GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

5. STRUCTURE OF THE MICROBIAL CELL

5.1. CELL WALLS OF MICROORGANISMS

5.1.1. Surface Structures of the Bacterial Cell Wall

Flagella and motility. Based on their ability to move, all Bacteria are divided into motile and non-motile. In most bacteria, The ability to move is due to the presence of flagella. Gliding bacteria (such as myxobacteria and cyanobacteria) and spirochetes can move without flagella.

The arrangement of flagella in motile eubacteria is a characteristic feature of specific groups and therefore has taxonomic significance. In rod-shaped bacteria, flagella may be arranged polarly or laterally (mono- and bipolar arrangement) (Fig. 5.1). Among bacteria with monopolar flagellation, only a few have a single, thick flagellum (monotrichous) (Vibrio). The MAJORITY of bacteria are polytrichous. Monopolar-polytrichous flagellation is also called lophotrichous (Pseudomonas, Chromatium), and bipolar-polytrichous is called amphitrichous (Spirillum). In peritrichous arrangement (enterobacteria, bacilli), flagella are located along the sides of The Cell or over its entire surface.

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Fig. 5.1. MAIN TYPES OF flagellation and movement patterns in bacteria

Flagella are helically coiled filaments. They vary among different bacteria in thickness (12–18 nm), length (up to 20 µm), as well as in filament length and amplitude. Flagellar filaments consist of a specific protein called flagellin. Flagellin has a Molecular Weight of up to 40,000.

In terms of function, flagellin is often compared to Myosin of Muscle Tissues (a contractile protein), which provides movement. However, unlike most contractile Proteins whose function is associated with ATP Hydrolysis (i.e., using the energy of ATP hydrolysis for movement), the basal body rotates around its axis using energy generated by the proton-motive force. A flagellum consists of three parts: a helical filament, a "hook" near the cell surface, and a basal body. The basal body anchors the flagellum in The Plasma Membrane and Cell wall.

Fimbriae and pili. The surface of some bacteria is covered with A large number (from 10 to several thousand) of long, thin, straight filaments 3–25 nm thick and up to 12 µm long, which are called fimbriae. Fimbriae are found in both flagellated and non-flagellated bacteria. One of the first studies on The chemical composition of fimbriae (in Escherichia coli) was conducted in 1960.

by A. Brinton. Experiments have shown that fimbriae contain Lectins—carbohydrate-binding proteins. Depending on the carbohydrate Specificity of the lectins, there are several types of fimbriae. For instance, the lectins of type 1 fimbriae are mannose-specific, while those of G-fimbriae are specific to N-acetylglucosamine. The Amino Acid Composition of type 1 fimbriae is dominated by dicarboxylic and aliphatic Amino Acids, whereas Sulfur-Containing Amino Acids are present in trace amounts. Fimbrial lectins contain up to 40–50% hydrophobic amino acids (Proline, Alanine, valine, leucine, isoleucine, phenylalanine). Although type 1 fimbriae are produced by a large number of enterobacteria, The amino acid composition of these proteins has been established only for certain strains. The molecular weight of the lectins comprising the fimbriae is approximately 16,000–25,000.

In addition to fimbriae, Cells of many bacteria contain sex pili (F-pili). There are no more than one or two of them per cell. Pili appear as hollow protein tubes with a length of 0.5 to 10 µm. Through sex pili, a male cell attaches to a female cell, forming a conjugation bridge (tunnel) through which DNA is transferred from the donor to the recipient.

Taxes. Taxes (from Greek *taxis* — arrangement) can be positive or negative, depending on whether the bacterium moves toward or away from a stimulus. There are several types of taxes.

Chemotaxis is movement induced by chemical substances. Based on their ability to induce (i.e., trigger or cause) positive or negative chemotaxis, substances are divided into two groups: attractants — substances that cause cells to accumulate in regions of higher compound concentration; repellents — substances that cause cells to accumulate in regions of lower concentrations. It should be noted that not all compounds used by microorganisms as nutrients are attractants.

