The required organelles are: nuclear apparatus, cytoplasm, cytoplasmic membrane.
Optional(minor) structural elements are: cell wall, capsule, spores, drank, flagella.
1.In the center of the bacterial cell is nucleoid- nuclear formation, represented most often by one annular chromosome. Consists of a double-stranded DNA strand. The nucleoid is not separated from the cytoplasm by the nuclear membrane.
2.Cytoplasm- a complex colloidal system containing various inclusions of metabolic origin (grains of volutin, glycogen, granulosa, etc.), ribosomes and other elements of the protein synthesizing system, plasmids (extra-nucleoid DNA), mesosomes(formed as a result of invagination of the cytoplasmic membrane into the cytoplasm, participate in energy metabolism, sporulation, formation of the intercellular septum during division).
3.Cytoplasmic membrane limits the cytoplasm from the outside, has a three-layer structure and performs a number of important functions - barrier (creates and maintains osmotic pressure), energy (contains many enzyme systems - respiratory, redox, carries out the transfer of electrons), transport (transfer of various substances into the cell and from the cage).
4.Cell wall- is inherent in most bacteria (except for mycoplasmas, acholeplasmas and some other microorganisms that do not have a true cell wall). It has a number of functions, first of all, it provides mechanical protection and a constant shape of cells; antigenic properties of bacteria are largely associated with its presence. The composition consists of two main layers, of which the outer one is more plastic, the inner one is rigid.
The main chemical compound of the cell wall, which is specific only to bacteria peptidoglycan(mureic acids). The structure and chemical composition of the bacterial cell wall determines the important for taxonomy trait of bacteria - relation to Gram staining... In accordance with it, two large groups are distinguished - gram-positive (“gram +”) and gram-negative (“gram -”) bacteria. The wall of gram-positive bacteria after Gram staining retains the iodine complex with gentian violet(colored in blue-violet color), gram-negative bacteria lose this complex and the corresponding color after treatment and are colored pink due to additional staining with magenta.
Features of the cell wall of gram-positive bacteria.
A powerful, thick, easily organized cell wall, which is dominated by peptidoglycan and teichoic acids, no lipopolysaccharides (LPS), and often no diaminopimelic acid.
Features of the cell wall of gram-negative bacteria.
The cell wall is much thinner than that of gram-positive bacteria, contains LPS, lipoproteins, phospholipids, diaminopimelic acid. The structure is more complicated - there is an outer membrane, so the cell wall is three-layered.
When processing gram-positive bacteria with enzymes that destroy peptidoglycan, structures completely devoid of the cell wall arise - protoplasts... Treatment of gram-negative bacteria with lysozyme destroys only the peptidoglycan layer without completely destroying the outer membrane; such structures are called spheroplasts... Protoplasts and spheroplasts have a spherical shape (this property is associated with osmotic pressure and is characteristic of all cellless forms of bacteria).
L- forms of bacteria.
Under the influence of a number of factors that adversely affect the bacterial cell (antibiotics, enzymes, antibodies, etc.), occurs L- transformation bacteria, leading to permanent or temporary loss of the cell wall. L-transformation is not only a form of variability, but also adaptation of bacteria to unfavorable conditions of existence. As a result of changes in antigenic properties (loss of O- and K-antigens), a decrease in virulence and other factors, L-forms acquire the ability to stay for a long time ( persist) in the host's body, maintaining a sluggish current infectious process. The loss of the cell wall makes the L-form insensitive to antibiotics, antibodies and various chemotherapy drugs, the point of application of which is the bacterial cell wall. Unstable L-shape capable reverse into the classic (original) forms of bacteria with a cell wall. There are also stable L-forms of bacteria, the absence of a cell wall and the inability to reverse them into classical forms of bacteria are genetically fixed. In a number of ways, they very much resemble mycoplasmas and others. mollicuts- bacteria in which the cell wall is absent as a taxonomic feature. Microorganisms belonging to mycoplasmas are the smallest prokaryotes, do not have a cell wall and, like all bacterial wallless structures, have a spherical shape.
To the surface structures of bacteria(optional, like the cell wall) include capsule, flagella, microvilli.
Capsule or a mucous layer surrounds the shell of a number of bacteria. Allocate microcapsule detected by electron microscopy in the form of a layer of microfibrils, and macrocapsule detected by light microscopy. The capsule is a protective structure (primarily from drying out), in a number of microbes it is a pathogenic factor, prevents phagocytosis, inhibits the first stages of defense reactions - recognition and absorption. Have saprophytes capsules are formed in the external environment, in pathogens - more often in the host's body. There are a number of methods for coloring capsules, depending on their chemical composition. The capsule often consists of polysaccharides (the most common color is according to Ginsu), less often from polypeptides.
Flagella. Motile bacteria can be gliding (moving on a hard surface as a result of wave-like contractions) or floating, moving due to filamentous, spirally curved proteins ( flagellin by chemical composition) formations - flagella.
According to the location and number of flagella, a number of forms of bacteria are distinguished.
1.Monotrichs have one polar flagellum.
2. Lophotrichs - have a polarly located bundle of flagella.
3. Amphitrichs - have flagella at diametrically opposite poles.
4. Peritrichus - have flagella along the entire perimeter of the bacterial cell.
The ability for purposeful movement (chemotaxis, aerotaxis, phototaxis) in bacteria is genetically determined.
Fimbriae or cilia- short filaments surrounding the bacterial cell in large numbers, with the help of which bacteria are fixed to substrates (for example, to the surface of mucous membranes). Thus, the fimbriae are factors of adhesion and colonization.
