, spiral, as well as various transitional options and star-shaped configuration).
- The presence of devices for movement (immobile, flagellated, due to the secretion of mucus).
- Articulation of cells with each other (isolated, linked in the form of pairs, granules, branching forms).
Among the structures formed by round bacteria (cocci), there are cells that are in pairs after division and then break up into single formations (micrococci) or remain together all the time (diplococci). A quadratic structure of four cells is formed by tetracocci, a chain by streptococci, a granule of 8-64 units by sarcina, and clusters by staphylococci.
Rod-shaped bacteria are represented by a variety of shapes due to the great variability in the length (0.1-15 µm) and thickness (0.1-2 µm) of the cell. The shape of the latter also depends on the ability of bacteria to form spores - structures with a thick shell that allows microorganisms to survive unfavorable conditions. Cells with this ability are called bacilli, and those that do not have such properties are simply called rod-shaped bacteria.
Special modifications of rod-shaped bacteria are filamentous (elongated) forms, chains and branching structures. The latter is formed by actinomycetes at a certain stage of development. “Curved” rods are called crimped bacteria, among which vibrios are isolated; spirilla having two bends (15-20 µm); spirochetes that resemble wavy lines. Their cell lengths are 1-3, 15-20 and 20-30 µm, respectively. In Fig. Figures 5 and 6 show the main morphological forms of bacteria, as well as the types of spore arrangement in the cell.
Rice. 5. Basic forms of bacteria.
Rice. 6. Bacteria according to the type of spore location in the cell. 1, 4 – in the center; 2, 3, 5 – end location; 6 – from the side.
The main cellular structures of bacteria: nucleoid (genetic material), ribosomes intended for protein synthesis, cytoplasmic membrane (part of the cell membrane), which in many representatives is additionally protected from above, capsule and mucous sheath (Fig. 7).
Rice. 7. Scheme of a bacterial cell.
According to the classification of bacteria, there are more than 20 types. For example, extremely thermophilic (high temperature lovers) Aquificae, anaerobic rod-shaped bacteria Bacteroidetes. However, the most dominant type, which includes many different representatives, is an Actinobacteria. It includes bifidobacteria, lactobacilli, and actinomycetes. The uniqueness of the latter lies in the ability to form mycelium at a certain stage of development.
In common parlance this is called mycelium. Indeed, the branching cells of actinomycetes resemble fungal hyphae. Despite this feature, actinomycetes are classified as bacteria, since they are prokaryotes. Naturally, their cells are less similar in structure to fungi.
Actinomycetes (Fig. 8) are slow-growing bacteria, and therefore do not have the ability to compete for readily available substrates. They are capable of decomposing substances that other microorganisms cannot use as a carbon source, in particular petroleum hydrocarbons. Therefore, actinomycetes are intensively studied in the field of biotechnology.
Some representatives are concentrated in areas oil fields, and create a special bacterial filter that prevents the penetration of hydrocarbons into the atmosphere. Actinomycetes are active producers of practically valuable compounds: vitamins, fatty acids, antibiotics.
Rice. 8. Representative actinomycete Nocardia.
Fungi in microbiology
The object of microbiology is only lower mold fungi (rhizopus, mucor, in particular). Like all mushrooms, they are not able to synthesize substances themselves and require a nutrient medium. The mycelium of the lower representatives of this kingdom is primitive, not divided by partitions. A special niche in microbiological research is occupied by yeast (Fig. 9), characterized by the absence of mycelium.
Rice. 9. Forms of colonies of yeast cultures on a nutrient medium.
Currently about them beneficial properties a wealth of knowledge has been collected. However, yeast continues to be studied for its ability to synthesize practically valuable organic compounds and is actively used as model organisms in genetic experiments. Since ancient times, yeast has been used in fermentation processes. Metabolism differs among different representatives. Therefore, some yeasts are more suitable for a particular process than others.
For example, Saccharomyces beticus, which is more resistant to high alcohol concentrations, is used to create strong wines (up to 24%). While, the yeast S. cerevisiae is able to produce lower concentrations of ethanol. According to the areas of their application, yeasts are classified into feed, bakers, brewers, spirits, and wines.
Pathogenic microorganisms
Disease-causing or pathogenic microorganisms are found everywhere. Along with well-known viruses: influenza, hepatitis, measles, HIV, etc., dangerous microorganisms are rickettsia, as well as streptococci and staphylococci, which cause blood poisoning. Among rod-shaped bacteria there are many pathogens. For example, diphtheria, tuberculosis, typhoid fever (Fig. 10). Many representatives of microorganisms dangerous to humans are found among protozoa, in particular malarial plasmodium, toxoplasma, leishmania, lamblia, trichomonas, and pathogenic amoebas.
Rice. 10. Photo of the bacterium Bacillus anthracis, which causes anthrax.
Many actinomycetes are not dangerous to humans and animals. However, many pathogenic representatives are found among mycobacteria that cause tuberculosis and leprosy. Some actinomycetes initiate a disease such as actinomycosis, accompanied by the formation of granulomas and sometimes an increase in body temperature. Certain types of mold fungi are capable of producing substances toxic to humans - mycotoxins. For example, some representatives of the genus Aspergillus, Fusarium. Pathogenic fungi cause a group of diseases called mycoses. Thus, candidiasis or, simply put, thrush is caused by yeast-like fungi (Fig. 11). They are always present in the human body, but are activated only when the immune system is weakened.
Rice. 11. Candida fungus is the causative agent of thrush.
Fungi can cause a variety of skin lesions, in particular all kinds of lichen, except for herpes zoster, which is caused by a virus. Malassezia yeast, permanent inhabitants of human skin, can cause a decrease in the activity of the immune system. Don't immediately rush to wash your hands. Yeasts and opportunistic bacteria, in good health, perform important function, prevent the development of pathogenic microorganisms.
