MICROBIOLOGY - M.H. Serhiichuk - 2008

CHAPTER 7. SYSTEMATICS OF MICROORGANISMS

BRIEF CHARACTERISTICS OF PROKARYOTIC TAXONOMIC GROUPS

Group 1. Spirochetes. This group includes the order Spirochaetales, which comprises two families: Spirochaetaceae and Leptospiraceae. The family Spirochaetaceae includes four genera: Spirochaeta, Cristispira, Treponema, and Borrelia. The family Leptospiraceae is represented by a single genus, Leptospira. These are small Bacteria that are easily distinguished from other Gram-negative forms by their Cell Morphology. The spirochete cell is strongly elongated, spiral-shaped, and covered by an elastic Cell wall with one or more helical turns. Spirochetes are characterized by a unique type of motility. The Cell contains a protoplasmic cylinder enclosing one or more pairs of axial filaments (fibrils). Each filament originates from a basal disk located at the pole of the cell (cylinder). Contraction of the fibrils generates a secondary wave along the cell surface, propelling it through the substrate.

Spirochetes reproduce by transverse Cell Division. They include aerobes (Leptospira interrogans), facultative anaerobes (Spirochaeta aurantia), and anaerobes (S. litoralis). Nutritionally, they are chemoorganotrophs. Cell dimensions vary widely. Large Cells (up to 250 µm in length) are found in spirochetes belonging to the genera Spirochaeta and Cristispira. Shorter cells are characteristic of the genera Treponema (5.0-20.0 µm), Borrelia (3.0-10.0 µm), and Leptospira (4.0-8.0 µm).

Representatives of the genera Treponema, Borrelia, and Leptospira are parasites of humans and other vertebrates: Treponema pallidum is the CAUSATIVE AGENT OF Syphilis; T. pertenue is the causative agent of yaws, a disease resembling syphilis (found in Central America, tropical Africa, and Indonesia); Borrelia recurrentis is the causative agent of relapsing fever in humans; Leptospira interrogans is the causative agent of leptospirosis, an acute infectious disease affecting humans, agricultural, wild, and fur-bearing animals, as well as rodents.

Group 2. Aerobic/microaerophilic, motile, helical/vibrioid Gram-negative bacteria. This is a rather heterogeneous group of vibrioid or spiral-shaped bacteria. In some species, Aging cultures undergo a transition of certain cells into coccoid bodies with a thin cell wall. Representatives of the genus Azospirillum are characterized by The formation of large cysts. Cells from young cultures stain Gram-negative. In older cultures, Cells of the genus Azospirillum may stain Gram-positive. When grown in liquid media, cells possess flagella located at a single pole (Azospirillum, Bdellovibrio, Cellvibrio, Vampirovibrio), both poles (Aquaspirillum, Spirillum), or either one or both poles of the bacterial cell (Campylobacter, Helicobacter). Nutritionally, they are chemoorganoheterotrophs, aerobes, or microaerophiles. Representatives of the genera Azospirillum and Herbaspirillum are capable of MOLECULAR Nitrogen Fixation under microaerophilic conditions. Aquaspirillum magnetotacticum cells exhibit magnetotaxis.

Bacteria of this group are found in soils, freshwater and marine habitats, on plant ROOT systems, and in the intestinal and oral cavities of humans and animals. Representatives of the genera Bdellovibrio, Micavibrio, and Vampirovibrio are predatory toward other microorganisms. For instance, Bdellovibrio bacteriovorus (from the Greek bdello – leech, vibrio – vibrio, i.e., leech-vibrio) possesses a single complex polar flagellum (up to 50 µm thick) that ensures rapid cell motility. Morphologically and physiologically, bdellovibrios exhibit a biphasic life cycle consisting of a predatory phase, during which the cells do not divide, and a reproductive phase that takes place within the host cell. B. bacteriovorus attaches to a host cell, sheds its flagellum, penetrates through The cell wall (by releasing lytic Enzymes that lyse the host cell wall) into the victim cell, and multiplies there. The intracellular development cycle lasts 3–5 hours, after which the parasite cells exit the lysed victim cell to either repeat the parasitic phase or form cysts as a resting form—small, oval, or spherical cells enclosed within a thick envelope.