F- drank (fertility factor)- apparatus conjugation of bacteria, are found in small quantities in the form of thin proteinaceous villi.
Endospores and sporulation.
Spore formation- a way to preserve certain types of bacteria in adverse environmental conditions. Endospores are formed in the cytoplasm, are cells with low metabolic activity and high resistance ( resistance) to drying, the action of chemical factors, high temperature and other undesirable environmental factors. Light microscopy often uses a spore detection method. by Ozheshko... High resistance is associated with a high content of calcium salt of dipicolinic acid in the shell of a dispute. The location and size of spores in different microorganisms is different, which has differential diagnostic (taxonomic) significance. The main phases of the "life cycle" of spores sporulation(includes the preparatory stage, the pre-dispute stage, the formation of the shell, maturation and dormancy) and germination ending in the formation of a vegetative form. The process of sporulation is genetically determined.
Non-cultivated forms of bacteria.
In many species of gram-negative bacteria that do not form spores, there is a special adaptive state - uncultivated forms. They have low metabolic activity and do not actively reproduce, i.e. do not form colonies on solid nutrient media, are not detected during sowing. They are highly resistant and can remain viable for several years. They are not detected by classical bacteriological methods, they are detected only using genetic methods ( polymerase chain reaction - PCR).
To study the structure of a bacterial cell, along with a light microscope, electron microscopic and microchemical studies are used to determine the ultrastructure of a bacterial cell.
A bacterial cell (Fig. 5) consists of the following parts: a three-layer membrane, cytoplasm with various inclusions, and a nuclear substance (nucleoid). Additional structural formations are capsules, spores, flagella, pili.
Rice. 5. Schematic representation of the structure of a bacterial cell. 1 - shell; 2 - mucous layer; 3 - cell wall; 4 - cytoplasmic membrane; 5 - cytoplasm; 6 - ribosome; 7 - polysome; 8 - inclusions; 9 - nucleoid; 10 - flagellum; 11 - drank
Shell the cell consists of the outer mucous layer, the cell wall and the cytoplasmic membrane.
The mucous capsular layer is located outside the cell and performs a protective function.
The cell wall is one of the main structural elements of the cell, retaining its shape and separating the cell from the environment. An important property of the cell wall is selective permeability, which ensures the penetration of essential nutrients (amino acids, carbohydrates, etc.) into the cell and the removal of metabolic products from the cell. The cell wall maintains a constant osmotic pressure inside the cell. The strength of the wall is provided by murein, a substance of a polysaccharide nature. Some substances destroy the cell wall, such as lysozyme.
Bacteria that are completely devoid of a cell wall are called protoplasts. They retain the ability to breathe, divide, and synthesize enzymes; to the effects of external factors: mechanical damage, osmotic pressure, aeration, etc. Protoplasts can be preserved only in hypertonic solutions.
Bacteria with a partially destroyed cell wall are called spheroplasts. If you suppress the synthesis of the cell wall with the help of penicillin, then L-forms are formed, which in all types of bacteria are spherical large and small cells with vacuoles.
The cytoplasmic membrane adheres tightly to the cell wall from the inside. It is very thin (8-10 nm) and consists of proteins and phospholipids. This is a semi-permeable boundary layer through which the cell is nourished. The membrane contains permease enzymes, which carry out active transport of substances, and respiratory enzymes. The cytoplasmic membrane forms mesosomes that take part in cell division. When a cell is placed in a hypertonic solution, the membrane can separate from the cell wall.
Cytoplasm- the internal contents of the bacterial cell. It is a colloidal system consisting of water, proteins, carbohydrates, lipids, various mineral salts. The chemical composition and consistency of the cytoplasm change depending on the age of the cell and environmental conditions. The cytoplasm contains nuclear matter, ribosomes and various inclusions.
Nucleoid, nuclear substance of a cell, its hereditary apparatus. The nuclear substance of prokaryotes, unlike eukaryotes, does not have its own membrane. The nucleoid of a mature cell is a double strand of DNA coiled into a ring. The DNA molecule encodes the genetic information of the cell. In genetic terminology, the nuclear substance is called the genophore or genome.
Ribosomes are located in the cytoplasm of the cell and perform the function of protein synthesis. The ribosome contains 60% RNA and 40% protein. The number of ribosomes in a cell reaches 10,000. Joining together, ribosomes form polysomes.
Inclusions - granules containing various reserve nutrients: starch, glycogen, fat, volutin. They are located in the cytoplasm.
Bacterial cells in the process of vital activity form protective organelles - capsules and spores.
Capsule- the outer thickened mucous layer adjacent to the cell wall. This is a protective organ that appears in some bacteria when they enter the body of humans and animals. The capsule protects the microorganism from the protective factors of the body (causative agents of pneumonia and anthrax). Some microorganisms have a permanent capsule (Klebsiella).
Controversy found only in rod-shaped bacteria. They are formed when a microorganism enters unfavorable environmental conditions (high temperatures, drying, pH change, a decrease in the amount of nutrients in the environment, etc.). Spores are located inside the bacterial cell and represent a compacted area of the cytoplasm with a nucleoid, dressed with its own dense membrane. In chemical composition, they differ from vegetative cells in a small amount of water, an increased content of lipids and calcium salts, which contributes to the high resistance of spores. Spore formation occurs within 18-20 hours; when a microorganism enters favorable conditions, the spore germinates into a vegetative form within 4-5 hours. In a bacterial cell, only one spore is formed, therefore, spores are not reproductive organs, but serve to experience unfavorable conditions.
Spore-forming aerobic bacteria are called bacilli, and anaerobic bacteria are called clostridia.