Viruses as an object of microbiology
Viruses are the most primitive organisms on earth. In a free state, no metabolic processes occur in them. Only when they enter a host cell do viruses begin to multiply. In all living organisms, the carrier of genetic material is deoxyribonucleic acid (DNA). Only among viruses are there representatives with a genetic sequence such as ribonucleic acid (RNA).
Viruses are often not classified as truly living organisms.
The morphology of viruses is very diverse (Fig. 12). Typically, their diametrical sizes range from 20-300 nm.
Rice. 12. Diversity of viral particles.
Some representatives reach a length of 1-1.5 microns. The structure of the virus consists of surrounding the genetic material with a special protein frame (capsid), characterized by a variety of shapes (helical, icosahedral, spherical). Some viruses also have an envelope on top formed from the host cell membrane (supercapsid). For example, (Fig. 13) is known as the causative agent of a disease called (AIDS). It contains RNA as genetic material and affects a certain type of immune system cell (helper T-lymphocytes).
Rice. 13. Structure of the human immunodeficiency virus.
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Despite their microscopic size and external simplicity of structure, bacteria are complex organisms in their structure. Their fine structure was established using electron microscopy and microchemical studies (Fig. 4).
By modern ideas The body of bacteria is built like plant cells and consists of a shell, internal contents (cytoplasm) and a nucleus.
The bacteria shell is thin and colorless. It determines the preservation by bacteria of the relative constancy of forms and the surface properties of the bacterial cell - surface tension, electric charge, osmotic state.
The shell has three layers: 1) inner - cytoplasmic membrane, 2) middle - cell wall and 3) outer - mucous. They protect the cell from harmful factors environment.
The cytoplasmic membrane lies beneath the cell wall and makes up the outer layer of the cytoplasm. Its thickness is 50-75 A. Various enzymes are located on its surface. Some of them take part in protein synthesis, others in the process of respiration.
The cell wall has a size of 100-200A. It is quite dense, gives bacteria a certain shape and has a selective ability for environmental substances and metabolic products of the cell itself. The cell wall of some gram-negative bacteria is characterized by a multilayer structure and consists of macromolecules of carbohydrates, lipoids and proteins. The same multilayer structure is also characteristic of gram-positive bacteria, in which the deep layer bordering the cytoplasmic membrane consists of proteins, and magnesium ribonucleate is located more superficially.
The presence of a cell wall in bacteria can be proven by the phenomena of plasmolysis or plasmoptysis a. The essence of plasmolysis is as follows.
Rice. 4. Schematic representation of the structure of a bacterial cell.
If bacteria are placed in a hypertonic sugar solution or table salt, then the cytoplasm becomes dehydrated, wrinkles and, together with the cytoplasmic membrane, moves away from the cell wall, which becomes clearly visible in both stained and unstained preparations. When the concentration ratio is reversed, i.e., when the bacteria are in hypotonic solutions or distilled water, the cells swell to extreme limits, rupture, and the cell wall is clearly visible in a colored or uncolored state (plasmoptysis).
Capsule. The cell wall is covered on the outside with a layer of mucus. In some bacteria, this mucous layer can reach a considerable thickness and then a kind of case - a capsule - is formed around the cell. The diameter of the capsule is sometimes many times greater than the size of the microbial body itself. The capsules are clearly visible with a special staining method according to Hins; a colored microbe stands out in the dark foyer of the preparation, surrounded by a rim of colorless mucus (see Fig. 19). Pathogenic bacteria (pneumococcus, anthrax bacilli, etc.) form a capsule only in the body of a sick person or animal. For some microorganisms (a group of capsular bacteria - pneumonia, ozena and rhinoscleroma bacteria), the presence of a capsule is their permanent feature, i.e. they form a capsule both in the body and outside it. Capsules for pathogenic bacteria are a useful formation that protects them from harmful effects macroorganism (phagocytosis, antibody action). This can be seen in the example of pneumococci and anthrax bacilli, which form a capsule in the body, and when released into the external environment, they lose it.
The chemical composition of the capsules is heterogeneous. In some bacteria, the mucous layer consists of high molecular weight polysaccharides and glucoproteins (for example, in pneumococcus), in others it is made of proteins (in anthrax bacilli).
The cytoplasm of microbes is a mixture of colloids and has a liquid consistency. It consists of water, proteins, carbohydrates, lipids, nucleic acids and other substances. Electron microscopy reveals small grains (granules) 100-200 A in diameter containing dehydrases, various cytochromes, and ribonucleic acid in the cytoplasm. Granules rich in RNA are called ribosomes, where proteins and enzymes are synthesized. Granular structures have biochemical activity. Often the cytoplasm contains various inclusions - sulfur, fat droplets (lipoprotein bodies), granulosa, glycogen, pigment grains. Of the protein substances, there are grains of volutin, which are a nucleoprotein and contain a significant amount of metaphosphates and other phosphorus compounds. These grains are especially well developed in Spirillum volutans and therefore they are called volutinous. Volutin grains are distinguished by metachromosia, i.e. they are stained darker than the cytoplasm or selectively take on a different color than the cytoplasm. Hence another name for them - metachromatic grains. The presence of volutin grains is one of the diagnostic signs when identifying microorganisms (for example, diphtheria bacillus). Lipoprotein bodies and volutin granules are used by bacteria when there is a lack of nutrients in the environment. Some microbes (E. coli, mycobacteria, salmonella, etc.) contain round or ellipsoidal grains of various sizes and containing phospholipids (mitochondria). Mitochondria have a system of redox enzymes and take part in respiratory processes and anaerobic fermentation of substances. In the cytoplasm there are also vacuoles (6-10 or more), consisting of various substances dissolved in water. The vacuoles are surrounded by a membrane - the tonoplast. The role of vacuoles in the life of bacteria has not been clarified. At the periphery of the cytoplasm, bodies are found - mesosomes, which play a large role in the formation of the cell wall and division.