Bdellovibrios predominantly lyse Gram-negative bacteria, primarily pseudomonads and enterobacteria.

Of considerable interest are representatives of the genus Helicobacter, which are straight, curved, or spiral-shaped rods (0.5–1.0 × 2.5–5.0 µm). Their rapid motility is driven by numerous sheathed flagella located at one or both poles of the cell, and occasionally laterally. They are microaerophiles. Growth is variable in an atmosphere enriched with 10% CO₂ and under anaerobic conditions. All strains rapidly hydrolyze urea. These bacteria were originally isolated from the gastric mucosa of mammals. The presence of certain Helicobacter species is associated with gastritis as well as gastric and duodenal ulcers, which has led to a shift in ulcer Treatment strategies in recent years. The type species of the genus Helicobacter is H. pylori.

Group 3. Non-motile (or rarely motile) Gram-negative curved bacteria. Representatives of this group feature curved, S-shaped, spiral, or ring-shaped cells. In some cases (bacteria of the genus Spirosoma), the ends of the cells may overlap to form a ring with an outer diameter of 1.5–3.0 µm, while the length of sinusoidally curved filaments can reach 50 µm. Occasionally, arc-shaped curved cells are arranged in a cloverleaf-like pattern (Brachyarcus), and some form clusters of motile cells bearing flagella (Pelosigma). They may exhibit a respiratory (Runella) or fermentative (Meniscus) type of METABOLISM. They are chemoorganotrophs, obligate aerobes (Brachyarcus thiophilus), or aerotolerant anaerobes (Meniscus glaucopis). They are found in soils, freshwater, and marine habitats.

Group 4. Gram-negative aerobic/microaerophilic rods and cocci. This is a fairly heterogeneous group of microorganisms, but all share a respiratory type of metabolism and utilize oxygen as the terminal electron acceptor. Nearly all representatives (with the exception of microaerophiles) grow in ambient air (21% O₂). Some are capable of Anaerobic Respiration using nitrate, fumarate, or other alternative TERMINAL ELECTRON ACCEPTORS. Bacteria from several genera can fix molecular nitrogen.

The family Pseudomonadaceae includes representatives of the genera Pseudomonas, Xanthomonas, and Xanthobacter. They are chemoorganotrophs, and some species are facultative chemolithotrophs. Bacteria of this family are widespread in soil, freshwater and marine habitats, mud, and wastewater, where they drive intensive mineralization processes of organic matter. They are utilized as producers of BIOLOGICALLY ACTIVE SUBSTANCES: organic acids (gluconic, α-ketoglutaric, pyruvic), Amino Acids (aspartic acid, valine, glutamine), and enzymes (asparaginase, peroxidase). This family includes plant pathogens (Xanthomonas) and human/animal pathogens (Pseudomonas aeruginosa—an opportunistic pathogen capable of causing meningitis, otitis, Pneumonia, and postoperative purulent infections). Some species possess entomopathogenic properties and exhibit antibiotic activity against bacteria, Yeasts, and Fungi.

The family Azotobacteriaceae includes species whose cells tend to exhibit polymorphism—altering their cell morphology depending on the physiological state of the culture and cultivation conditions. Some species form cysts. These are obligate aerobes and chemoorganoheterotrophs found in soils, on plant surfaces, and in aquatic environments. A characteristic feature of the family is the ability of its members to fix molecular nitrogen. Azotobacter chroococcum is the first free-living aerobic nitrogen fixer described by M. Beijerinck in 1901. Its cells are oval and pleomorphic, ranging from rod-shaped to coccoid. They occur in pairs or form short chains surrounded by a common capsule. Upon aging, resting structures called cysts are formed. Representatives of the genus Azomonas (A. agilis) do not form cysts.