Aerotaxis is the movement of bacteria toward (or away from) molecular oxygen. In motile bacteria, the type of METABOLISM (aerobic or anaerobic) can be determined by their aerotactic movement and accumulation of cells at specific distances from the coverslip. Under these conditions, strict anaerobes will gather in the center of the slide, aerobes near its edges or near air bubbles, and facultative anaerobes between the aerobes and anaerobes.

Phototaxis is the movement of bacteria driven by light energy. Thus, phototrophic bacteria, which require light to obtain energy, accumulate in illuminated areas As a result of phototaxis. If a preparation in which a cell suspension of Chromatium is evenly distributed under a coverslip is kept in the dark and then a beam of light is directed onto it, the bacteria will gather within the illuminated spot.

Magnetotaxis is the movement of bacteria along the magnetic field lines of the Earth or a magnet, caused by the presence of large amounts of iron (up to 0.4% of dry weight) in the form of ferromagnetic iron oxide within special granules called magnetosomes. Magnetosomes are located near the flagellar attachment sites.

Thermotaxis is the movement of bacteria directed by a heat source.

Viscositaxis is the movement of bacteria toward an increase or decrease in solution viscosity. For example, for Human and Animal parasitic spirochetes that migrate toward the surface of mucous membranes, this property has an adaptive character. The Mechanism of this process has not yet been established.

Capsule and slime layer. The capsule is located on the outer surface of The cell wall. It can be observed under a Light Microscope by staining the preparation with Dyes such as nigrosin, congo red, and India ink, which do not penetrate the capsule. This results in negative staining: the light capsule stands out against a dark Background.

Microcapsules are distinguished by a thickness of up to 0.2 µm. Microcapsules are invisible under a light microscope and can only be detected immunologically (by Swelling when mixed with specific Antibodies). A macrocapsule with a thickness of more than 0.2 µm is clearly visible under a light microscope. The slime layer exceeds the cell size many times over in thickness. It represents a hydrated, viscous mass that accumulates on the cell surface.

The capsule is easily separated from the cell mechanically, for example, by centrifugation or extraction into aqueous, buffered, or weakly alkaline solutions.

According to their chemical composition, capsules are divided into:

polysaccharide capsules, consisting of Homopolysaccharides (composed of a single type of monosaccharide, e.g., glucose in *Leuconostoc mesenteroides*, and galactose in bacteria of the genus *Klebsiella*) and Heteropolysaccharides (composed of different monosaccharide residues, e.g., glucose, galactose, mannose, rhamnose, and glucuronic acid residues in *Pseudomonas aeruginosa*);

capsules consisting of Polypeptides and Polysaccharides, for example, in Bacillus megaterium.

The capsule can be regarded as an adaptive Structure in both saprophytic and pathogenic bacteria. Capsule formation is stimulated by the presence of living tissue in pathogens (Bacillus anthracis), the presence of CARBOHYDRATES and low temperatures (Salmonella typhi), and the availability of sucrose (Azotobacter). The capsular polysaccharides of certain bacteria act as Antigens and can also contribute to bacterial virulence (for instance, encapsulated pneumococcal strains cause Pneumonia in mice, whereas unencapsulated strains lose this ability).

The capsule is anchored to the surface of the cell wall via both ionic and covalent bonds.

The polysaccharides that form the capsule belong to exopolysaccharides (EPS). In microbial polysaccharide biotechnology, they are referred to as capsular polysaccharides, whereas those released into the culture medium are termed exopolysaccharides.

The ability to synthesize exopolysaccharides is characteristic of many microorganisms belonging to various physiological and taxonomic groups. EPS are synthesized by Fungi (Aureobasidium pullulans producing pullulan, Sclerotium rolfsii producing scleroglucan), Yeasts (Cryptococcus laurentii, Hansenula), and bacteria. Bacterial EPS producers include phytopathogenic bacteria (Xanthomonas campestris, Pseudomonas, Erwinia), nitrogen-fixing bacteria (Azotobacter, Beijerinckia), and methylotrophic bacteria (Methylocystis parvus, Methylomonas mucosa). Similar to capsular polysaccharides, EPS include both homo- and heteropolysaccharides. They are further classified into neutral EPS (composed exclusively of monosaccharide residues), acidic EPS (containing residues of uronic, pyruvic, and other acids), and basic EPS (containing amino sugar residues).