In some bacteria, it is possible to separate the cytoplasm from the cell wall by exposure to penicillin, lysozyme or bacteriophage. Bacteria lacking a cell wall are called protoplasts. They are spherical bodies that, under appropriate osmotic conditions, have the ability to grow, respire, synthesize enzymes, proteins, nucleic acids, and sporulate. Unlike bacteria, protoplasts do not reproduce and are very sensitive to changes in osmotic pressure, mechanical stress and aeration, and the action of antibacterial substances.
With partial loss of the cell wall, the formation of flippers is observed by the sphere. Unlike protoplasts, spheroplasts retain the ability to adsorb bacteriophages on themselves.
Core. In bacteria (nucleoid), the nucleus is the most important organelle of the cell, taking part in vital activity, division and sporulation. The compact nucleus is found in large microorganisms (yeast and yeast-like fungi) and is clearly visible in a conventional optical microscope using special staining methods. In bacteria, the nucleus is small and becomes visible only in an electron microscope. Using an electron microscope, it was proved not only that bacteria have a nucleus in the form of a dense chromatin strand, but also changes in its shape, size and degree of compactness were revealed. It has also been proven that more mature and older cell forms have more compact nuclei. The presence of chromatin substance in bacteria has also been proven using a microchemical nucleic reaction to thymonuclepnic acid, which is an essential part of chromatin.
The nuclear structure of a microbial cell consists of deoxyribonucleic acid (DNA) and performs the main functions of the nucleus - self-duplication, control of the synthesis of specific proteins, transmission of hereditary characteristics to offspring.
Controversy. Many bacteria are characterized by the formation of spores. Unlike vegetative forms, which represent the active stage of a bacterial cell, when all physiological functions are vigorously manifested, spores are a dormant form of a microorganism. Sporulation usually occurs when external environment unfavorable conditions are created for bacteria: accumulation harmful products exchange, appropriate temperature, action sun rays, aging of a microorganism culture on a nutrient medium, etc. Sporulation is not observed in the human and animal body.
The fact that sporulation occurs under conditions unfavorable for the life of bacilli indicates that the spore is an adaptation that serves not for cell reproduction, but for the preservation of the species. This is also confirmed by the fact that one bacterial cell is capable of forming only one spore.
The spores of different bacilli differ from each other in shape, size and location in the cell. For example, the spore of the tetanus bacillus is round, located at the end (terminally) and its diameter is greater than the diameter of the body of the bacilli (see Fig. 17), and the spores of the anthrax causative agent are located centrally, have an oval shape and their diameter does not exceed the diameter of the microbial cell (see Fig. 97). The subterminal (closer to one end) location of the spore is characteristic of the causative agents of gas gangrene and botulism.
The process of sporulation occurs in bacteria in relatively short term(within approximately 24 hours). The essence of the process is that the contents of the microbial cell are gradually compacted and, concentrating in one place, are covered with a dense shell. Gradually, the vegetative part of the bacterial cell dies. The spore shell consists of two layers: outer and inner. Outer layer It is difficult to permeate water and various substances; the bacterial cell wall is formed from the internal spore during germination. Once in conditions favorable for development (presence of the necessary nutrient medium, humidity, temperature), the spores quickly germinate and turn into vegetative forms. In this case, the spore shell first swells and then dissolves or breaks due to the action of enzymes, and a bacterial seedling emerges from it, dressed in a thin shell and then turning into a vegetative cell.
The process of spore germination usually takes 4-5 hours. The low permeability of the spore shell is the reason that with conventional staining methods they remain colorless. To paint them, you need to apply vigorous pressure, leading to loosening of the shell. The chemical composition of the spore is characterized by a small amount of free water and a high content of lipoids, which largely determines the resistance of the spores to a number of unfavorable physical and chemical factors. For example, a temperature of 100° does not kill spores; To destroy them, exposure to higher temperatures is necessary. Low temperatures The spores can withstand drying for many years. Sporulation is characteristic of bacilli; among coccal forms it is rare (in urinary sarcipa, enterococcus).
Flagella. All bacteria are divided into mobile and immobile. Among the mobile ones there are crawling and swimming ones. Crawling bacteria move slowly (crawl) along a supporting surface as a result of wave-like contractions of their body (for example, Myxobacterium). Floating bacteria move freely in a liquid environment using flagella. Flagella are the thinnest, elastic convoluted filaments, starting from the basal granules in the cytoplasm and extending out through the membrane. Their diameter is measured in hundredths of a micron (0.02-0.05 microns), and their length is sometimes many times greater than the size of the body of the microbe itself. Flagella are fragile formations and are quickly detached from cells when shaken, other mechanical impacts and treatment with various chemicals. The number and location of flagella vary among different microorganisms. In this regard, motile bacteria are divided into several types (Fig. 5).
- Monotrichs are bacteria with one flagellum at the end (for example, Vibrio cholerae).
Rice. 5. Flagella in bacteria.
- Lophotrichs are bacteria with a bundle of flagella at one of the cell poles (for example, Bact. faecalis alcaligenes).
- Peritrichous bacteria are bacteria with flagella located around the entire body of the microbe (for example, bacteria of the typhoid-paratyphoid group).
The flagella are so thin that they cannot be seen during conventional microscopy of preparations with living or killed microbes. You can detect flagella as follows:
a) when microscopying in a darkened field of view with a special condenser that creates strong lateral illumination, in a prepared “hanging” or “crushed” drop (see page 53).
b) in a preparation with mobile bacteria after special treatment with a mordant, for example a tannin solution. This results in swelling of the flagella and an increase in their size (diameter). After staining with the appropriate dye of such a preparation, the flagella become visible under normal microscopy.
Rice. 6. Flagella under an electron microscope.
Rice. 7. Fringes (cilia) under an electron microscope.