The family Rhizobiaceae in the 9th edition of Bergey's Manual of Systematic Bacteriology is represented by two genera: Rhizobium and Agrobacterium, though recently new genera of root-nodule bacteria have been described and officially recognized (Azorhizobium, Bradyrhizobium, etc.). The genus Rhizobium unites symbiotic N₂ fixers. These are Gram-negative, chemoorganotrophic, motile (possessing a single polar or subpolar flagellum, or peritrichous flagellation) rod-shaped bacteria characterized by their ability to induce root tissue proliferation with nodule formation and to fix atmospheric nitrogen while in a symbiotic relationship with leguminous plants (Fig. 7.6). The Classification of root-nodule bacteria is based on their host plant Specificity: R. leguminosarum bv. viae (infecting peas, vetches, and field beans), R. leguminosarum bv. trifolii (clover), R. leguminosarum bv. phaseoli (beans), R. loti (lupine), R. meliloti (alfalfa), R. galegae (goat's rue). Representatives of the genus Bradyrhizobium (B. japonicum) are found in tropical and subtropical latitudes. Bacteria of the genus Agrobacterium are tumor-inducing plant pathogens (except for A. radiobacter). They cause plant tissue proliferations in the form of galls on roots and stems—such as crown gall, hairy root, and bacterial stem gall. Tumor induction correlates with the presence of a large tumor-inducing plasmid (Ti plasmid) within the bacterial cells. The type species is A. tumefaciens.

The family Methylococcaceae unites bacteria that utilize single-carbon Organic compounds—specifically methane or methanol—as their sole carbon source. Representatives of the genera Methylomonas (M. methanica, motile) and Methylococcus (M. capsulatus, non-motile) differ in morphology and motility.

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Fig. 7.6. Root-nodule bacteria of the genus Rhizobium:

a – Rhizobium cells penetrating plant tissue; b – formed nodules on a legume plant

The family Halobacteriaceae comprises halotolerant bacteria of the genus Halomonas (H. elongata), which are capable of growing in the presence of 0.05–20% NaCl in the medium. Their metabolism is respiratory, though some representatives can grow anaerobically using nitrate as a terminal electron acceptor. These were isolated from crude salt production facilities.

A characteristic feature of representatives of the family Acetobacteriaceae is their ability to oxidize alcohol solutions to acetic acid. The cells are rod-shaped, ellipsoid, or curved, occurring singly, in pairs, or forming chains. They are obligate aerobes, either motile (with peritrichous or lateral flagellation) or non-motile. As chemoorganotrophs, they are found in wine, cider, kefir, etc. The family is represented by two genera: Acetobacter (A. aceti) and Gluconobacter (G. oxydans).

The family Neisseriaceae includes four genera: Neisseria, Moraxella, Kingella, and Acinetobacter. Representatives of the first three genera are parasites of mucosal membranes: Neisseria gonorrhoeae is the causative agent of Gonorrhea, N. meningitidis is the causative agent of meningitis, and Moraxella lacunata and Kingella kingae are parasites of the mucous membranes of humans and warm-blooded animals. These bacteria require complex nutrient media for growth. The genus Acinetobacter is represented by saprophytic forms that are widespread in nature.

Group 5. Facultatively anaerobic Gram-negative rods. This group unites representatives of three families: Enterobacteriaceae, Vibrionaceae, and Pasteurellaceae. The family Enterobacteriaceae includes motile (peritrichous), asporogenous, chemoorganotrophic, saprophytic, and pathogenic organisms. Their metabolism is either respiratory or fermentative. Widely distributed in soil and Water, they are also found on vegetables, plants, and within the bodies of humans and animals. Certain serotypes of E. coli and representatives of the genera Klebsiella, Enterobacter, Proteus, and Providencia can cause nosocomial (hospital-acquired) infections. The type genus is Escherichia, and the type species is E. coli (Escherichia coli). Representatives of this genus grow well on standard nutrient media at 37 ⁰C, forming two colony types: S-colonies (smooth)—slightly convex, moist with a shiny surface and an even edge, grayish, and easily emulsifiable in saline; R-colonies (rough)—dry, and not emulsifiable in saline. Endo Agar is used for differentiation, on which E. coli forms pink colonies with a metallic sheen, alongside serological tests. The genus Escherichia also includes E. blattae.