The spores vary in shape, size, and position in the cage. They can be located centrally, subterminally and terminal (Fig. 6). In the causative agent of anthrax, the spore is located centrally, its size does not exceed the diameter of the cell. The spore of the causative agent of botulism is located closer to the end of the cell - subterminal and exceeds the width of the cell. In the causative agent of tetanus, a rounded spore is located at the end of the cell - terminally and significantly exceeds the width of the cell.
Flagella- organs of movement, characteristic of rod-shaped bacteria. These are thin filamentous fibrils composed of a protein called flagellin. Their length significantly exceeds the length of the bacterial cell. Flagella depart from the basal body located in the cytoplasm and emerge onto the cell surface. Their presence can be detected by determining the mobility of cells under a microscope, in a semi-liquid nutrient medium or by staining with special methods. The ultrastructure of the flagella was studied using an electron microscope. According to the location of the flagella, bacteria are divided into groups (see Fig. 6): monotrichs - with one flagellum (the causative agent of cholera); amphitrichs - with bundles or single flagella at both ends of the cell (spirilla); lofotrichi - with a bundle of flagella at one end of the cell (fecal alkali-forming agent); peritrichous - flagella are located over the entire surface of the cell (intestinal bacteria). The speed of movement of bacteria depends on the number and location of flagella (monotrichs are the most active), on the age of bacteria and the influence of environmental factors.
Rice. 6. Variants of the arrangement of spores and flagella in bacteria. I - disputes: 1 - central; 2 - subterminal; 3 - terminal; II - flagella: 1 - monotrichs; 2 - amphitrichs; 3 - lophotrichs; 4 - peritrichs
Drank or fimbria- villi located on the surface of bacterial cells. They are shorter and thinner than flagella and also have a spiral structure. Drank consists of protein - pilin. Some drank (there are several hundred of them) serve to attach bacteria to the cells of animals and humans, while others (single) are associated with the transfer of genetic material from cell to cell.
Mycoplasma
Mycoplasmas are cells that do not have a cell wall, but are surrounded by a three-layer lipoprotein cytoplasmic membrane. Mycoplasmas can be spherical, oval, in the form of filaments and stars. Mycoplasmas, according to Berga's classification, are separated into a separate group. Nowadays, more and more attention is paid to these microorganisms as causative agents of inflammatory diseases. Their sizes are different: from a few micrometers to 125-150 nm. Small mycoplasmas pass through bacterial filters and are called filterable forms.
Spirochetes
Spirochetes (see Fig. 52) (from Lat. Speira - bend, chaite - hair) are thin, convoluted, mobile unicellular organisms ranging in size from 5 to 500 microns in length and 0.3-0.75 microns in width. With the simplest, they are related by the method of movement by reducing the internal axial filament, consisting of a bundle of fibrils. The nature of the movement of the spirochetes is different: translational, rotational, flexion, wavy. The rest of the cell structure is typical for bacteria. Some spirochetes are weakly stained with aniline dyes. Spirochetes are divided into genera according to the number and shape of the thread curls and its end. In addition to saprophytic forms common in nature and the human body, among the spirochetes there are pathogens - causative agents of syphilis and other diseases.
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According to scientists, bacteria are more than 3.5 billion years old. They existed on Earth long before the appearance of highly organized organisms. Being at the origins of life, bacterial organisms received an elementary structure according to the prokaryotic type, characterized by the absence of a formed nucleus and nuclear envelope. One of the factors that influenced the formation of their biological properties is the bacterial membrane (cell wall).
The bacterial wall has several fundamental functions:
Certain types of bacteria have an outer capsule that is durable and serves to preserve the integrity of the microorganism for a long time. In this case, the membrane in bacteria is an intermediate form between the cytoplasm and the capsule. Some bacteria (for example, leukonostok) have the peculiarity of enclosing several cells in one capsule. This is called a zoogel.
The chemical composition of the capsule is characterized by the presence of polysaccharides and a large amount of water. The capsule can also allow the bacteria to attach to a specific object.
The degree of its assimilation by the bacteria depends on how easily the substance penetrates through the membrane. Molecules with long chain sections that are resistant to biodegradation are more likely to penetrate.
The bacterial membrane consists of lipopolysaccharides, proteins, lipoproteins, teichoic acids. The basic component is murein (peptidoglycan).
The thickness of the cell wall can be different and reach 80 nm. The surface is not solid, it has pores of various diameters through which the microbe receives nutrients and excretes the products of its vital activity.
The importance of the outer wall is evidenced by its significant weight - it can vary from 10 to 50% of the dry mass of the entire bacterium. The cytoplasm can bulge, changing the external relief of the bacteria.
From above, the shell can be covered with cilia or flagella can be located on it, which consist of flagellin, a specific protein substance. For attachment to the bacterial membrane, flagella have special structures - flat discs. Bacteria with one flagellum are called monotrichs, with two - amphitrichs, with a bundle - lophotrichs, with many bundles - peritrichs. Microorganisms that do not have flagella are called atrichia.
The cell membrane has an inner part, which begins to form after the completion of cell growth. Unlike outside, it consists of much less water and has greater elasticity and strength.
The process of synthesis of the walls of microorganisms begins inside the bacteria. For this, it has a network of polysaccharide complexes that alternate in a certain sequence (acetylglucosamine and acetylmuramic acid) and are linked by strong peptide bonds. The assembly of the wall is carried out outside, on the plasma membrane, where the shell is located.
Since the bacterium does not have a nucleus, it does not have a nuclear envelope either.
