Bacterial cell drawing with captions. bacteria
Bacteria (grains)
The shape and structure of bacterial cells
Bacteria are the most ancient group of living organisms, having dimensions that most often do not exceed 0.5 microns. Their structure can only be seen under an electron microscope (Fig. 2.1). Bacteria do not have mitochondria, lysosomes, Golgi complex, endoplasmic reticulum. They have no plastids, no formalized nucleus, and the nuclear substance (DNA) is represented by one ring-shaped chromosome (nucleoid), located directly in the cytoplasm, but attached to the cytoplasmic membrane at one point. There are many ribosomes in the cytoplasm, in which protein synthesis proceeds intensively. Most bacteria are colorless, but some are green or purple. Bacteria are the most common organisms in nature, they are classified as prokaryotes, i.e. prenuclear organisms.
Rice. 2.1. Bacterium
The shape of bacteria is varied. Some of them look like single balls - cocci, which can pair up - diplococci, four - tetracocci, form chains - streptococci. Accumulations of cocci have the form of packages - sarcina or bunch of grapes - staphylococci. Some bacteria are elongated in the form of rods - bacilli, others are bent in the form of a comma - vibrios, or several times along the entire length - spirilla (Fig. 2.2).
Rice. 2.2. Bacteria cell shapes:
1 - cocci; 2, 3 - diplococci; 4 - streptococci; 5 - tetracocci; 6 - staphylococci; 7 - sarcins; 8, 9 - bacilli; 10 - chains of bacilli; 11 - vibrios; 12 - spirilla; 13 - flagellated, 14 - ciliated
Many bacteria have organelles of movement- one or more flagella. Bacteria that do not have flagella, but are covered with mucus on the outside, are also capable of gliding movement. Some water and soil bacteria, in particular cyanobacteria, can rise and fall by regulating the amount of gas in the gas vacuoles present in the cytoplasm.
The bacterial cell is covered with a membrane cytoplasmic membrane and cell wall(Fig. 2.3). The membrane is made up of proteins and lipids. It is semi-permeable and ensures selective entry of substances into the cell and release of decay products into the environment. On the surface of the invaginations of the cytoplasmic membrane inside the bacterium, called mesosomes, there are oxidative enzymes that take part in the respiration process. Such invaginations of the membrane play the role of mitochondria and some other organelles that are absent in the bacterial cell. In bacteria capable of photosynthesis (cyanobacteria, green bacteria, etc.), photosynthetic pigments are localized on the mesosomes.
Rice. 2.3. Structure diagram bacterial cell:
1 - ribosomes; 2 - cell membrane; 3 - mucous capsule; 4 - nucleoid; 5 - cell wall; 6 - flagellum; 7 - mesosome
The cell wall is also permeable to nutrients and waste products. It has a strong lattice of mureins (peptidoglycans), gives the bacteria a certain shape and protects it from environmental influences. In some bacteria, the cytoplasmic membrane and cell wall take part in the formation of another, outer layer of the membrane - capsules. A capsule is a semi-liquid mucous mass that covers the outside of the cell wall. It performs a protective function.
The structure and chemical composition of the bacterial
cells
The general structure of a bacterial cell is shown in Figure 2. The internal organization of a bacterial cell is complex. Each systematic group of microorganisms has its own specific structural features.
Cell wall. The bacterial cell is covered with a dense membrane. This surface layer, located outside the cytoplasmic membrane, is called the cell wall (Fig. 2, 14). The wall performs protective and supporting functions, and also gives the cell a permanent, characteristic shape (for example, the shape of a rod or coccus) and is the outer skeleton of the cell. This dense shell makes bacteria related to plant cells, which distinguishes them from animal cells that have soft shells.
Inside the bacterial cell, the osmotic pressure is several times, and sometimes tens of times higher than in external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as the cell wall.
The thickness of the cell wall is 0.01-0.04 µm. It is from 10 to 50% of the dry mass of bacteria. The amount of material from which the cell wall is built changes during bacterial growth and usually increases with age.
The main structural component of the walls, the basis of their rigid structure in almost all bacteria studied so far is murein
mucopeptide). This is an organic compound complex structure, which includes sugars that carry nitrogen - amino sugars and 4-5 amino acids. Moreover, the amino acids of cell walls have an unusual shape (D-stereoisomers), which is rarely found in nature.
The constituent parts of the cell wall, its components, form a complex strong structure (Fig. 3, 4 and 5).
Using the method of staining, first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-positive and
gram negative. Gram-positive organisms are able to bind certain aniline dyes, such as crystal violet, and retain the iodine-dye complex after treatment with iodine and then alcohol (or acetone). The same bacteria that are under the influence ethyl alcohol this complex is destroyed (cells become discolored), they are gram-negative.
Chemical composition cell walls of gram-positive and gram-negative bacteria is different.
In gram-positive bacteria, the cell walls include, in addition to mucopeptides, polysaccharides (complex, high-molecular sugars), teichoic acids
(complex in composition and structure compounds consisting of sugars, alcohols, amino acids and phosphoric acid). Polysaccharides and teichoic acids are associated with the framework of the walls - murein. We do not yet know what structure these constituent parts of the cell wall of gram-positive bacteria form. With the help of electronic photographs, thin sections (layering) were not found in the walls of gram-positive bacteria.
Probably, all these substances are very closely related to each other.
The walls of gram-negative bacteria are more complex in chemical composition, they contain a significant amount of lipids (fats) associated with proteins and sugars in complex complexes - lipoproteins and lipopolysaccharides. In general, there is less murein in the cell walls of gram-negative bacteria than in gram-positive bacteria.
The wall structure of Gram-negative bacteria is also more complex. Using an electron microscope, it was found that the walls of these bacteria are multilayered (Fig.
6).
The inner layer is murein. Above it is a wider layer of loosely packed protein molecules. This layer is in turn covered by a layer of lipopolysaccharide. The top layer is made up of lipoproteins.
The cell wall is permeable: through it, nutrients freely pass into the cell, and metabolic products are released into the environment. Large molecules with high molecular weight do not pass through the shell.
Capsule. The cell wall of many bacteria is surrounded from above by a layer of mucous material - a capsule (Fig. 7). The thickness of the capsule can be many times greater than the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule.
The capsule is not an obligatory part of the cell, it is formed depending on the conditions in which the bacteria enter. It serves as a protective cover of the cell and participates in water exchange, protecting the cell from drying out.
By chemical composition, capsules are most often polysaccharides.
Sometimes they consist of glycoproteins (complex complexes of sugars and proteins) and polypeptides (genus Bacillus), in rare cases - of fiber (genus Acetobacter).
Mucous substances secreted into the substrate by some bacteria determine, for example, the mucous-viscous consistency of spoiled milk and beer.
Cytoplasm. The entire contents of a cell, with the exception of the nucleus and cell wall, is called the cytoplasm. The liquid, structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, mitochondria, plastids and other structures, as well as reserve nutrients. The cytoplasm has an extremely complex, fine structure (layered, granular). With the help of an electron microscope, many interesting details of the structure of the cell have been revealed.