With electron microscopy (Fig. 6), you can not only clearly see the flagella, but also become familiar with the details of their structure. They have a spiral shape and helical structure. The axial filament of the flagella consists of two intertwined filaments covered with a sheath. The flagella contain a special protein - flagellin, the contraction of which determines the intensity and nature of the movement of bacteria. The fastest and most linear movements are made by monotrichs and lophotrichs. For example, Vibrio cholerae moves at a speed of 30 microns per second, which is 30 times its size. Peritrichs are characterized by less energetic and more erratic movements. The nature of the movement of bacteria also depends on the age and properties of the culture (young cells move more energetically), temperature ( optimal temperature promotes movement), availability chemical substances and other factors. Under unfavorable living conditions, bacteria can lose their flagella, remaining in this state for a number of subsequent generations. The body surface of a number of bacteria is covered with numerous fimbriae - cilia, villi (Fig. 7). The role of fimbriae in the life of bacteria has not yet been clarified. It is believed that microbes attach to the surface of certain substrates with fimbriae and possibly take part in the nutrition of the bacterial cell.
SPIROCHETES
The group of spirochetes includes microorganisms that have a convoluted shape and, in their biological properties, occupy an intermediate position between bacteria and protozoa. The body of the spirochete consists of cytoplasm, which is arranged in a spiral around the central axial filament, forming primary curls. The bends of the axial thread form secondary curls, the number and shape of which are characteristic of various types spirochete. They do not have a cell membrane and a compact, formed nucleus. Some spirochetes have long, thin cords arranged in bundles at the ends of the body and visible in an electron microscope. Spirochetes are mobile, which is determined by the contractility of their cytoplasm and the elasticity of the axial filament. There are rotational, flexion and forward movements. Spirochetes reproduce by simple transverse division and are difficult to cultivate on nutrient media. Pathogenic spirochetes include the causative agent of syphilis - spirochete pallidum, spirochete of relapsing fever (Fig. 8 inset) and leptospira, which causes icteric and anicteric leptospirosis in humans.
Rice. 8. Spirochetes of relapsing fever in the blood.
RICKETSIA
- Type a, or coccoid rickettsia, in the form of very small ovoids or ellipsoids (“coccobacilli”) with a diameter of about 0.5 microns, often forming diploforms or chains.
- Type B, or rod-shaped rickettsia, are in the form of delicate short rods with a diameter of 1 to 1.5 microns.
- Type c, or bacillary (long rod-shaped) rickettsia, in the form of elongated and usually curved thin rods measuring 3-4 microns.
- Type d, or filamentous rickettsia, in the form of long, often giant, intricately curved filaments, reminiscent of large spirilla measuring 10-20-40 microns and larger.
Pathogenic rickettsia cause diseases in humans and animals; a typical representative of rickettsia is Rickettsia provaceki, the causative agent of typhus.
A number of infectious diseases of humans, animals and plants are caused by such microbes, the sizes of which are calculated in millimicrons. TO viral diseases include: smallpox, rabies, polio, influenza, measles, etc. Currently, there are more than 500 viruses that infect humans and animals.
The structure of viruses can only be studied at high magnifications of an electron microscope. Their shape is round, rod-shaped, cuboidal and thread-like. The sizes of viruses range from 10 to 350 mmk. The largest viruses are called elementary bodies. If their value is greater than 0.2 μm, then they are visible in a conventional optical microscope using an immersion system. Elementary bodies in smallpox (their size is about 0.2 microns) can be detected in an optical microscope only with a special processing method (Morozov method).
According to electron microscopic studies, central part The virus is a nucleic acid - a nucleoid, a formation resembling a nucleus, enclosed in a shell - a capsid, consisting of individual protein subunits - capsomes. More complex viruses, in addition to the capsid, have an outer shell, which includes carbohydrates and lipids. During some viral infections, special inclusions are found in the cells of the affected tissues, the structure and location of which are always characteristic of a particular virus. So, with rabies in the cytoplasm nerve cells In the brain, inclusions are found in the form of Babesh-Negri bodies, in the cytoplasm of epithelial cells in smallpox - Guarnieri bodies, etc. The size of intracellular inclusions ranges from 0.25 to 20-30 microns, their shape is round and oval, pear-shaped, spindle-shaped or crescent-shaped . They are located in the cytoplasm or nuclei of cells, and sometimes in the cytoplasm and nucleus. The nature of intracellular inclusions is not precisely understood. They are believed to be reactive formations of cells in which viral particles (virus colonies) accumulate. To detect intracellular formations, which is of great diagnostic importance, smears or tissue sections are prepared and stained special methods(according to Morozov, Turevich, or Muromtsev).
Microorganisms are represented by precellular forms (viruses - kingdom Vira) and cellular forms (bacteria, archaebacteria, fungi and protozoa). In a new way highest level in the classification hierarchy among cellular life forms, 3 domains (or “empires”) are distinguished: “Bacteria”, “Archaea”, “Eukarya”:
domain “Bacteria” - prokaryotes, represented by real bacteria (eubacteria);
domain “Archaea” - prokaryotes, represented by archaebacteria;
domain "Eukarya" - eukaryotes, whose cells have a nucleus with a nuclear envelope and nucleolus, and the cytoplasm consists of highly organized organelles - mitochondria, Golgi apparatus, etc. The domain "Eukarya" includes the kingdom Fungi (fungi); animal kingdom Animalia (includes protozoa - subkingdom Protozoa); plant kingdom Plantae.
Domains include kingdoms, phyla, classes, orders, families, genera, and species. One of the main taxonomic categories is species. A species is a collection of individuals united by similar properties, but differing from other representatives of the genus.
A set of homogeneous microorganisms isolated on a nutrient medium, characterized by similar morphological, tinctorial, cultural, biochemical and antigenic properties, is called a pure culture.
Strain is a pure culture of microorganisms isolated from a specific source and different from other representatives of the species. A strain is a narrower concept than a species or subspecies. Close to the concept of a strain is the concept of a clone. A clone is a collection of descendants grown from a single microbial cell.
1.3. Structure and classification of bacteria (prokaryotes).
The decision of the International Code for bacteria recommended the following taxonomic categories: class, division, order, family, genus, species. The species name corresponds to binary nomenclature, i.e. consists of two words. For example, the causative agent of diphtheria is written as Corynebacterium diphtheriae, the causative agent of meningitis is Neisseria meningitides, and the causative agent of tuberculosis is Mycobacterium tuberculosis. The first word is the name of the genus and is written with a capital letter, the second word denotes the species and is written with a lowercase letter.