The envelope is an unstained thin structure, which cannot even be seen without a special staining of the cells. For this, plasmolysis and a darkened field of view are used.
To study the detailed structure of a cell in 1884, Christian Gram proposed a special way of coloring it, which was later named after him. Gram staining divides all microorganisms into gram-positive and gram-negative. Each species has its own biochemical and biological properties. Different coloration is also due to the structure of the cell wall:
The specific gravity of gram-positive microbes that are harmless to humans is much higher than gram-negative ones. To date, three groups of gram-negative microorganisms have been classified that cause diseases in humans:
Between the bacterial wall and the cytoplasmic membrane is the periplasmic space, which consists of enzymes. This component is an obligatory structure, it makes up 10-12% of the dry mass of the bacteria. If the membrane collapses for some reason, the cell dies. Genetic information is located directly in the cytoplasm, not separated from it by the nuclear envelope.
Regardless of whether the microbe is gram-positive or gram-negative, it is the osmotic barrier of the microorganism, the transporter of organic and inorganic molecules deep into the cell. The definite role of periplasm in the growth of a microorganism has also been proven.
Bacteria, despite their apparent simplicity, have a well-developed cell structure that is responsible for many of their unique biological properties. Many structural details are unique to bacteria and are not found among archaea or eukaryotes. However, despite the relative simplicity of bacteria and the ease of growing individual strains, many bacteria cannot be grown in laboratory conditions, and their structures are often too small to study. Therefore, although some of the principles of bacterial cell structure are well understood and even applied to other organisms, most of the unique features and structures of bacteria are still unknown.
Most bacteria are either spherical in shape, the so-called coca (from the Greek word kókkos- grain or berry), or rod-shaped, the so-called bacilli (from the Latin word bacillus- stick). Some rod-shaped bacteria (vibrios) are slightly bent, while others form spiral curls (spirochetes). All this variety of forms of bacteria is determined by the structure of their cell wall and cytoskeleton. These forms are important for bacteria to function because they can interfere with the bacteria's ability to get nutrients, attach to surfaces, move around, and escape predators.
Bacteria can come in a wide variety of shapes and sizes (or morphologies). Bacterial cells are usually 10 times smaller in size than eukaryotic cells, of course only 0.5-5.0 microns in their largest size, although giant bacteria such as Thiomargarita namibiensis and Epulopiscium fishelsoni, can grow up to 0.5 mm in size and be visible to the naked eye. The smallest free-living bacteria are mycoplasmas, members of the genus Mycoplasma, only 0.3 µm in length, roughly the size of the largest viruses.
Small size is important for bacteria because it results in a large surface area-to-volume ratio, aids in the rapid transport of nutrients and the release of waste. Low surface area to volume ratios, on the other hand, limit the metabolic rate of the microbe. The reason for the large cells' existence is unknown, although it seems that the large volume is primarily used to store additional nutrients. However, there is also the smallest size of free-living bacteria. According to theoretical calculations, a spherical cell with a diameter of less than 0.15-0.20 microns becomes incapable of independent reproduction, since it physically does not fit all the necessary biopolymers and structures in sufficient quantities. Nanobacteria (and similar nanobes and ultramicrobacteria), smaller than the "permissible" size, although the existence of such bacteria is still questionable. They, unlike viruses, are capable of independent growth and reproduction, but they require a number of nutrients that they cannot synthesize from the host cell.
As in other organisms, the bacterial cell wall provides the structural integrity of the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor caused by much higher concentrations of proteins and other molecules inside the cell compared to those around it. The bacterial cell wall differs from the wall of all other organisms by the presence of peptidoglycan (the role of N-acetylglucosamine and N-acetomuramic acid), which is located directly outside the cytoplasmic membrane. Peptidoglycan is responsible for the stiffness of the bacterial cell wall and, in part, for determining the shape of the cell. It is relatively porous and does not resist the penetration of small molecules. Most bacteria have cell walls (with a few exceptions, such as mycoplasma and related bacteria), but not all cell walls have the same structure. There are two main types of bacterial cell walls, gram-positive and gram-negative bacteria, which are distinguished by Gram staining.
The cell wall of Gram-positive bacteria is characterized by the presence of a very thick layer of peptidoglycan, which is responsible for the loss of gentian violet dye during the Gram staining procedure. Such a wall is found exclusively in organisms belonging to the types Actinobacteria (or gram-positive bacteria with a high% G + C content) and Firmicutes (or gram-positive bacteria with a low% G + C content). The bacteria in the Deinococcus-Thermus group can also stain positively according to Gram, but contain some of the cell wall structures typical of gram-negative organisms. In the cell wall of gram-positive bacteria, there are embedded polyalcohols called tehoic acid, some of which are bound to lipids to form lipotechoevic acid. Since lipotechoevic acids bind covalently to lipids within the cytoplasmic membrane, they are responsible for binding peptidoglycan to the membrane. Tehoic acid provides Gram-positive bacteria with a positive electrical effect due to phosphodiesteric bonds between the tehoic acid monomers.
Unlike gram-positive bacteria, gram-negative bacteria contain a very thin layer of peptidoglycan, which is responsible for the inability of the cell walls to contain the crystal violet dye during the Gram staining procedure. In addition to the peptidoglycan layer, gram-negative bacteria have a second, so-called outer membrane, located outside the cell wall and assembling phospholipids and lipopolysaccharide on their outer side. The negatively charged Lipopolysaccharide also provides the cell with a negative electrical charge. Chemical structure The lipopolysaccharide of the outer membrane is often unique to individual strains of bacteria and is often responsible for the reaction of antigens with representatives of these strains.