The outer lipoprotective layer of the bacterial protoplast, which has special physical and chemical properties called the cytoplasmic membrane (Fig.
2, 15).
Inside the cytoplasm are all vital structures and organelles.
The cytoplasmic membrane plays a very important role - it regulates the flow of substances into the cell and the release of metabolic products to the outside.
Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes. In addition, the membrane synthesizes some constituent parts cells, mainly components of the cell wall and capsule.
Finally, the most important enzymes (biological catalysts) are located in the cytoplasmic membrane. The orderly arrangement of enzymes on membranes makes it possible to regulate their activity and prevent the destruction of some enzymes by others. Ribosomes are attached to the membrane - structural particles on which protein is synthesized.
The membrane is made up of lipoproteins. It is strong enough and can provide the temporary existence of a cell without a shell. The cytoplasmic membrane makes up to 20% of the dry mass of the cell.
In electron photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75 Å thick, consisting of a light layer
(lipids) enclosed between two darker ones (proteins). Each layer has a width
20-30A. Such a membrane is called elementary (Table 30, Fig. 8).
Between the plasma membrane and the cell wall there is a connection in the form of desmoses
- bridges. The cytoplasmic membrane often gives invaginations - invaginations into the cell. These invaginations form special membrane structures in the cytoplasm, called
mesosomes. Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside such membranous sacs (Fig. 2). These structures perform a variety of functions in bacteria. Some of these structures are analogues of mitochondria. Others perform the functions of the endoplasmic reticulum or the Golgi apparatus. By invagination of the cytoplasmic membrane, the photosynthetic apparatus of bacteria is also formed.
After invagination of the cytoplasm, the membrane continues to grow and forms stacks (Table 30), which, by analogy with plant chloroplast granules, are called thylakoid stacks. In these membranes, which often fill most cytoplasm of a bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes are localized
(cytochromes) that carry out the process of photosynthesis.
,
The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cage. Ribosomes are made up of RNA and protein. In bacteria, many ribosomes are located freely in the cytoplasm, some of them can be associated with membranes.
Ribosomes are the centers of protein synthesis in the cell. At the same time, they often combine with each other, forming aggregates called polyribosomes or polysomes.
The cytoplasm of bacterial cells often contains granules of various shapes and sizes.
However, their presence cannot be regarded as a constant sign microorganism, usually to a large extent associated with the physical and chemical conditions of the environment. Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These reserve substances are formed when the body is supplied with a sufficient amount of nutrients, and, conversely, are used when the body enters conditions that are less favorable in terms of nutrition.
In many bacteria, the granules are composed of starch or other polysaccharides - glycogen and granulosa. Some bacteria, when grown on a sugar-rich medium, have droplets of fat inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules are composed of polymetaphosphate (reserve substance, including phosphoric acid residues).
Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria accumulate volutin more often under unusual nutritional conditions, such as on a medium that does not contain sulfur. Sulfur droplets are found in the cytoplasm of some sulfur bacteria.
In addition to various structural components, the cytoplasm consists of a liquid part - a soluble fraction. It contains proteins, various enzymes, t-RNA, some pigments and low molecular weight compounds - sugars, amino acids.
As a result of the presence of low molecular weight compounds in the cytoplasm, a difference arises in the osmotic pressure of the cellular contents and the external environment, and this pressure may be different for different microorganisms. The highest osmotic pressure was noted in gram-positive bacteria - 30 atm, in gram-negative bacteria it is much lower - 4-8 atm.
Nuclear device. In the central part of the cell, the nuclear substance is localized - deoxyribonucleic acid a (DNA).
,
Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have its analogue -
"nuclear equivalent" - nucleoid(see Fig. 2, 8), which is an evolutionarily more primitive form of organization of nuclear matter. Microorganisms that do not have a real nucleus, but have its analogue, belong to prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, most of the DNA is concentrated in one or more places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The nucleus is surrounded by a shell membrane.
In bacteria, DNA is less densely packed than in true nuclei; A nucleoid does not have a membrane, a nucleolus, or a set of chromosomes. Bacterial DNA is not associated with the main proteins - histones - and is located in the nucleoid in the form of a bundle of fibrils.
Flagella. Some bacteria have adnexal structures on their surface; the most widespread of them are flagella - the organs of movement of bacteria.
The flagellum is anchored under the cytoplasmic membrane by two pairs of discs.
Bacteria can have one, two, or many flagella. Their location is different: at one end of the cell, at two, over the entire surface, etc. (Fig. 9). Bacterial flagella have a diameter
0.01-0.03 microns, their length can be many times greater than the length of the cell. Bacterial flagella Consist of a protein - flagellin - and are twisted helical filaments.
On the surface of some bacterial cells there are thin villi -
fimbriae.
Plant life: in 6 volumes. - M.: Enlightenment. Edited by A. L. Takhtadzhyan, chief
editor USSR Academy of Sciences, prof. A.A. Fedorov. 1974
- The structure and chemical composition of a bacterial cell
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CYTOPLASMA (CPU)
Participate in spore formation.
MESOSOME
With excessive growth, in comparison with the growth of the CS, the CPM forms invaginates (invaginations) - mesosomes. Mesosomes are the center of the energy metabolism of the prokaryotic cell. Mesosomes are analogues of eukaryotic mitochondria, but are simpler in structure.
Well-developed and complexly organized mesosomes are characteristic of Gram+ bacteria.
Cell wall of bacteria
In Gram bacteria, mesosomes are less common and are simply organized (in the form of a loop). Mesosome polymorphism is observed even in the same species of bacteria. Rickettsia have no mesosomes.
Mesosomes vary in size, shape, and location within the cell.
Mesosomes are distinguished by shape:
- - lamellar (lamellar),
- - vesicular (having the form of bubbles),
- - tubular (tubular),
- mixed.
According to the location in the cell, mesosomes are distinguished:
- - formed in the zone of cell division and the formation of the transverse septum,
- - to which the nucleoid is attached;
– – formed as a result of invagination of the peripheral parts of the CPM.
Mesosome functions:
1. Strengthen the energy metabolism of cells, as they increase the total "working" surface of the membranes.
2. Participate in secretory processes(in some Gram+ bacteria).
3. Participate in cell division. During reproduction, the nucleoid moves to the mesosome, receives energy, doubles and divides by amitosis.
Detection of mesosomes:
1. Electron microscopy.
Structure. Cytoplasm (protoplasm) is the content of the cell, surrounded by the CPM and occupying the bulk of the bacterial cell. CPU is internal environment cells and is a complex colloidal system consisting of water (about 75%) and various organic compounds(proteins, RNA and DNA, lipids, carbohydrates, minerals).
The layer of protoplasm located under the CPM is denser than the rest of the mass in the center of the cell. The fraction of the cytoplasm, which has a homogeneous consistency and contains a set of soluble RNA, enzyme proteins, products and substrates of metabolic reactions, is called cytosol. Another part of the cytoplasm is represented by a variety of structural elements: nucleoid, plasmids, ribosomes and inclusions.