Bacteria are prokaryotes, i.e. prenuclear microorganisms, since they have a primitive nucleus without a shell, nucleolus, histones, and the cytoplasm lacks highly organized organelles (mitochondria, Golgi apparatus, lysosomes, etc.).
According to the second edition (2001) of Bergey's Manual of Systematic Bateriology, bacteria are divided into 2 domains: "Bacteria" and "Archaea". The following bacteria can be distinguished in the “Bacteria” domain: 1) bacteria with a thin cell wall – gram-negative; 2) bacteria with a thick cell wall - gram-positive; 3) bacteria without a cell wall (mycoplasma). Archaebacteria do not contain peptidoglycan in their cell wall. The term “archaebacteria” appeared in 1977. This is one of the ancient forms of life, as indicated by the prefix “archaean”. There are no pathogens of infectious diseases among them.
The Bacteria domain includes 22 types, of which the following are of medical importance:
Phylum Proteobacteria
Class Alphaproteobacteria. Genera: Rickettsia, Orientia, Ehrlichia, Bartonella, Brucella.
Class Betaproteobacteria. Genera: Burkholderia, Alcaligenes, Bordetella, Neisseria, Kingella, Spirillum.
Class Gammaproteobacteria. Genera: Francisella, Legionella, Coxiella, Pseudomonas, Moraxella, Acinetobacter, Vibrio, Enterobacter, Callimatobacterium, Citrobacter, Edwardsiella, Erwinia, Escherichia, Hafnia, Klebsiella, Morganella, Proteus, Providensia, Salmonella, Serracia, Shigella, Yersinia, Pasteurella.
Class Deltaproteobacteria. Genus Bilophila.
Class Epsilonproteobacteria. Genera: Campylobacter, Helicobacter, Wolinella.
Phylum Firmicutes (mainly gram-positive)
Class Clostridia. Genera: Clostridium, Sarcina, Peptostreptococcus, Eubacterium, Peptococcus, Veillonella.
Class Mollicutes. Genera: Mycoplasma, Ureaplasma.
Class Bacilli. Genera: Bacillus, Sporosarcina, Listeria, Staphylococcus, Gemella, Lactobacillus, Pediococcus, Aerococcus, Leuconostoc, Streptococcus, Lactococcus.
Phylum Actinobacteria
Class Actinobacteria. Genera: Actinomyces, Arcanodacterium, Mobiluncus, Micrococcus, Rothia, Stomatococcus, Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, Bifidobacterium, Gardnerella.
Phylum Clamydiae
Class Clamydiae. Genera: Clamydia, Clamidophila.
Type Spirochaetes
Class Spirochaetes. Genera: Spirochaeta, Borrelia, Treponema, Leptospira.
Phylum Bacteroidetes
Class Bacteroidetes. Genera: Bacteroides, Porphyromonas, Prevotella.
Class Flavobacteria. Genus: Flavobacterium.
Shapes of bacteria. There are several main forms of bacteria - coccoid, rod-shaped, convoluted and branching, filamentous.
Spherical forms, or cocci, are spherical bacteria, which, according to their relative positions, are divided into micrococci, diplococci, streptococci, tetracocci, sarcina and staphylococci.
Micrococci are characterized by single, paired or random arrangement of cells. They are saprophytes, inhabitants of water and air.
Diplococci divide in one plane and form paired cocci, connected by two individuals. Diplococci include: meningococcus, gonococcus, pneumococcus.
Streptococci are divided in one plane and are arranged in chains of varying lengths. Streptococci pathogenic to humans cause purulent-inflammatory diseases.
Tetracocci are arranged in fours, because divided in two mutually perpendicular planes. Does not cause human disease.
Sarcinas are divided in three mutually perpendicular planes and look like bales of 8, 16 or more cells. They are often found in the air and are not causative agents of infectious diseases.
Staphylococci - cluster-shaped cocci, dividing in different planes, cause purulent-inflammatory diseases in humans.
Rod-shaped bacteria are divided into bacteria, bacilli and clostridia. Bacteria include rod-shaped microorganisms that usually do not form spores (intestinal, typhoid, dysentery, diphtheria, tuberculosis, etc.). Bacilli (lat. vacillus - rod) and clostridia (lat. closter - spindle) include microbes that form spores (anthrax, tetanus bacilli, etc.). In shape, rod-shaped bacteria are short (tularemia, pertussis, brucellosis), long (anthrax), with rounded ends (most rods), with pointed ends (fusobacteria), with club-shaped thickenings at the ends (diphtheria).
Twisted shapes of bacteria. This group of bacteria includes vibrios, spirillum, campylobacteria, helicobacteria, and spirochetes.
Vibrios are cells whose bend is equal to ¼ of a spiral, having the shape of a comma. A pathogenic representative is Vibrio cholerae, the causative agent of cholera.
Spirilla are convoluted forms of bacteria that have bends with one or more turns of the spiral. Of the pathogenic species, one known species is Spirillum minor - the causative agent of sodoku - a disease transmitted through the bite of rats and other rodents.
Campylobacter, Helicobacter - have curves like the wing of a flying seagull. Campylobacters are the causative agents of zoonotic bacterial infections primarily affecting the digestive tract. Helicobacter bacteria are opportunistic microorganisms that can cause chronic damage to the gastric and duodenal mucosa.
Spirochetes are represented by 3 genera pathogenic for humans: Treponema, Borrelia, Leptospira.
Treponemas have the appearance of thin, corkscrew-twisted threads with 8-12 evenly small curls. The pathogenic representative is T. Pallidum, the causative agent of syphilis.
Borrelia, unlike Treponema, are longer and have 3-8 large curls. These include the causative agent of tick-borne borreliosis or Lyme disease – B. Burgdorferi.