Like any double layer of phospholipids, the outer membrane is sufficiently impermeable to all charged molecules. However, protein channels (plunge) are present in the outer membrane, permitting the passive transport of many ions, sugars and amino acids across the outer membrane. Thus, these molecules are present in the periplasmic, the layer between the outer and cytoplasmic membranes. The periplasmic layer contains a layer of peptidoglycan and many proteins, which are responsible for hydrolysis and reception of extracellular signals. It reads that perivlasma is gel-like, not liquid, due to its high protein and peptidoglycan content. Signals and life-giving substances from the periplasmic enter the cytoplasm of the cell using transport proteins in the cytoplasmic membrane.
The bacterial cyoplasmic membrane is composed of a double layer of phospholipids, and therefore has all the common functions of the cytoplasmic membrane, acting as a permeability barrier for most molecules and enclosing transport proteins that regulate the transport of molecules into cells. In addition to these functions, energy cycle reactions also take place on the bacterial cytoplasmic membranes. Unlike eukaryotes, bacterial membranes (with some exceptions, such as in mycoplasmas and methanotrophs) generally do not contain sterols. However, many bacteria contain structurally related compounds called hopanoids, presumably serving the same function. Unlike eukaryotes, bacteria can have a wide variety of fatty acids in their membranes. Together with typical saturated and unsaturated fatty acids, bacteria can contain fatty acids with additional methyl, hydroxy, or even cyclic groups. The bacterium can adjust the relative proportions of these fatty acids to maintain optimal membrane fluidity (for example, with changes in temperature).
Villi and fimbria (pili, fimbriae)- surface structures of bacteria oriental in structure. At first, these terms were introduced separately, but now such structures are classified as type I, IV villi and genital villi, but many other types remain unclassified.
Sexual villi are very long (5-20 microns) and are present on the bacterial cell in small numbers. They are used for DNA exchange in bacterial conjugation.
The villi or fimbriae of type I are short (1-5 microns), extend from the outer membrane in many directions, are tubular, present in the limbs of the Proteobacteria type. These villi are commonly used to attach to a surface.
The villi or fimbriae of type IV are of medium length (about 5 microns), located at the poles of the bacteria. Type IV villi help to attach to surfaces (eg, during biofilm formation), or to other cells (eg, animal cells during pathogenesis)). Some bacteria (eg Myxococcus) use type IV villi as a movement mechanism.
On the surface, outside the peptidiglycan layer or outer membrane, the protein S-layer is often located. Although the function of this layer is not fully understood, it is believed that this layer provides chemical and physical protection to the cell surface and may serve as a macromolecular barrier. It is also believed that the S-layers can have other functions, for example, they can serve as factors of pathogenicity in Campylobacter and contain external enzymes in Bacillus stearothermophilus.
Many bacteria secrete extracellular polymers outside of their cell walls. These polymers are usually composed of polysaccharides and sometimes proteins. Capsules are relatively impermeable structures that cannot be dyed with many dyes. They are generally used for adhering bacteria to other cells or inanimate surfaces in biofilm formation. They have a different structure from the disorganized mucous layer of cellular polymers to highly structured membrane capsules. Sometimes these structures are involved in protecting cells from being absorbed by eukaryotic cells such as macrophages. Also, the secretion of mucus has a signal function for slow-moving bacteria and, possibly, is used directly for the movement of bacteria.
Perhaps the most easily recognizable extracellular structures of a bacterial cell are flagella. Bacterial flagella are filamentous structures that actively rotate around their axis using a flagellar motor and are responsible for the movement of many bacteria in a liquid medium. The location of the flagella depends on the type of bacteria and is of several types. Cell flagella are complex structures composed of many proteins. The filament itself is composed of include flagellin (FlaA), which forms a tubular filament. The basal motor is a large protein complex that encloses the cell wall and both membranes (if any) to form a rotational motor. This motor is driven by an electric potential across the cytoplasmic membrane.
In addition, specialized secretion systems are located in the cytoplasmic membrane and cell membrane, the structure of which depends on the type of bacteria.
In comparison with eukaryotes, the intracellular structure of a bacterial cell is somewhat simpler. Bacteria contain almost no membrane organelles, like eukaryotes. Of course, the chromosome and ribosomes are the only easily visible intracellular structures found in all bacteria. Although some groups of bacteria contain complex specialized intracellular structures, some of them are discussed below.
The entire interior of a bacterial cell within the inner membrane is called the cytoplasm. The homogeneous fraction of the cytoplasm containing a set of soluble RNA, proteins, products and substrates of metabolic reactions is called the cytosol. Another part of the cytoplasm is represented by various structural elements, including the chromosome, ribosomes, bacterial cytoskeleton, and others. Until recently, it was believed that bacteria do not have a cytoskeleton, but now bacteria have found orthologs or even homologues of all types of eukaryotic filaments: microtubules (FtsZ), actin (MreB and ParM), and intermediate filaments (Crescentin). The cytoskeleton has many functions, often being responsible for cell shape and intracellular transport.
Unlike eukaryotes, the bacterial chromosome is not located in the inner part of the membrane-limited nucleus, but is located in the cytoplasm. This means that the transfer of cellular information through the processes of translation, transcription and replication occurs within the same compartment and its components can interact with other structures of the cytoplasm, in particular, ribosomes. The unpackaged bacterial chromosome uses histones like eukaryotes, but instead exists in a compact supercoiled structure called a nucleoid. The bacterial chromosomes themselves are circular, although there are examples of linear chromosomes (for example, in Borrelia burgdorferi). Along with chromosomal DNA, most bacteria also contain small, independent pieces of DNA called plasmids, which often encode individual proteins that are beneficial but not essential to the host bacterium. Plasmids can be easily acquired or lost by bacteria and can be transferred between bacteria as a form of horizontal gene transfer.