Closely related to E. coli are coliform bacteria of the genera Salmonella, Shigella, and Citrobacter. The genus Salmonella includes species that can be causative agents of typhoid fever (S. typhi), paratyphoid fever (S. paratyphi), and nosocomial salmonelloses (S. typhimurium). Shigellae are the causative agents of bacterial dysentery: S. dysenteriae, S. flexneri, and S. sonnei.

Certain species of the genus Proteus can cause postoperative complications. Phytopathogenic species are found among bacteria of the genera Erwinia, Serratia, Hafnia, and Enterobacter. Bacteria of the genus Yersinia possess oval or rod-shaped cells and are motile, though their motility depends on the cultivation Temperature. Y. pestis (the causative agent of plague) was described in 1894 by Yersin.

The family Vibrionaceae comprises straight or curved motile rods found in aquatic environments, on the surfaces of aquatic animals, and within their bodies. Bacteria of the genus Aeromonas inhabit freshwater environments and can cause diseases in frogs and fish or infect humans, leading to diarrhea and bacteremia. Vibrio cholerae is the causative agent of cholera, an acute infectious disease with a high potential for epidemic spread. Most strains of the genus Photobacterium grow in mineral media based on seawater in the presence of D-glucose and NH4Cl. Two species within this genus (P. leiognathi, P. phosphoreum) are capable of Bioluminescence. These bacteria

can be isolated from marine fish. If a marine fish is covered with salt water (~ 3 % NaCl) in a shallow dish and left in a refrigerator for several days, luminescent colonies appear on its surface.

The family Pasteurellaceae is represented by pleomorphic, non-motile, aerobic or facultatively anaerobic bacteria. Cells range from coccoid to straight rods, which may swell or form filamentous structures. They are chemoorganotrophs with respiratory and fermentative Types of Metabolism. These are parasites of vertebrates, predominantly mammals and birds. Pasteurella multocida is the causative agent of hemorrhagic septicemia in cattle, fowl cholera, and pneumonia in farm animals. Haemophilus influenzae is a major causative agent of meningitis in children and can also cause otitis media, Chronic Bronchitis, and pneumonia. This species was first described in 1889 as the etiological factor of Influenza. H. ducreyi is the causative agent of Chancroid, or soft chancre.

Group 6. Gram-negative, anaerobic, straight, curved, and helical bacteria. These are obligate anaerobes, chemoorganotrophs, and non-motile or motile rods that tend toward pleomorphism. Bacteria of various genera differ in their End products of Fermentation: Butyrovibrio produce butyrate from glucose; Succinovibrio produce succinate, acetate, formate, etc.; Fusobacterium produce butyric acid; Leptotrichia produce lactic acid; Bacteroides produce a mixture of acids (succinic, acetic, formic, etc.). Bacteria of this group are found in the digestive tract of humans and animals (Selenomonas sputigena is a human Oral Cavity inhabitant; S. ruminantium inhabits the rumen of ruminants). Certain species of Fusobacterium are isolated from Blood and suppurative foci.

Group 7. Bacteria that perform dissimilatory sulfate or sulfur reduction. This group includes chemolithotrophic obligate anaerobes. Sulfate-reducing bacteria reduce sulfate or sulfur to H2S. Electrons are donated by H₂ or organic compounds. The oxidation of organic compounds can be incomplete (yielding acetate as the end product) or complete (yielding CO₂). In many species, electron Donors include H₂, lactate, Fatty acids, ethanol, or dicarboxylic acids. Autotrophic growth via the assimilation of H₂, CO₂, and sulfates is possible. Typical habitats include anaerobic sediments or the bottom layer in freshwater, marine, or saline water bodies. Thermophilic species are found in hot springs and hydrothermal vent channels. The group is heterogeneous in its morphological, physiological, and biochemical characteristics. Thermophilic sulfate-reducing archaea and extremely thermophilic archaea are classified in groups 32 and 35, respectively. This group includes representatives of several genera: Desulfolobus, Desulfomicrobium, Desulfomonas, Desulfovibrio, and Thermodesulfobacterium.