Functions of the cytoplasm:
1. Contains cell organelles.
Detection of the cytoplasm:
1. Electron microscopy.
Structure. Nucleoid - the equivalent of the eukaryotic nucleus, although it differs from it in its structure and chemical composition. The nucleoid is not separated from the CP by a nuclear membrane, does not have nucleoli and histones, contains one chromosome, has a haploid (single) set of genes, and is not capable of mitotic division.
The nucleoid is located in the center of the bacterial cell, contains a double-stranded DNA molecule, a small amount of RNA and proteins. In most bacteria, a double-stranded DNA molecule about 2 nm in diameter, about 1 m long, with a molecular weight of 1–3x109 Da is closed into a ring and tightly packed like a coil. In mycoplasmas, the molecular weight of DNA is the smallest for cellular organisms (0.4–0.8 × 109 Da).
Prokaryotic DNA is built in the same way as in eukaryotes (Fig. 25).
Rice. 25. DNA structure of prokaryotes:
BUT- a fragment of a DNA strand formed by alternating residues of deoxyribose and phosphoric acid. A nitrogenous base is attached to the first carbon atom of deoxyribose: 1 - cytosine; 2 - guanine.
B- DNA double helix: D- deoxyribose; F - phosphate; A - adenine; T - thymine; G - guanine; C - cytosine
The DNA molecule carries many negative charges, since each phosphate residue contains an ionized hydroxyl group. In eukaryotes, negative charges are neutralized by the formation of a DNA complex with the main proteins - histones. There are no histones in prokaryotic cells; therefore, charge neutralization is carried out by the interaction of DNA with polyamines and Mg2+ ions.
By analogy with eukaryotic chromosomes, bacterial DNA is often referred to as a chromosome. It is represented in the cell in the singular, since bacteria are haploid. However, before cell division, the number of nucleoids doubles, and during division it increases to 4 or more. Therefore, the terms "nucleoid" and "chromosome" do not always coincide. When cells are exposed to certain factors (temperature, pH, ionizing radiation, salts of heavy metals, some antibiotics, etc.) the formation of many copies of the chromosome occurs. When the influence of these factors is eliminated, as well as after the transition to the stationary phase, one copy of the chromosome is found in the cells.
For a long time, it was believed that no regularity could be traced in the distribution of DNA strands of the bacterial chromosome. Special studies have shown that prokaryotic chromosomes are a highly ordered structure. Part of the DNA in this structure is represented by a system of 20–100 independently supercoiled loops. Supercoiled loops correspond to currently inactive DNA regions and are located in the center of the nucleoid. On the periphery of the nucleoid there are despiralized areas where messenger RNA (mRNA) is synthesized. Since the processes of transcription and translation proceed simultaneously in bacteria, the same mRNA molecule can be simultaneously associated with DNA and ribosomes.
In addition to the nucleoid, the cytoplasm of a bacterial cell can contain plasmids - factors of extrachromosomal heredity in the form of additional autonomous circular molecules of double-stranded DNA with a lower molecular weight. Also encoded in plasmids hereditary information, however, it is not vital for a bacterial cell.
Functions of Nucleiod:
1. Storage and transmission of hereditary information, including the synthesis of pathogenicity factors.
Nucleoid detection:
1. Electron microscopy: on electron diffraction patterns of ultrathin sections, the nucleoid has the form of light zones of lower optical density with fibrillar, filamentous DNA structures (Fig. 26). Despite the absence of a nuclear membrane, the nucleoid is quite clearly separated from the cytoplasm.
2. Phase-contrast microscopy of native preparations.
3. Light microscopy after staining with DNA-specific methods according to Felgen, according to Pashkov or according to Romanovsky-Giemsa:
- the preparation is fixed with methyl alcohol;
- a Romanovsky-Giemsa dye (a mixture of equal parts of three colors - azure, eosin and methylene blue, dissolved in methanol) is poured onto a fixed preparation for 24 hours;
- the paint is drained, the preparation is washed with distilled water, dried and microscoped: the nucleoid is stained in purple and is located diffusely in the cytoplasm, stained in pale pink.
Read also:
Features of the chemical composition of bacterial cells
The structure of a bacterial cell. Main differences between prokaryotes and eukaryotes. Functions of individual structural elements of a bacterial cell. Features of the chemical composition of the cell walls of gram-positive and gram-negative bacteria.
A bacterial cell consists of a cell wall, cytoplasmic membrane, cytoplasm with inclusions, and a nucleus called a nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili. Some bacteria under adverse conditions are able to form spores.
Differences in the structure of the cell
1) Prokaryotes do not have a nucleus, but eukaryotes do.
2) Prokaryotes from organelles have only ribosomes (small, 70S), while eukaryotes, in addition to ribosomes (large, 80S), have many other organelles: mitochondria, ER, cell center, etc.
3) A prokaryotic cell is much smaller than a eukaryotic cell: 10 times in diameter, 1000 times in volume.
1) DNA is circular in prokaryotes and linear in eukaryotes
2) In prokaryotes, DNA is naked, almost not connected to proteins, while in eukaryotes, DNA is connected to proteins in a 50/50 ratio, a chromosome is formed
3) In prokaryotes, DNA lies in a special region of the cytoplasm called the nucleoid, while in eukaryotes, DNA lies in the nucleus.
Permanent components of a bacterial cell.
Nucleoid - the equivalent of the nucleus of prokaryotes
The cell wall is different in Gr+ and Gr-bacteria. Determines and maintains a constant form, provides communication with the external environment, determines the antigenic specificity of bacteria, and has important immunospecific properties; violation of the synthesis of the cell wall leads to the formation of L-forms of bacteria.
Gr+: this coloration is associated with the content of teichoic and dipoteichoic acids in the CS, which penetrate it through and fix it in the cytoplasm. Peptidoglycan is thick, composed of a plasma membrane bound by beta-glycosidic bonds.
Gr -: a thin layer of peptidoglycans, the outer membrane is represented by lipopolysaccharide glycocoproteins, glycolipids.
CPM - consists of lipoproteins. Perceives all chemical information entering the cell. It is the main barrier. Participates in the process of nucleoid and plasmid replication; contains a large number of enzymes; Participates in the synthesis of cell wall components.
Mesosomes are analogs of mitochondria in a bacterial cell
Ribosomes 70S are numerous small granules located in the cytoplasm.
UNPERMANENT:
Flagella: composed of the protein flagellin, originate from the CMP, the main function is motor.
Pili: due to them, attachment to the host cell occurs
Plasmids. Capsule, Spores, Inclusions.
Main article: Supramembrane complex
The supra-membrane apparatus of bacteria is represented by a cell wall, the specific organization of which serves as the basis for dividing them into two non-taxonomic groups (gram-positive and gram-negative forms) and correlates with a very large number of morphofunctional, metabolic and genetic characters. The cell wall of prokaryotes is essentially a polyfunctional organoid, derived from the protoplast and carrying a significant proportion of the metabolic load of the cell.