In most bacteria, the numerous intracellular structures of the ribosome are the site of protein synthesis in all living organisms. The ribosomes of bacteria are also somewhat different from the ribosomes of eukaryotes and archaea and have a sedimentation constant of 70S (as opposed to 80S in eukaryotes). Although ribosomes are the most common intracellular protein complex in bacteria, sometimes other large complexes are observed using electron microscopy, although in most cases their purpose is unknown.
One of the main differences between a bacterial cell and a eukaryotic cell is the absence of a nuclear membrane and, often, the absence of membranes at all inside the cytoplasm. Many important biochemical reactions, such as energy cycle reactions, occur through ionic gradients across membranes, creating a potential difference like a battery. The absence of internal membranes in bacteria means that these reactions, such as electron transfer in reactions of the electron transport chain, occur across the cytoplasmic membrane, between the cytoplasm and the periplasmic. However, in some photosynthetic bacteria, there is a developed network of cytoplasmic-derived photosynthetic membranes. In purple bacteria (for example, Rhodobacter) they have retained a connection with the cytoplasmic membrane, it is easily detected on sections under an electron microscope, but in cyanobacteria this connection is either difficult to be found or lost in the process of evolution.
Some bacteria form intracellular granules to store nutrients such as glycogen, polyphosphate, sulfur, or polyhydroxyalkanoates, allowing bacteria to store these nutrients for later use.
Gas vesicles are spindle-shaped structures found in some planktonic bacteria that provide buoyancy to the cells of these bacteria, reducing their overall density. They consist of a protein shell, which is very impermeable to water, but penetrates most gases. By adjusting the amount of gas present in its gas vesicles, bacteria can increase or decrease their total density and thus move up or down within the water column, maintaining themselves in an environment optimal for growth.
Carboxysomes are intracellular structures found in many autotrophic bacteria such as Cyanobacteria, nitrous bacteria, and Nitrobacteria. These are protein structures that resemble the heads of viral particles in morphology, and contain enzymes for fixing carbon dioxide in these organisms (especially ribulose-bisphosphate-carboxylase / oxygenase, RuBisCO, and carbonic anhydrase). It is believed that the high local concentration of enzymes together with the rapid conversion of bicarbonate to carbonic acid carbonic anhydrase allows faster and more efficient fixation of carbon dioxide than is possible within the cytoplasm.
It is known that such structures contain the coenzyme B12-containing glycerol dehydratase, a key enzyme in the fermentation of glycerol to 1,3-propanediol in some members of the Enterobacteriaceae family (for example Salmonella).
A well-known class of membrane organelles of bacteria, which are more reminiscent of eukaryotic organelles, but possibly also associated with the cytoplasmic membrane, are magnetosomes, which are present in magnetotactic bacteria.
With the participation of bacteria, fermented milk products (kefir cheeses) otsotic acid are obtained. Certain groups of bacteria are used to make antibiotics and vitamins. Used for pickling cabbage and tanning leather. And in agriculture, bacterii are used for the manufacture and storage of green animal feed.
Bacterii can spoil food. By settling in products, they produce toxic substances for both humans and animals. If serum is NOT applied in a timely manner and the drugs are poisoned, a person may die! Therefore, be sure to wash vegetables and fruits before use!
Some bacteria such as Firmicutes are capable of forming endospores, allowing them to withstand extreme environmental and chemical conditions (for example, gram-positive Bacillus, Anaerobacter, Heliobacterium and Clostridium). In almost all cases, one endospora is formed, so this is not a reproduction process, although Anaerobacter can form up to seven endospores per cell. Endospores have a central nucleus composed of cytoplasm containing DNA and ribosomes, surrounded by a cork layer and protected by an impenetrable and rigid membrane. Endospores show no metabolism and can withstand extreme physical and chemical pressures such as high levels of ultraviolet radiation, gamma rays, detergents, disinfectants, heat, pressure and drying. In such an inactive state, these organisms, in some cases, can remain viable for millions of years and survive even in outer space. Endospores can cause disease, for example anthrax can be caused by inhalation of endospores Bacillus anthracis.
Methane oxidizing bacteria in the genus Methylosinus also form spores resistant to drying, the so-called exospores, because they are formed by budding at the end of the cell. Exospores do not contain diaminopicolinic acid, a characteristic component of endospores. Cysts are other inactive, thick-walled structures formed by members of the genus Azotobacter, Bdellovibrio (bdelocysts), and Myxococcus (mixospores). They are resistant to drying and other hazards, but to a lesser extent than endopores. When cysts are formed by representatives Azotobacter, cell division ends with the formation of a thick multilayer wall and a shell surrounding the cell. Filamentous Actinobacteria form reproductive spores of two categories: air conditioners, which are chains of spores formed from filament mycelium, and sporangiespores, which are formed in specialized pouches, sporangia.
From the point of view of modern science, prokaryotes have a primitive structure. But it is precisely this "simplicity" that helps them survive in the most unexpected conditions. For example, in hydrogen sulphide sources or at nuclear test sites. Scientists have calculated that the total mass of all terrestrial microorganisms is 550 billion tons.
Bacteria are unicellular... This does not mean, however, that bacterial cells fall behind the cells of animals or plants. Microbiology already has knowledge of hundreds of thousands of microbial species. Nevertheless, representatives of science discover new types and features of them every day.