Leptospira have shallow and frequent curls - in the form of a twisted rope. The ends of these spirochetes are curved like hooks with thickenings at the ends. The pathogenic representative L interrogans causes leptospirosis.
Filamentous (sulfur bacteria, iron bacteria - inhabitants of water bodies; actinomycetes - branching, filamentous or rod-shaped gram-positive bacteria, like fungi, form mycelium). These include bacteria of the genera Corynebacterium, Mycobacterium, and Nocardia). Pathogenic actinomycetes cause actinomycosis, nocardia - nocardiosis, mycobacteria - tuberculosis and leprosy, corynebacteria - diphtheria.
Chlamydia are obligate intracellular coccoid gram-negative bacteria. In humans, chlamydia causes damage to the eyes (trachoma, conjunctivitis), urogenital tract, and lungs.
Mycoplasmas are small bacteria; due to the lack of a cell wall, they have a variety of shapes: coccoid, filamentous, flask-shaped.
The bacterial cell is surrounded by a membrane consisting of a cell wall and a cytoplasmic membrane. Under the shell is protoplasm, consisting of cytoplasm with inclusions and a nucleus called the nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili. Some bacteria are capable of forming spores under unfavorable conditions.
The cell wall is a strong, elastic structure that gives the bacterium a certain shape, is involved in the process of cell division and transport of metabolites, and has receptors for bacteriophages. The main component of the cell wall of Gram-positive bacteria is multilayer peptidoglycan (murein) to which teichoic acids are covalently linked. In gram-negative bacteria, the main component is a lipid bilayer. The inner layer of the outer membrane is composed of phospholipids, and the outer layer contains lipopolysaccharide. The ability of Gram-positive bacteria to retain gentian violet in combination with iodine when stained according to Gram (blue-violet color of bacteria) is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, subsequent treatment of a bacterial smear with alcohol causes a narrowing of the pores in the peptidoglycan and thereby retains the dye in the cell wall. Gram-negative bacteria, after exposure to alcohol, lose their dye, which is due to a smaller amount of peptidoglycan in the cell wall; they are discolored by alcohol and, when treated with fuchsin, acquire a red color. When the synthesis of the bacterial cell wall is disrupted under the influence of lysozyme, penicillin, etc., cells with an altered spherical shape are formed - protoplasts - bacteria completely devoid of a cell wall. Bacteria with a partially preserved cell wall are spheroplasts. Bacteria of the sphero- or protoplast type, which have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to reproduce, are called L-forms (from the name of the D. Lister Institute, where they were first studied). L-forms can also arise as a result of mutations. L-forms can be produced by many pathogens of infectious diseases.
The cytoplasmic membrane is a dynamic structure with mobile components, so it is thought of as a mobile fluid structure. It surrounds the outer part of the bacterial cytoplasm and is involved in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell. With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complex twisted membrane structures, called mesosomes.
The cytoplasm occupies the main volume of the bacterial cell and consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes, responsible for protein synthesis. Bacterial ribosomes have a size of about 20 nm and a sedimentation coefficient of 70S, in contrast to eukaryotic cells (80S). Therefore, some antibiotics, by binding to bacterial ribosomes, suppress bacterial protein synthesis without affecting protein synthesis in eukaryotic cells.
The cytoplasm contains various inclusions: volutin granules, lipoprotein bodies, glycogen, granulosa, pigment accumulations, sulfur, calcium. The biological significance of volutin granules and lipoprotein inclusions is that they serve as reserve nutritional material and are used by bacteria when there is a lack of nutrients. The characteristic arrangement of volutin granules is revealed in diphtheria bacilli when stained by Neisser.
Nucleiod is the equivalent of a nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA closed in a ring. The nucleus of bacteria does not have a nuclear envelope, nucleolus and basic proteins (histones). A bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring.
Capsule, microcapsule, mucus. When pathogenic bacteria enter a macroorganism, they can form capsules more than 0.2 microns thick. The capsule is a thick mucous layer around the cell wall. It is detected using special methods of staining a smear according to Burri-Gins (anthrax bacillus, Klebsiella, pneumococcus). The capsule consists of polysaccharides or polypeptides. The capsule prevents the phagocytosis of bacteria; it is an antigen: antibodies against the capsule cause its enlargement (capsule swelling reaction). Many bacteria form a microcapsule - a mucous formation less than 0.2 µm, detectable only by electron microscopy. Mucus should be distinguished from the capsule - mucoid exopolysaccharides that do not have clear external boundaries. Mucoid exopolysaccharides are characteristic of mucoid strains of Pseudomonas aeruginosa, often found in the sputum of patients with cystic fibrosis. The capsule and mucus protect bacteria from damage and drying out.
Controversy. When exposed to unfavorable environmental conditions (drying, nutrient deficiency), bacteria form spores. Sporulation occurs in the external environment (soil, nutrient media) and is not observed in human and animal tissues. Finding favorable conditions, the spores germinate and turn back into vegetative forms. Spore-forming bacteria of the genus Bacillus, in which the size of the spore does not exceed the diameter of the cell, are called bacilli. Spore-forming bacteria in which the spore size exceeds the diameter of the cell, causing them to take the shape of a spindle, are called clostridia, for example bacteria of the genus Clostridium. The spores are acid-resistant, therefore, according to Ozheshko’s method, they are painted red, and the vegetative cell is painted blue. The shape of the spores can be oval, spherical; location in the cell is terminal, i.e. at the end of the stick (in the causative agent of tetanus), subterminal - closer to the end of the stick (in the causative agents of botulism, gas gangrene) and central (in the anthrax bacillus).
Bacterial flagella determine the motility of the bacterial cell. These are thin filaments originating from the cytoplasmic membrane and are longer than the cell itself. Flagella consist of a protein - flagellin, which is an antigen - the so-called H - antigen. The number of flagella in bacteria of various species varies from one (monotrich) in Vibrio cholerae to tens and hundreds of flagella extending along the perimeter of the bacterium (peretrich) in Escherichia coli, Proteus, etc. Lophotrichs have a bundle of flagella at one end of the cell. Amphitrichy has one flagellum or a bundle of flagella at opposite ends of the cell. Flagella are detected using electron microscopy.