Cell wall of Gram-positive bacteria
The structure of the cell wall
In gram-positive bacteria (Fig. 12, A), the cell wall is generally more simple. The outer layers of the cell wall are formed by protein in combination with lipids. In some species of bacteria, a layer of surface protein globules has been relatively recently discovered, the shape, size, and location of which are specific to the species. Inside the cell wall, as well as directly on its surface, enzymes are placed that break down substrates to low molecular weight components, which are subsequently transported through the cytoplasmic membrane into the cell. It also contains enzymes that synthesize extracellular polymers, such as capsular polysaccharides.
Polysaccharide capsule
The polysaccharide capsule, which envelops the cell wall of a number of bacteria from the outside, is mainly of particular adaptive significance, and its presence is not necessary to preserve the vital activity of the cell. Thus, it ensures the attachment of cells to the surface of dense substrates, accumulates some mineral substances and, in pathogenic forms, prevents their phagocytosis.
Murein
A rigid murein layer adjoins directly to the cytoplasmic membrane.
Murein, or peptidoglycan, is a copolymer of acetylglucosamine and acetylmuramic acid with oligopeptide crosslinks. It is possible that the murein layer is one giant bag molecule that ensures the rigidity of the cell wall and its individual shape.
Teichoic acids
In close contact with the mureic layer is the second polymer of the wall of gram-positive bacteria - teichoic acids. They are credited with the role of an accumulator of cations and a regulator of ion exchange between the cell and the environment.
Cell wall of Gram-negative bacteria
The structure of the cell wall
Compared with gram-positive forms, the cell wall of gram-negative bacteria is more complex and its physiological significance is incomparably wider. In addition to the murein layer, the second protein-lipid membrane is located closer to the surface (Fig. 12, B, C), which includes lipopolysaccharides. It is covalently linked to murein by cross-links from lipoprotein molecules. The main function of this membrane is the role of a molecular sieve; in addition, enzymes are located on its outer and inner surfaces.
3. Structure of a bacterial cell.
The space bounded by the outer and cytoplasmic membranes is called periplasmic and is a unique property of gram-negative bacteria. A whole set of enzymes is localized in its volume - phosphatases, hydrolases, nucleases, etc. They break down relatively high-molecular nutrient substrates, and also destroy their own cellular material released into the environment from the cytoplasm. To a certain extent, the periplasmic space can be likened to the eukaryotic lysosome. In the periplasmic zone, it is possible not only to carry out the most efficient enzymatic reactions, but also to isolate compounds from the cytoplasm that pose a threat to its normal functioning. Material from the site http://wiki-med.com
Functions of the bacterial cell wall
Both in gram-positive and gram-negative forms, the cell wall plays the role of a molecular sieve, selectively carrying out passive transport of ions, substrates and metabolites. In bacteria that have the ability to actively move due to flagella, the cell wall is a component of the locomotor mechanism. Finally, individual sections of the cell wall are closely associated with the cytoplasmic membrane in the zone of nucleoid attachment and play an important role in its replication and segregation.
In one of the bacterial species, the process of destruction of the old cell wall, which occurs during cell division, is ensured by the work of at least four systems of hydrolytic enzymes present in the cell wall in a latent state. During cell division, a regular and strictly sequential activation of these systems occurs, leading to the gradual destruction and desquamation of the old ("mother") shell of the bacterial cell.
Material from the site http://Wiki-Med.com
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.the main component of the cell wall of gram-positive bacteria is
bacterial cell wall functions
features of the structure of the bacterial cell wall
cell wall structure
characterization of the bacterial cell wall
The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, proteins. The main component of the cell wall of these bacteria is a multilayer peptidoglycan (murein, mucopeptide), which makes up 40-90% of the mass of the cell wall. Teichoic acids (from Greek teichos - wall) are covalently bound to the peptidoglycan of the cell wall of gram-positive bacteria.
The cell wall of gram-negative bacteria includes an outer membrane linked by a lipoprotein to the underlying layer of peptidoglycan. On ultrathin sections of bacteria, the outer membrane has the form of a wavy three-layer structure similar to the inner membrane, which is called cytoplasmic. The main component of these membranes is a bimolecular (double) layer of lipids. The inner layer of the outer membrane is represented by phospholipids, and the outer layer contains lipopolysaccharide (LPS). Lipopolysaccharide of the outer membrane consists of three fragments: lipid A - a conservative structure, almost the same in gram-negative bacteria; core, or core, core part (lat. core - core), relatively conservative oligosaccharide structure (the most constant part of the LPS core is ketodeoxyoctonic acid); highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences (O-antigen). Proteins in the matrix of the outer membrane permeate it in such a way that protein molecules called porins border hydrophilic pores through which water and small hydrophilic molecules pass.
In violation of the synthesis of the bacterial cell wall under the influence of lysozyme,
penicillin, protective factors of the body, cells with a modified (often spherical) shape are formed: protoplasts - bacteria completely devoid of a cell wall; spheroplasts are bacteria with a partially preserved cell wall. Sphero- or protoplast-type bacteria that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-forms.
They are osmotically sensitive, spherical, flask-shaped cells of various sizes, including those passing through bacterial filters. Some L-forms (unstable) when the factor that led to changes in the bacteria is removed, can reverse, "returning" to the original bacterial cell.
Between the outer and cytoplasmic membranes is the periplasmic space, or periplasm, containing enzymes (proteases, lipases, phosphatases, nucleases, beta-lactomases) and components of transport systems.
The cytoplasmic membrane under electron microscopy of ultrathin sections is a three-layer membrane (2 dark layers 2.5 nm thick are separated by a light one - intermediate). In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of phospholipids with embedded surface and integral proteins, as if penetrating through the membrane structure. With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complexly twisted membrane structures, called mesosomes. Less complex twisted structures are called intracytoplasmic membranes.
Cytoplasm
The cytoplasm consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes responsible for the synthesis (translation) of proteins. Bacterial ribosomes are about 20 nm in size and have a sedimentation coefficient of 70S, in contrast to the 80S ribosomes characteristic of eukaryotic cells. Ribosomal RNA (rRNA) are conservative elements of bacteria ("molecular clock" of evolution). 16S rRNA is part of the small subunit of ribosomes, and 23S rRNA is part of the large subunit of ribosomes. The study of 16S rRNA is the basis of gene systematics, making it possible to assess the degree of relatedness of organisms.
In the cytoplasm there are various inclusions in the form of glycogen granules, polysaccharides, beta-hydroxybutyric acid and polyphosphates (volutin).
cell wall
They are reserve substances for the nutrition and energy needs of bacteria. Volyutin has an affinity for basic dyes and is easily detected using special staining methods (for example, according to Neisser) in the form of metachromatic granules. The characteristic arrangement of volutin granules is revealed in diphtheria bacillus in the form of intensely stained poles of the cell.
Nucleoid
Nucleoid is the equivalent of the nucleus in bacteria. It is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a ball. The nucleus of bacteria, unlike eukaryotes, does not have a nuclear membrane, nucleolus and basic proteins (histones). Usually, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring.