It is no wonder that for the full development of the Earth's surface, microorganisms have to take various forms:
The size of bacteria is measured in nanometers and micrometers. Their average value is 0.8 microns. But among them there are giant prokaryotes, reaching 125 microns and more. The real giants among the midgets are the spirochetes 250 microns long. Now compare with them the size of the smallest prokaryotic cell: mycoplasmas "grow" quite a bit and reach 0.1-0.15 microns in diameter.
It is worth saying that giant bacteria do not so easily survive in the environment. They find it difficult to find enough nutrients for themselves to successfully perform their function. But on the other hand, they are not easy prey for bacteria-predators, which feed on their counterparts - single-celled microorganisms, "flowing around" and eating them.
Under unfavorable environmental conditions, bacteria form a capsule. The microcapsule fits snugly against the wall. It can only be seen with an electron microscope. The macrocapsule is often formed by pathogenic microbes (pneumococci). In Klebsiella pneumonia, a macrocapsule is always found.
The capsule-like membrane is a formation that is loosely associated with the cell wall. Thanks to bacterial enzymes, the capsule-like shell is covered with carbohydrates (exopolysaccharides) of the external environment, which ensures the adhesion of bacteria with different surfaces, even completely smooth ones. For example, streptococci, entering the human body, are capable of sticking to teeth and heart valves.
The functions of the capsule are diverse:
They provide immersion and ascent. In the soil, the bacterial cell moves along the soil channels.
The entire contents of a cell, with the exception of the nucleus and the cell wall, are called 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). Many interesting details of the cell structure have been revealed with the help of an electron microscope.
The outer lipoprotein layer of the protoplast of bacteria, which has special physical and chemical properties, is called the cytoplasmic membrane. All vital structures and organelles are located inside the cytoplasm. The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products outside. Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes.
In addition, the synthesis of some constituent parts of the cell occurs in the membrane, mainly the components of the cell wall and capsule. Finally, the cytoplasmic membrane contains the most important enzymes (biological catalysts). The ordered arrangement of enzymes on membranes allows you to regulate their activity and prevent the destruction of some enzymes by others. Ribosomes are associated with the membrane - structural particles on which protein is synthesized. The membrane consists of lipoproteins. It is strong enough and can provide the temporary existence of a cell without a shell. The cytoplasmic membrane accounts for up to 20% of the dry mass of the cell.
In electronic photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75A thick, consisting of a light layer (lipids) enclosed between two darker ones (proteins). Each layer is 20-30A wide. Such a membrane is called elementary.
The cytoplasm of bacterial cells often contains granules of various shapes and sizes. However, their presence cannot be considered as some kind of permanent sign of a microorganism, usually it is largely associated with the physical and chemical conditions of the environment.
Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These storage substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body is placed in conditions that are less favorable in terms of nutrition.
In many bacteria, granules are composed of starch or other polysaccharides - glycogen and granulose. In some bacteria, when grown in a medium rich in sugars, droplets of fat are found inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules are composed of polymetaphosphate (a storage substance containing phosphoric acid residues). Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria are more likely to accumulate volutin in unusual feeding conditions, such as in a sulfur-free environment. Sulfur droplets are found in the cytoplasm of some sulfur bacteria.
There is a connection between the plasma membrane and the cell wall in the form of desmoses - bridges. The cytoplasmic membrane often gives invagination - invagination 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 membrane sacs. These structures perform a wide variety of functions in bacteria. Some of these structures are mitochondrial analogs.
Others perform the functions of the endoplasmic reticulum or the Golgi apparatus. The photosynthetic apparatus of bacteria is also formed by invagination of the cytoplasmic membrane. After the invagination of the cytoplasm, the membrane continues to grow and forms stacks, which, by analogy with plant chloroplast granules, are called thylakoid stacks. In these membranes, which often fill most of the cytoplasm of a bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes (cytochromes) that carry out the process of photosynthesis are localized.
Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have its analogue - the "nuclear equivalent" - the nucleoid, which is an evolutionarily more primitive form of organization of nuclear matter. It consists of one closed in a ring double-stranded DNA strand with a length of 1.1-1.6 nm, which is considered as a single bacterial chromosome, or genophore. The nucleoid in prokaryotes is not separated from the rest of the cell by a membrane - it does not have a nuclear envelope.
The structure of the nucleoid includes RNA polymerase, basic proteins and no histones; the chromosome is fixed on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosomes. The bacterial chromosome replicates in a polyconservative way: the parental DNA double helix unwinds and a new complementary strand is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the divergence of daughter nuclei is provided by the growth of the cytoplasmic membrane.
The bacterial nucleus is a differentiated structure. Depending on the stage of development of the cell, the nucleoid can be discrete (discontinuous) and consist of separate fragments. This is due to the fact that the division of a bacterial cell in time occurs after the completion of the replication cycle of the DNA molecule and the formation of daughter chromosomes.
The nucleoid contains the bulk of the genetic information of the bacterial cell. In addition to the nucleoid, extrachromosomal genetic elements - plasmids, represented by small circular DNA molecules capable of autonomous replication, were found in the cells of many bacteria.
Plasmids are autonomous molecules, coiled, double-stranded DNA. Their mass is much less than the mass of a nucleotide. Despite the fact that hereditary information is encoded in the DNA of plasmids, they are not vital and necessary for the bacterial cell.
The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cell. Ribosomes are made up of RNA and protein. In bacteria, many ribosomes are located freely in the cytoplasm, some of them may be associated with membranes.
Ribosomes are the centers of protein synthesis in the cell. Moreover, they often join together, forming aggregates called polyribosomes or polysomes.