Group 8. Anaerobic Gram-negative cocci. Cocci with a diameter ranging from 0.3-0.5 to 2.5 µm. They occur in pairs or singly, forming non-characteristic clusters or chains of non-motile cells. Gram-negative, yet they tend to be resistant to decolorization. Chemoorganotrophs with fastidious nutritional requirements. Representatives of the genera Acidaminococcus, Megasphaera, and Veillonella exhibit a fermentative type of metabolism. Some genera are characterized by the fermentation of lactate to yield CO₂, H₂, and various volatile fatty acids with 2-6 carbon atoms. Species of the genus Syntrophococcus possess a respiratory type of metabolism and require sugars in the medium as electron donors, as well as formate as an exogenous electron-acceptor system, for growth. They are found predominantly in ruminants, rodents, pigs, and humans. Megasphaera cerevisiae is found in beer during prolonged storage.

Group 9. Rickettsiae and Chlamydiae. Divided into two orders: Rickettsiales and Chlamydiales. Rickettsiae are represented by rod-shaped, Gram-negative, non-motile organisms that reproduce by binary fission inside host cells where they parasitize, failing to grow on ordinary nutrient media. However, each of these features may have certain exceptions: some rickettsiae are motile (possess a single flagellum); in stained preparations, they appear ring-shaped; and they may stain Gram-positive. Parasitic species of rickettsiae are associated with reticuloendothelial cells, vascular endothelial cells, or blood erythrocytes of vertebrates. Under unfavorable conditions, they are capable of forming resting and degenerate forms. The resting form is a cell with a thickened cell wall and Cytoplasm. Based on their host specificity, they are divided into three families: Rickettsiaceae, Anaplasmataceae, and Bartonellaceae.

The family Rickettsiaceae includes intracellular parasites divided into three tribes: Rickettsieae (genera Rickettsia, Rochalimaea, Coxiella) - pathogenic forms; Ehrlichieae (genera Ehrlichia, Cowdria, Neorickettsia) - species non-pathogenic to humans; Wolbachieae (genera Wolbachia, Rickettsiella) - represented by species non-pathogenic to vertebrates that are not cultivated on cell-free media.

Rickettsiae of the tribe Rickettsieae are the causative agents of many dangerous human diseases: epidemic typhus (R. typhi, R. prowazekii), tick-borne spotted fevers (R. sibirica), and Q fever (from query), caused by Coxiella burnetii.

The family Anaplasmataceae is represented by obligate parasites found within erythrocytes, On the surface of erythrocytes, or in the Blood Plasma of wild and domestic vertebrates - cows, goats, deer (genera Anaplasma, Aegyptianella, Haemobartonella, Eperythrozoon).

Rickettsiae of the family Bartonellaceae have rod-shaped, spherical, or ring-shaped cells found within or on The surface of human erythrocytes, in tissue cells (genus Bartonella), as well as in animal erythrocytes (genus Grahamella).

The order Chlamydiales includes the family Chlamydiaceae and the genus Chlamydia. These are coccoid microorganisms (0.2-1.5 µm) with a characteristic intracellular mode of reproduction. The First stage of the infectious process is the adsorption of the elementary body (EB) of chlamydiae onto the Plasmalemma of a susceptible host cell. Elementary bodies are the infectious form of chlamydiae, measuring 0.2-0.4 µm, which are incapable of reproduction but capable of infecting (penetrating) the host cell. Chlamydial penetration occurs via endocytosis within 7-10 hours. Upon entry, the EB utilizes cellular ATP for growth and transforms into a reticulate body (RB) within 6-8 hours. Reticulate bodies (the non-infectious form) are large (0.8-1.5 µm) reticular spheroids capable of active division. RBs form microcolonies - chlamydial inclusions (Halberstaedter-Prowazek bodies). For 18-24 hours, they are localized within a cytoplasmic vacuole formed from the host cell membrane. Microcolonies may contain from 100 to 500 cells. The maturation process (intermediate bodies) and the transformation of RBs into EBs via division takes 36-42 hours. The cycle concludes with the destruction of the infected cell. Chlamydiae can be released from the infected cell while preserving host cell viability (when EBs exit via exocytosis), which leads to an asymptomatic course of the disease (Fig. 7.7).