In addition to the nucleoid, represented by one chromosome, the bacterial cell contains extrachromosomal factors of heredity - plasmids, which are covalently closed DNA rings.
Capsule, microcapsule, mucus
A capsule is a slimy structure more than 0.2 µm thick, firmly attached to the bacterial cell wall and having clearly defined outer boundaries. The capsule is distinguishable in smears-imprints from pathological material. In pure cultures of bacteria, the capsule is formed less frequently. It is revealed at special methods staining of the smear (for example, according to Burri-Gins), creating a negative contrast of the substances of the capsule: the ink creates a dark background around the capsule. The capsule consists of polysaccharides (exopolysaccharides), sometimes of polypeptides, for example, in the anthrax bacillus, it consists of polymers of D-glutamic acid. The capsule is hydrophilic and prevents phagocytosis of bacteria. The capsule is antigenic: antibodies against the capsule cause it to enlarge (capsule swelling reaction).
Many bacteria form a microcapsule - a mucous formation less than 0.2 microns thick, which can be detected only with electron microscopy. From the capsule should be distinguished slieb - mucoid exopolysaccharides that do not have clear boundaries. Slime is soluble in water.
Bacterial exopolysaccharides are involved in adhesion (sticking to substrates), they are also called glycocalyx. Beyond synthesis
exopolysaccharides by bacteria, there is another mechanism for their formation: through the action of extracellular bacterial enzymes on disaccharides. As a result, dextrans and levans are formed.
Flagella
Bacterial flagella determine the motility of the bacterial cell. Flagella are thin filaments that originate from the cytoplasmic membrane and are longer than the cell itself. The flagella are 12–20 nm thick and 3–15 µm long. They consist of 3 parts: a spiral thread, a hook and a basal body containing a rod with special discs (1 pair of discs in Gram-positive and 2 pairs of discs in Gram-negative bacteria). The discs of the flagella are attached to the cytoplasmic membrane and cell wall. This creates the effect of an electric motor with a motor rod that rotates the flagellum. Flagella consist of a protein - flagellin (from flagellum - flagellum); is an H antigen. Flagellin subunits are coiled.
The number of flagella in bacteria various kinds varies from one (monotrich) in Vibrio cholerae to ten or hundreds of flagella extending along the perimeter of the bacterium (peritrich) in coli, proteus, etc. Lofotrichous have a bundle of flagella at one end of the cell. Amphitrichous have one flagellum or a bundle of flagella at opposite ends of the cell.
drinking
Pili (fimbriae, villi) - filamentous formations, thinner and shorter (3-10 nm x 0.3-10 microns) than flagella. Pili extend from the cell surface and consist of the pilin protein, which has antigenic activity. There are pili responsible for adhesion, that is, for attaching bacteria to the affected cell, as well as pili responsible for nutrition, water-salt metabolism and sexual (F-pili), or conjugation pili. Drinks were plentiful - several hundred per cage. However, sex pili are usually 1-3 per cell: they are formed by so-called "male" donor cells containing transmissible plasmids (F-, R-, Col-plasmids). Distinctive feature sex pili is the interaction with special "male" spherical bacteriophages, which are intensively adsorbed on the sex pili.
controversy
Spores are a peculiar form of dormant firmicute bacteria, i.e. bacteria
with a gram-positive type of cell wall structure. Spores are formed under unfavorable conditions for the existence of bacteria (drying, nutrient deficiency, etc.. One spore (endospore) is formed inside the bacterial cell. The formation of spores contributes to the preservation of the species and is not a method of reproduction, like in fungi. Spore-forming bacteria of the genus Bacillus have spores that do not exceeding the diameter of the cell.Bacteria whose spore size exceeds the diameter of the cell are called clostridium, for example, bacteria of the genus Clostridium (lat. Clostridium - spindle).The spores are acid-resistant, therefore, they are stained red according to the Aujeszky method or according to the Ziehl-Neelsen method, and the vegetative cell into blue.
The shape of the dispute can be oval, spherical; the 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 pathogens of botulism, gas gangrene) and central (in anthrax bacilli). The spore persists for a long time due to the presence of a multi-layered shell, calcium dipicolinate, low water content and sluggish metabolic processes. Under favorable conditions, spores germinate through three successive stages: activation, initiation, germination.
Bacteria: habitats, structure, life processes, significance
2. b) The structure of a bacterial cell
The cell wall of bacteria determines their shape and ensures the preservation of the internal contents of the cell. According to the peculiarities of the chemical composition and structure of the cell wall, bacteria are differentiated by staining by gram ...
Biopolymers of the bacterial cell wall
The structure of a bacterial cell
The structure of bacteria is studied using electron microscopy of whole cells and their ultraviolet sections. The main structures of a bacterial cell are: cell wall, cytoplasmic membrane, cytoplasm with inclusions and nucleus ...
Humoral regulation of the body
3. Features of the structure, properties and functions of cell membranes
Diversity of living cells
1.1 The general plan of the structure of eukaryotic cells, which also characterizes the structure of an animal cell
A cell is a structural and functional unit of the living. All eukaryotic cells are characterized by the presence following structures: 1) The cell membrane is an organoid that limits the contents of the cell from the environment ...
Diversity of living cells
1.2 Structural features of a plant cell
In plant cells, there are organelles that are also characteristic of animals, for example, the nucleus, the endoplasmic reticulum, ribosomes, mitochondria, the Golgi apparatus (see Fig. 2). They lack a cell center, and the function of lysosomes is performed by vacuoles ...
Diversity of living cells
1.3 Structural features of the fungal cell
In most fungi, the cell in its structure and functions performed by it is generally similar to the plant cell. It consists of a hard shell and internal contents, which is a cytoplasmic system ...
Diversity of living cells
1.4 The general plan of the structure of prokaryotic cells, which also characterizes the structure of a bacterial cell
The prokaryotic cell is arranged as follows. The main feature of these cells is the absence of a morphologically pronounced nucleus, but there is a zone in which DNA (nucleoid) is located.
Structure of a bacterial cell
The cytoplasm contains ribosomes...
Fundamentals of microbiology
1. Describe the structure of a bacterial cell. Draw the cell organelles
Bacteria are microscopic plant organisms. Most of them - unicellular organisms that do not contain chlorophyll and reproduce by division. The shape of the bacteria are spherical, rod-shaped and convoluted ...
Features of visual and auditory sensory systems
13. Simple, complex and supercomplex cells and their functions
"Simple" and "complex" cells. Neurons responding to simple linear stimuli (slits, edges, or dark bands) were called "simple", while those responding to stimuli of complex configuration and moving stimuli were called "complex"...
Features of the structure of the cell
1. Cell as an elementary structural unit of the body. The main components of the cell
The cell is the basic structural and functional unit of life, limited by a semipermeable membrane and capable of self-reproduction. In a plant cell, first of all, it is necessary to distinguish between the cell membrane and the contents ...
Distribution and dynamics of the wild boar population in the Bryansk region
1.1 Structural features
The wild boar (Sus scrofa L.) is a massive animal with low, relatively thin legs. The body is relatively short, the front part is very massive, the back of the shoulder blades is strongly raised, the neck is thick, short, almost motionless ...