Villi or pili (fimbriae) are thread-like formations, thinner and shorter than flagella. The pili extend from the cell surface and are composed of the protein pilin. There are pili responsible for adhesion, i.e. for the attachment of bacteria to the affected cell, as well as pili, responsible for nutrition, water-salt metabolism and sexual (F- pili), or conjugation.
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Bacteria are prokaryotes (Fig. 1.2) and differ significantly from plant and animal cells (eukaryotes). They belong to single-celled organisms and consist of a cell wall, cytoplasmic membrane, cytoplasm, nucleoid (obligatory components of a bacterial cell). Some bacteria may have flagella, capsules, spores ( optional components bacterial cell).
Rice. 1.2. Combined schematic representation of a prokaryotic (bacterial) cell with flagella.
1 - polyhydroxybutyric acid granules; 2 - fat droplets; 3 - sulfur inclusions; 4 - tubular thylakoids; 5 - lamellar thylakoids; 6 - bubbles; 7 - chromatophores; 8 - nucleus (nucleoid); 9 - ribosomes; 10 - cytoplasm; 11 - basal body; 12 - flagella; 13 - capsule; 14 - cell wall; 15 - cytoplasmic membrane; 16 - mesosoma; 17 - gas vacuoles; 18 - lamellar structures; 19 - polysaccharide granules; 20 - polyphosphate granules
Cell wall
The cell wall is the outer structure of bacteria, 30-35 nm thick, the main component of which is peptidoglycan (murein). Peptidoglycan is a structural polymer consisting of alternating subunits of N-acetylglucosamine and N-acetylmuramic acid connected by glycosidic bonds (Fig.1.3).
Rice. 1.3. Schematic representation of the single-layer structure of peptidoglycan
Parallel polysaccharide (glycan) chains are connected to each other by cross peptide bridges (Fig. 1.4).
Rice. 1.4. Detailed structure of peptidoglycan structure Light and black short arrows indicate bonds cleaved by lysozyme (muramidase) and specific muroendopeptidase, respectively
The polysaccharide framework is easily destroyed by lysozyme, an antibiotic of animal origin. Peptide bonds are targets for penicillin, which inhibits their synthesis and prevents cell wall formation. The quantitative content of peptidoglycan affects the ability of bacteria to Gram stain. Bacteria with a significant thickness of the murein layer (90-95%) are persistently stained blue-violet with gentian violet and are called gram-positive bacteria.
Gram-negative bacteria with a thin layer of peptidoglycan (5-10%) in the cell wall lose their gentian violet color after exposure to alcohol and are additionally stained magenta pink. The cell walls of gram-positive and gram-negative prokaryotes differ sharply in both chemical composition(Table 1.1), and by ultrastructure (Fig. 1.5).
Rice. 1.5. Schematic representation of the cell wall in gram-positive (a) and gram-negative (b) prokaryotes: 1 - cytoplasmic membrane; 2 - peptidoglycan; 3 - periplasmic space; 4 - outer membrane; 5 - DNA
In addition to peptidoglycan, the cell wall of gram-positive bacteria contains teichoic acids (polyphosphate compounds), and in smaller quantities - lipids, polysaccharides, and proteins.
Table 1.1. Chemical composition of cell walls of gram-positive and gram-negative prokaryotes 
Gram-negative prokaryotes have an outer membrane, which includes lipids (22%), proteins, polysaccharides, and lipoproteins.
The cell wall in bacteria mainly performs form-building and protective functions, provides rigidity, forms a capsule, determines the ability of cells to adsorb phages.
All bacteria, depending on their relationship to Gram staining, are divided into gram-positive and gram-negative.
Gram staining technique
1. Place filter paper on the smear and pour in a carbolic solution of gentian violet for 1-2 minutes.2. Remove the paper, drain the dye and, without washing the smear with water, pour in Lugol’s solution for 1 minute.
3. Drain Lugol’s solution and decolorize the preparation in 96% alcohol for 30 seconds.
4. Rinse with water.
5. Paint for 1-2 minutes with an aqueous solution of fuchsin.
6. Wash with water and dry.
As a result of staining, gram-positive bacteria are stained purple, gram-negative bacteria are stained red.
Reason different attitude bacteria to Gram staining is explained by the fact that after treatment with Lugol's solution, an alcohol-insoluble complex of iodine with gentian violet is formed. This complex in gram-positive bacteria, due to the weak permeability of their walls, cannot diffuse, while in gram-negative bacteria it is easily removed by washing them with ethanol and then with water.
Bacteria that are completely devoid of a cell wall are called protoplasts; they are spherical in shape and have the ability to divide, respire, and synthesize proteins, nucleic acids, and enzymes. Protoplasts are unstable structures, very sensitive to changes in osmotic pressure, mechanical stress and aeration, do not have the ability to synthesize the constituent parts of the cell wall, are not infected by bacterial viruses (bacteriophages) and do not have active motility.
If, under the influence of lysozyme and other factors, partial dissolution of the cell wall occurs, then the bacterial cells turn into spherical bodies, called spheroplasts.
Under the influence of certain external factors, bacteria are capable of losing their cell wall, forming L-forms (named after the D. Lister Institute, where they were first isolated); such transformation can be spontaneous (for example, in chlamydia) or induced, for example, under the influence of antibiotics. There are stable and unstable L-forms. The former are not capable of reversion, while the latter revert to their original forms after removing the causative factor.
Cytoplasmic membrane
The cytoplasm of a bacterial cell is bounded from the cell wall by a thin, semi-permeable structure 5-10 nm thick called the cytoplasmic membrane (CPM). The CPM consists of a double layer of phospholipids permeated with protein molecules (Fig. 1.6).