Inclusions are metabolic products of nuclear and non-nuclear cells. They represent a supply of nutrients: glycogen, starch, sulfur, polyphosphate (valutin), etc. Inclusions often, when colored, acquire a different appearance than the color of the dye. Values can be used to diagnose diphtheria bacillus.
Since a bacterium is a prokaryotic microorganism, many organelles are always absent in bacterial cells, which are inherent in eukaryotic organisms:
It is also worth remembering that bacteria do not have a cell wall, therefore, processes such as pinocytosis and phagocytosis cannot proceed.
As a special microorganism, bacteria are adapted to exist in conditions where oxygen may be absent. And the very same breathing in them occurs due to the mesosomes. It is also very interesting that green organisms are able to photosynthesize in exactly the same way as plants. But it is important to take into account that in plants the process of photosynthesis occurs in chloroplasts, and in bacteria, on membranes.
Reproduction in a bacterial cell occurs in the most primitive way. A mature cell divides in two, they reach maturity after a while, and this process is repeated. In favorable conditions, a change of 70-80 generations can occur per day. It is important to remember that bacteria, due to their structure, do not have access to such methods of reproduction as mitosis and meiosis. They are inherent only in eukaryotic cells.
It is known that the formation of spores is one of several ways of propagation of fungi and plants. But bacteria also know how to form spores, which is inherent in few of their species. They have this ability in order to survive particularly adverse conditions that can be dangerous to their lives.
Such species are known that are able to survive even in space. This can not be repeated by any living organisms. Bacteria became the progenitors of life on Earth due to their simplicity of structure. But the fact that they exist to this day shows how important they are for the world around us. With their help, people can get as close as possible to the answer to the question of the origin of life on Earth, constantly studying bacteria and learning something new.
Staphylococcus aureus (Staphylococcus aureus) is a common bacteria that affects about 30 percent of all people. In some people, it is part of the microbiome (microflora) and is found both inside the body and on the skin or mouth. While there are harmless strains of Staphylococcus aureus, others, such as Methicillin-resistant Staphylococcus aureus, pose serious health problems, including skin infections, cardiovascular disease, meningitis, and diseases of the digestive system.
Researchers at Vanderbilt University found that staphylococcus bacteria prefer human blood over animal blood. These bacteria are partial to iron, which is found in hemoglobin found in red blood cells. Staphylococcus aureus ruptures blood cells to reach the iron inside them. It is believed that genetic variations in hemoglobin may make some people more desirable to Staphylococcus bacteria than others.
The researchers found that bacteria in the atmosphere may play a role in the production of rain and other forms of rainfall. This process begins when bacteria from plants are blown into the atmosphere. At high altitude, ice forms around them and they begin to grow. Once the frozen bacteria reach a certain growth threshold, the ice begins to melt and returns to the ground in the form of rain. The bacteria of the species Psuedomonas syringae have even been found in the center of large hail particles. They produce a special protein in cell membranes that allows them to bind water in a unique way, promoting ice formation.
Researchers have found that certain strains of acne-causing bacteria can actually help prevent acne. The bacteria that causes acne, Propionibacterium acnes, lives in the pores of our skin. When these bacteria provoke an immune response, the area on the skin swells and pimples form.
However, certain strains of bacteria have been found to be less likely to cause acne. These strains may be the reason why people with healthy skin rarely get acne. By studying the genes of the Propionibacterium acnes strains collected from people with acne and healthy skin, the researchers identified a strain that was common on clear skin and rarely found on skin with acne. Future research will include attempts to develop a drug that only kills the acne-causing strains of the bacteria Propionibacterium acnes.
Who would have thought that brushing your teeth regularly could help prevent heart disease? Earlier studies have found a link between gum disease and cardiovascular disease. Now scientists have found a specific link between these diseases.
Both bacteria and humans are thought to produce certain types of proteins called stress proteins. These proteins are formed when cells experience various types of stressful conditions. When a person has a gum infection, the cells of the immune system begin to attack the bacteria. Bacteria produce stress proteins when attacked, and white blood cells also attack stress proteins.
The problem is that white blood cells cannot differentiate between stress proteins produced by bacteria and those produced by the body. As a result, the cells of the immune system also attack the stress proteins produced by the body, which causes the accumulation of white blood cells in the arteries and leads to atherosclerosis. A calcified heart is the leading cause of cardiovascular disease.
Did you know that spending time in the garden or gardening can help you learn better? According to the researchers, the soil bacterium Mycobacterium vaccae can improve learning in mammals.
Probably, these bacteria enter our body by swallowing or breathing. According to scientists, the bacterium Mycobacterium vaccae improves learning by stimulating the growth of neurons in the brain, which leads to an increase in serotonin levels and a decrease in anxiety.
The study was carried out using mice fed with live bacteria, Mycobacterium vaccae. The results showed that mice that ate the bacteria moved the maze much faster and with less anxiety than mice that did not eat the bacteria. Scientists suggest that Mycobacterium vaccae plays a role in improving new challenges and reducing stress levels.
Researchers at Argonne National Laboratory have found that the bacterium Bacillus subtilis has the ability to rotate very small gears. These bacteria are aerobic, meaning they need oxygen to grow and develop. When they are placed in a solution with microbubbles of air, bacteria float in the teeth of the gear and cause it to turn in a specific direction.
It takes several hundred bacteria working in unison to start the gear. It was also found that bacteria can turn several gears connected to each other. The researchers were able to control the speed at which the bacteria turned the gears by adjusting the amount of oxygen in the solution. The decrease in the amount of oxygen led to the slowing down of bacteria. Removing oxygen causes them to stop moving completely.