Structure, properties and functions of proteins
2. Functions of cell organelles
Cell organelles and their functions: 1. Cell membrane - consists of 3 layers: 1. rigid cell wall; 2. a thin layer of pectin; 3. thin cytoplasmic filament. The cell wall provides mechanical support and protection...
4.1 Structural features
Thallus is a plasmodium capable of amoeba-like movements on the surface or inside the substrate. During sexual reproduction, plasmodia turn into fruiting bodies called sporocarps ...
Taxonomic group of slime molds
5.1 Structural features
A vegetative body in the form of a multinucleated protoplast that is not capable of independent movement and is located inside the host plant cell. Special sporulations are not formed. The wintering stage is represented by spores ...
The energy system of the cell. Classification of muscle tissue. The structure of the sperm
The energy system of the cell. The general plan of the structure of mitochondria and plastids, their functions. The hypothesis of the symbiotic origin of mitochondria and chloroplasts
Eukaryotic cells have a unique organelle, the mitochondrion, in which ATP molecules are formed in the process of oxidative phosphorylation. Mitochondria are often said to be the powerhouses of the cell (Figure 1)…
The main differences between a prokaryotic (bacterial) cell and a eukaryotic one are: the absence of a formalized nucleus (i.e., nuclear membrane), the absence of intracellular membranes, nucleoli, the Golgi complex, lysosomes, and 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). The length of the DNA is about 1 mm. The amount of information is about 1000 genes (features). The nucleoid is not separated from the cytoplasm by a membrane.
The cytoplasm is a colloid, i.e. aqueous solution of proteins, carbohydrates. Lipids, minerals, in which there are ribosomes, inclusions, plasmids.
Protein synthesis takes place on ribosomes. Ribosomes of prokaryotes differ from eukaryotic ones in smaller sizes (70 S).
Inclusions are reserve nutrients of a bacterial cell, as well as accumulations of pigments. Reserve nutrients include: granules of volutin (inorganic polyphosphate), glycogen, granulosa, starch, fat drops, accumulations of pigment, sulfur, calcium. Inclusions are usually formed when bacteria are grown on rich nutrient media and disappears on starvation.
Cell membrane - limits the cytoplasm. Consists of a double layer of phospholipids and embedded membrane proteins. CMs, in addition to the barrier and transport functions, play the role of a center of metabolic activity (in contrast to the eukaryotic cell). Membrane proteins responsible for transporting essential substances into the cell are called permeases. On the inner surface of the CM there are enzyme ensembles, i.e. ordered accumulations of enzyme molecules responsible for the synthesis of energy carriers - ATP molecules. CM can form invaginations into the cytoplasm, which are called mesosomes. There are two types of mesosomes:
Septal - form transverse partitions in the process of cell division.
Lateral - serve to increase the surface of the CM 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 history. 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 the different coloration of bacteria according to the Gram method was elucidated relatively recently - about 35 years ago. It turned out that G- and G+ bacteria have different cell wall structures. The structure of the cell wall of G+ bacteria. The basis of the cell wall of G+ bacteria is made up of 2 polymers: peptidoglycan and teichoic acids. Peptidoglycan is a linear polymer with alternating muramic acid and acetylglucosamine residues. A tetrapeptide (protein) is covalently bound to muramic acid. The strands of peptidoglycan are interconnected through peptides and form a strong framework - the basis of the cell wall. Between the strands of peptidoglycan is another polymer - teichoic acids (glycerol TK and ribitol TK) - a polymer of polyphosphates. Teichoic acids act on the surface of the cell wall and are the main antigens of G+ bacteria. In addition, Mg ribonucleate is included in the cell wall of G+ bacteria. 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. LPS 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. At the same time, a protoplast is formed from G+ bacteria, and a spheroplast is formed from G-bacteria (because the outer layer of the cell wall is preserved).
Under certain conditions of cultivation, cells lacking 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 turn into 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 sheath. This is a capsule. The function of the capsule is to protect bacteria from phasocytosis.
Surface structures of a bacterial cell.
The organs of attachment to the substrate (adhesion) are pili (fimbriae) or cilia. They start from the cell membrane. Composed of pilin protein. The number of pili can reach 400 per 1 cell.
The organs of transmission of hereditary information are F-drank or sex-drank. F-pills are formed only if the cell is odd to the plasmid, because F-pili proteins encode 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. The flagella are made up of the protein flagellin. The base of the flagellum is connected to the cell membrane through the basal body. The basal body consists of a system of rings that, while rotating, transmit rotational motion to the flagellum. According to the location of the flagellum, bacteria are divided into mono-, lopho-, amphi-, peritrichous.
Reading time: 6 minModern science has made fantastic progress in recent centuries. However, some mysteries still excite the minds of prominent scientists.
Today, the answer to the urgent question has not been found - how many varieties of bacteria exist on our vast planet?
Bacterium- an organism with a unique internal organization, which is characterized by all the processes characteristic of living organisms. The bacterial cell has many amazing features, one of which is the variety of shapes.
A bacterial cell can be spherical, rod-shaped, cubic, or star-shaped. In addition, the bacteria are slightly bent or form a variety of curls.
The shape of the cell plays an important role in the proper functioning of the microorganism, as it can affect the ability of the bacterium to attach to other surfaces, obtain the necessary substances and move.
The minimum cell size is usually 0.5 µm, however, in exceptional cases, the size of the bacterium can reach 5.0 µm.
The structure of the cell of any bacterium is strictly ordered. Its structure differs significantly from the structure of other cells, such as plants and animals. Cells of all types of bacteria do not have such elements as: a differentiated nucleus, intracellular membranes, mitochondria, lysosomes.
Bacteria have specific structural components- permanent and non-permanent.
Permanent components include: cytoplasmic membrane (plasmolemma), cell wall, nucleoid, cytoplasm. Non-permanent structures are: capsule, flagella, plasmids, pili, villi, fimbriae, spores.
cytoplasmic membrane
Any bacterium is enveloped by a cytoplasmic membrane (plasmolemma), which includes 3 layers. The membrane contains globulins responsible for the selective transport of various substances into the cell.
The plasma membrane also performs the following important functions:
- mechanical- ensures the autonomous functioning of the bacterium and all structural elements;
- receptor- proteins located in the plasmalemma act as receptors, that is, they help the cell to perceive various signals;
- energy Some proteins are responsible for the function of energy transfer.
Violation of the functioning of the plasma membrane leads to the fact that the bacterium collapses and dies.
cell wall
The structural component inherent only in bacterial cells is the cell wall. This is a rigid permeable membrane, which acts as an important component of the structural skeleton of the cell. It is located on the outside of the cytoplasmic membrane.
The cell wall performs the function of protection, and in addition gives the cell a permanent shape. Its surface is covered with numerous spores that let in the necessary substances and remove decay products from the microorganism.
Protection of internal components from osmotic and mechanical effects is another function of the wall. It plays an indispensable role in the control of cell division and the distribution of hereditary traits in it. It contains peptidoglycan, which gives the cell valuable immunobiological characteristics.