Fig.1.6. Structure of the plasma membrane Two layers of phospholipid molecules, with hydrophobic poles facing each other and covered with two layers of globular protein molecules.
Many enzymes and proteins involved in the transfer of nutrients, as well as enzymes and electron carriers of the final stages of biological oxidation (dehydrogenases, cytochrome system, ATPase) are associated with the CPM.
Enzymes that catalyze the synthesis of peptidoglycan, cell wall proteins, and own structures. The membrane is also the site of energy conversion during photosynthesis.
Periplasmic space
The periplasmic space (periplasm) is the zone between the cell wall and the CPM. The thickness of the periplasm is about 10 nm; the volume depends on environmental conditions and, above all, on the osmotic properties of the solution.The periplasm can include up to 20% of all water in the cell; some enzymes (phosphatases, permeases, nucleases, etc.) and transport proteins that carry the corresponding substrates are localized in it.
Cytoplasm
The contents of the cell, surrounded by the CPM, constitute the bacterial cytoplasm. That part of the cytoplasm that has a homogeneous colloidal consistency and contains soluble RNA, enzymes, substrates and metabolic products is designated as cytosol. The other part of the cytoplasm is represented by various structural elements: mesosomes, ribosomes, inclusions, nucleoid, plasmids.Ribosomes are submicroscopic ribonucleoprotein granules with a diameter of 15-20 nm. Ribosomes contain approximately 80-85% of all bacterial RNA. Prokaryotic ribosomes have a sedimentation constant of 70 S. They are built from two particles: 30 S (small subunit) and 50 S (large subunit) (Fig. 1.7).
Rice. 1.7. Ribosome (a) and its subparticles - large (b) and small (c) Ribosomes serve as the site of protein synthesis.
Cytoplasmic inclusions
Often, various inclusions are found in the cytoplasm of bacteria that are formed during life: droplets of neutral lipids, wax, sulfur, glycogen granules, β-hydroxybutyric acid (especially in the genus Bacillus). Glycogen and β-hydroxybutyric acid serve as a reserve source of energy for bacteria.Some bacteria have protein crystals in their cytoplasm that have a toxic effect on insects.
Some bacteria are capable of accumulating phosphoric acid in the form of polyphosphate granules (volutin grains, metachromatic grains). They play the role of phosphate depots and are detected in the form of dense formations in the shape of a ball or ellipse, located mainly at the poles of the cell. Usually there is one granule at each pole.
Nucleoid
Nucleoid is the nuclear apparatus of bacteria. Represented by a DNA molecule corresponding to one chromosome. It is closed, located in a nuclear vacuole, and does not have a membrane limiting it from the cytoplasm.A small amount of RNA and RNA polymerase are associated with DNA. DNA is coiled around a central core made of RNA and forms a highly ordered compact structure. The chromosomes of most prokaryotes have molecular weight within 1-3 x109, sedimentation constant 1300-2000 S. A DNA molecule includes 1.6x10 nucleotide pairs. Differences in the genetic apparatus of prokaryotic and eukaryotic cells determine its name: in the former it is a nucleoid (a formation similar to a nucleus), in contrast to the nucleus in the latter.
The nucleoid of bacteria contains the basic hereditary information, which is realized in the synthesis of specific protein molecules. Systems of replication, repair, transcription and translation are associated with the DNA of a bacterial cell.
The nucleoid in a prokaryotic cell can be detected in stained preparations using a light or phase contrast microscope.
In many bacteria, extrachromosomal genetic elements - plasmids - are found in the cytoplasm. They are double-stranded DNA closed in rings, consisting of 1500-40000 nucleotide pairs and containing up to 100 genes.
Capsule
Capsule is a mucous layer of the bacterial cell wall, consisting of polysaccharides or polypeptides. Most bacteria can form a microcapsule (less than 0.2 microns thick).Flagella
Flagella act as an organ of movement, allowing bacteria to move at a speed of 20-60 µm/sec. Bacteria may have one or more flagella, located over the entire surface of the body or collected in bundles at one pole or at different poles. The thickness of the flagella is on average 10-30 nm, and the length reaches 10-20 µm.The basis of the flagellum is a long spiral thread (fibril), which at the surface of the cell wall turns into a thickened curved structure - a hook and is attached to the basal granule, embedded in the cell wall and the CPM (Fig. 1.8).
Rice. 1.8. Schematic model of the basal end of the E. coli flagellum, based on electron micrographs of the isolated organelle
Basal granules have a diameter of about 40 nm and consist of several rings (one pair in gram-positive bacteria, four in gram-negative prokaryotes). Removal of the peptidoglycan layer of the cell wall leads to the loss of the bacteria's ability to move, although the flagella remain intact.
Flagella consist almost entirely of the protein flagellin, with some carbohydrates and RNA.
Controversy
Some bacteria are capable of forming spores at the end of the period of active growth. This is preceded by depletion of the environment nutrients, changes in its pH, accumulation of toxic metabolic products. As a rule, one bacterial cell forms one spore - the localization of the spores is different (central, terminal, subterminal - Fig. 1.9).
Rice. 1.9. Typical forms of spore-forming cells.
If the size of the spores does not exceed the transverse size of the rod-shaped bacterium, then the latter is called a bacillus. When the diameter of the spore is larger, the bacteria have a spindle shape and are called clostridia.
In terms of chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. The disputes contain less water and more lipids.
In the spore state, microorganisms are metabolically inactive and can withstand high temperature(140-150°C) and exposure to chemical disinfectants, persist for a long time in the environment.
Once in the nutrient medium, the spores germinate into vegetative cells. The process of spore germination includes three stages: activation, initial stage and growth stage. Activating agents that disrupt the state of dormancy include elevated temperature, acidic reaction of the environment, mechanical damage, etc. The spore begins to absorb water and, with the help of hydrolytic enzymes, destroys many of its own structural components. After the destruction of the outer layers, a period of formation of a vegetative cell begins with activation of biosynthesis, ending with cell division.
L.V. Timoschenko, M.V. Chubik