The thickness of the cell wall ranges from 0.01 to 0.04 µm. With age, bacteria grow and the amount of material from which it is built increases accordingly.
Nucleoid
Nucleoid is a prokaryote, which stores all the hereditary information of a bacterial cell. The nucleoid is located in the central part of the bacterium. Its properties are equivalent to the kernel.
A nucleoid is a single DNA molecule closed in a ring. The length of the molecule is 1 mm, and the amount of information is about 1000 features.
The nucleoid is the main carrier of material about the properties of the bacterium and the main factor in the transmission of these properties to offspring. The nucleoid in bacterial cells does not have a nucleolus, membrane, or basic proteins.
Cytoplasm
Cytoplasm- an aqueous solution containing the following components: mineral compounds, nutrients, proteins, carbohydrates and lipids. The ratio of these substances depends on the age and type of bacteria.
The cytoplasm contains various structural components: ribosomes, granules and mesosomes.
- Ribosomes are responsible for protein synthesis. Their chemical composition includes RNA molecules and protein.
- Mesosomes are involved in spore formation and cell reproduction. May be in the form of a bubble, loop, tubule.
- Granules serve as an additional energy resource for bacterial cells. These elements come in a variety of forms. They contain polysaccharides, starch, fat droplets.
Capsule
Capsule It is a mucous structure tightly bound to the cell wall. Examining it under a light microscope, one can see that the capsule envelops the cell and its outer boundaries have a clearly defined contour. In a bacterial cell, the capsule serves as a protective barrier against phages (viruses).
Bacteria form a capsule when environmental conditions become aggressive. The capsule includes in its composition mainly polysaccharides, and in certain cases it may contain fiber, glycoproteins, polypeptides.
The main functions of the capsule:
- adhesion with cells in the human body. For example, streptococci stick together with tooth enamel and, in alliance with other microbes, provoke caries;
- protection from negative environmental conditions: toxic substances, mechanical damage, elevated oxygen levels;
- participation in water metabolism (protection of cells from drying out);
- creation of an additional osmotic barrier.
The capsule forms 2 layers:
- internal - part of the cytoplasm layer;
- external - the result of the excretory function of the bacterium.
The classification was based on the structural features of the capsules. They are:
- normal;
- complex capsules;
- with cross-striped fibrils;
- discontinuous capsules.
Some bacteria also form a microcapsule, which is a mucous formation. The microcapsule can be detected only under an electron microscope, since the thickness of this element is only 0.2 microns or even less.
Flagella
Most bacteria have surface structures of the cell that provide its mobility and movement - flagella. These are long processes in the form of a left-handed spiral, built from flagellin (a contractile protein).
The main function of flagella is that they allow bacteria to move in a liquid environment in search of more favorable conditions. The number of flagella in one cell can vary: from one to several flagella, flagella on the entire surface of the cell or only on one of its poles.
There are several types of bacteria, depending on the number of flagella in them:
- Monotrichous- they have only one flagellum.
- lophotrichous- have a certain number of flagella at one end of the bacterium.
- amphitriches- characterized by the presence of flagella at polar opposite poles.
- Peritrichi- flagella are located over the entire surface of the bacterium, they are characterized by slow and smooth movement.
- Atrichi- flagella are absent.
Flagella perform motor activity, making rotational movements. If bacteria do not have flagella, it is still able to move, or rather, slide with the help of mucus on the surface of the cell.
Plasmids
Plasmids are small mobile DNA molecules separate from chromosomal heredity factors. These components usually contain genetic material that makes the bacterium more resistant to antibiotics.
They can transfer their properties from one microorganism to another. Despite all their features, plasmids do not act as important elements for the life of a bacterial cell.
Pili, villi, fimbriae
These structures are localized on the surfaces of bacteria. They count from two units to several thousand per cell. These structural elements has both a bacterial motile cell and a non-motile cell, since they do not have any effect on the ability to move.
AT quantitatively, pili reach several hundred per bacterium. There are pili that are responsible for nutrition, water-salt metabolism, as well as conjugation (sex) pili.
The villi are characterized by a hollow cylindrical shape. It is through these structures that viruses enter the bacterium.
Villi are not considered essential components of a bacterium, since even without them the process of division and growth can be successfully completed.
Fimbria are located, as a rule, at one end of the cell. These structures allow the microorganism to be fixed in the tissues of the body. Some fimbriae have special proteins that are in contact with the receptor endings of the cells.
Fimbria differ from flagella in that they are thicker and shorter, and also do not realize the function of movement.
controversy
Spores are formed in the event of negative physical or chemical manipulation of the bacterium (as a result of drying or lack of nutrients). They are diverse in spore size, since they can be completely different in different cells. The shape of the spores also differs - they are oval or spherical.
By location in the cell, spores are divided into:
- central - their position in the very center, as, for example, in anthrax;
- subterminal - located at the end of the stick, giving the shape of a club (in the causative agent of gas gangrene).
In a favorable environment, the spore life cycle includes the following stages:
- preparatory stage;
- activation stage;
- initiation stage;
- germination stage.
Spores are distinguished by their special vitality, which is achieved due to their shell. It is multilayered and consists mainly of protein. The increased resistance of spores to negative conditions and external influences is ensured precisely due to proteins.
Structural components of a cell are the shell of bacteria, consisting of a cell wall, a cytoplasmic membrane, and sometimes a capsule; cytoplasm; ribosomes; various cytoplasmic inclusions; nucleoid (nucleus). Some types of bacteria also have spores, flagella, cilia (pili, fimbriae) (Fig. 2).
cell wall obligatory formation of bacteria of most species. Its structure depends on the type and affiliation
bacteria to groups differentiated by Gram staining. The mass of the cell wall is about 20% of the dry mass of the entire cell, the thickness is from 15 to 80 nm.
Rice. 3. Scheme of the structure of a bacterial cell
1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - cytoplasm; 5 - mesosomes; 6 - ribosomes; 7 - nucleoid; 8 - intracytoplasmic membrane formations; 9 - fat drops; 10 - polysaccharide granules; 11 - polyphosphate granules; 12 - sulfur inclusions; 13 - flagella; 14 - basal body
The cell wall has pores up to 1 nm in diameter, so it is a semi-permeable membrane through which nutrients penetrate and metabolic products are released.
These substances can penetrate into the microbial cell only after preliminary hydrolytic cleavage by specific enzymes secreted by bacteria into the external environment.
The chemical composition of the cell wall is heterogeneous, but it is constant for a certain type of bacteria, which is used for identification. Nitrogenous compounds, lipids, cellulose, polysaccharides, pectin substances were found in the composition of the cell wall.
The most important chemical component of the cell wall is a complex polysaccharide peptide. It is also called peptidoglycan, glycopeptide, murein (from lat. Murus - wall).
Murein is a structural polymer composed of glycan molecules formed by acetylglucosamine and acetylmuramic acid. Its synthesis is carried out in the cytoplasm at the level of the cytoplasmic membrane.
Cell wall peptidoglycan of various types has a specific amino acid composition and, depending on this, a certain chemotype, which is taken into account when identifying lactic acid and other bacteria.
In the cell wall of Gram-negative bacteria, peptidoglycan is represented by a single layer, while in the wall of Gram-positive bacteria it forms several layers.
In 1884, Gram proposed a tissue staining method that was used to stain prokaryotic cells. If, during Gram staining, fixed cells are treated with an alcoholic solution of crystal violet dye, and then with an iodine solution, then these substances form a stable colored complex with murein.
In homopositive microorganisms, the stained violet complex does not dissolve under the influence of ethanol and, accordingly, does not discolor; when stained with fuchsin (red dye), the cells remain dark purple.
In gram-negative species of microorganisms, gentian violet is dissolved in ethanol and washed out with water, and when stained with fuchsin, the cell turns red.
The ability of microorganisms to stain with analine dyes and according to the Gram method is called tinctorial properties . They need to be studied in young (18-24 hour) cultures, as some Gram-positive bacteria in old cultures lose their ability to stain positive by the Gram method.
The significance of peptidoglycan lies in the fact that, thanks to it, the cell wall has rigidity, i.e. elasticity, and is the protective frame of the bacterial cell.
When peptidoglycan is destroyed, for example, under the action of lysozyme, the cell wall loses its rigidity and collapses. The contents of the cell (cytoplasm), together with the cytoplasmic membrane, acquires a spherical shape, that is, it becomes a protoplast (spheroplast).
Many synthesizing and degrading enzymes are associated with the cell wall. Cell wall components are synthesized in the cytoplasmic membrane and then transported into the cell wall.
cytoplasmic membrane located under the cell wall and tightly adjacent to its inner surface. It is a semi-permeable membrane surrounding the cytoplasm and the inner contents of the cell - the protoplast. The cytoplasmic membrane is the thickened outer layer of the cytoplasm.
The cytoplasmic membrane is the main barrier between the cytoplasm and environment, violation of its integrity leads to cell death. It consists of proteins (50-75%), lipids (15-45%), in many species - carbohydrates (1-19%).
The main lipid component of the membrane are phospho- and glycolipids.
The cytoplasmic membrane, with the help of enzymes localized in it, performs various functions: it synthesizes membrane lipids - components of the cell wall; membrane enzymes - selectively transfer various organic and inorganic molecules and ions through the membrane, the membrane is involved in the transformation of cellular energy, as well as in chromosome replication, in the transfer of electrochemical energy and electrons.
Thus, the cytoplasmic membrane provides selective entry into the cell and removal from it of various substances and ions.
Derivatives of the cytoplasmic membrane are mesosomes . These are spherical structures formed when the membrane is twisted into a curl. They are located on both sides - at the site of the formation of the cell septum or near the zone of localization of nuclear DNA.
Mesosomes are functionally equivalent to mitochondria in cells of higher organisms. They participate in the redox reactions of bacteria, play an important role in the synthesis of organic substances, in the formation of the cell wall.
Capsule is a derivative of the outer layer of the cell coat and is a mucous membrane surrounding one or more microbial cells. Its thickness can reach 10 microns, which is many times greater than the thickness of the bacterium itself.
The capsule performs a protective function. The chemical composition of the bacterial capsule is different. In most cases, it consists of complex polysaccharides, mucopolysaccharides, sometimes polypeptides.
Capsulation is usually a species trait. However, the appearance of a microcapsule often depends on the conditions of bacterial cultivation.
Cytoplasm- a complex colloidal system with a large amount of water (80-85%), in which proteins, carbohydrates, lipids, as well as mineral compounds and other substances are dispersed.
The cytoplasm is the contents of a cell surrounded by a cytoplasmic membrane. It is divided into two functional parts.
One part of the cytoplasm is in the state of a sol (solution), has a homogeneous structure and contains a set of soluble ribonucleic acids, enzyme proteins and metabolic products.
The other part is represented by ribosomes, inclusions of various chemical nature, genetic apparatus, and other intracytoplasmic structures.
Ribosomes- these are submicroscopic granules, which are spherical nucleoprotein particles with a diameter of 10 to 20 nm, a molecular weight of about 2-4 million.
Ribosomes of prokaryotes consist of 60% RNA (ribonucleic acid), located in the center, and 40 % protein that covers nucleic acid outside.
Cytoplasmic inclusions are metabolic products, as well as reserve products, due to which the cell lives in conditions of nutrient deficiency.
The genetic material of prokaryotes consists of a double strand of deoxyribonucleic acid (DNA) of a compact structure located in the central part of the cytoplasm and not separated from it by a membrane. Bacterial DNA does not differ in structure from eukaryotic DNA, but since it is not separated from the cytoplasm by a membrane, the genetic material is called nucleoid or genophore. Nuclear structures are spherical or horseshoe shaped.
controversy bacteria are a dormant, non-reproducing form. They form inside the cell, are round or oval formations. Spores form predominantly gram-positive bacteria, rod-shaped with aerobic and anaerobic type of respiration in old cultures, as well as under adverse environmental conditions (lack of nutrients and moisture, accumulation of metabolic products in the medium, changes in pH and cultivation temperature, the presence or absence of atmospheric oxygen and etc.) can switch to an alternative development program, resulting in disputes. In this case, one spore is formed in the cell. This indicates that sporulation in bacteria is an adaptation for the preservation of the species (individual) and is not a way of their reproduction. The process of sporulation occurs, as a rule, in the external environment within 18-24 hours.
A mature spore is approximately 0.1 of the volume of the mother cell. Spores in different bacteria differ in shape, size, location in the cell.
Microorganisms whose spore diameter does not exceed the width of the vegetative cell are called bacilli, bacteria that have spores, the diameter of which is 1.5-2 times larger than the diameter of the cell, are called clostridia.
Inside the microbial cell, the spore can be located in the middle - the central position, at the end - terminal and between the center and the end of the cell - subterminal location.
Flagella bacteria are locomotor organs (organs of movement), with the help of which bacteria can move at a speed of up to 50-60 microns / s. At the same time, for 1 s, the bacteria cover the length of their body by 50-100 times. The length of the flagella exceeds the length of bacteria by 5-6 times. The thickness of the flagella is on average 12-30 nm.
The number of flagella, their size and location are constant for certain types of prokaryotes and therefore are taken into account when identifying them.
Depending on the number and location of flagella, bacteria are divided into monotrichous (monopolar monotrichous) - cells with one flagellum at one end, lofotrichous (monopolar polytrichous) - a bundle of flagella is located at one of the ends, amphitrichous (bipolar polytrichous) - flagella are located at each of poles, peritrichous - flagella are located over the entire surface of the cell (Fig. 4) and atrichous - bacteria devoid of flagella.
The nature of the movement of bacteria depends on the number of flagella, age, characteristics of the culture, temperature, the presence of various chemical substances and other factors. Monotrichous have the highest mobility.
Flagella are more often found in rod-shaped bacteria; they are not vital cell structures, since there are non-flagellated variants of motile bacteria.