Fig. 7.7. The life cycle of chlamydiae:

a - adsorption of the chlamydial elementary body (EB) onto The Introduction/5.html">Eukaryotic Cell plasmalemma; b - penetration of the EB into the cell; c - transformation of EB and RB; d - limited division; e - aberrant forms of chlamydiae; f - multiplication via binary fission; g - transformation of RB into EB

When studying the mechanisms of chlamydial persistence, data were obtained indicating that In addition to fully infectious (elementary) bodies, cytoplasmic inclusions contain abnormal forms: giant forms, including spheroplast-like bodies up to 5000 nm in diameter; small structures lacking a cell wall, measuring 100-250 nm; and vesicular forms with a diameter of 30-80 nm.

It is believed that in certain features, chlamydiae occupy an intermediate position between bacteria and Viruses. The complex of bacteria-like properties of chlamydiae provides: the preservation of morphological essence throughout the entire life cycle; division of vegetative forms; presence of a Gram-negative type Bacterial cell wall; presence of DNA, RNA, and enzymatic activity; ability to stain with aniline Dyes; susceptibility to broad-spectrum Antibiotics; and the presence of a genus-specific antigen common to all chlamydiae.

They are linked to viruses by: small size (they pass through bacterial filters); inability to grow on artificial nutrient media (cell-free substrates); and The ability to persist long-term in the host Organism, exhibiting energy parasitism.

Chlamydiae are parasites of humans, mammals, and birds; they cause A number of diseases in humans: trachoma, lymphogranuloma venereum, inclusion Conjunctivitis (C. trachomatis), and psittacosis (C. psittaci). In 1988, the Judicial Commission of the International Committee on Systematic Bacteriology officially recognized a third chlamydial species - C. pneumoniae, which causes human respiratory pathology. In 1990, Japanese microbiologists described a fourth species - C. pecorum, which is associated with The Development of joint pathology. In 2000, a new international classification of chlamydiae was approved, based on the presence of > 95 % Homology in The nucleotide sequence of 16S and 23S rRNA genes for all representatives of the genus and > 90 % for the family. According to the new classification, the order Chlamydiales includes four families: Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, and Waddliaceae. The most radical changes occurred in the systematics of the family Chlamydiaceae, which currently comprises two genera: Chlamydia (C. trachomatis, C. suis, C. muridarum) and Chlamydophila (C. pneumoniae, C. pecorum, C. psittaci, C. abortus, C. caviae, C. felis). The other three families each include a single genus with a single representative: Parachlamydia (P. acanthamoebae); Simkania (S. negevensis); Waddlia (W. chondrophila).

Group 10. Anoxygenic phototrophic bacteria. Photosynthetic bacteria characterized by a specific set of pigments (bacteriochlorophylls and carotenoids) and a special type of Photosynthesis not accompanied by oxygen evolution (anoxygenic). Morphologically, this is a heterogeneous group comprising cocci, rods, spiral forms, non-motile, and motile cells. Cells occur singly or in regular or irregular aggregates. Unicellular or single-row multicellular filamentous forms are also encountered. Cell length ranges from 1.0-5.0 µm (sometimes reaching 100 µm), and its thickness ranges from 0.3-6.0 µm. Endospores are not formed; they are Gram-negative, reproduce by binary fission, and some species (Rhodopseudomonas palustris, R. viridis) reproduce by budding.

Photosynthetic Pigments may be localized in: the cytoplasmic membrane (Heliobacillus - possessing flagella, Heliobacterium - possessing gliding motility); cytoplasmic membrane structures (Amoebobacter, Chromatium, Thiospirillum); chlorosomes (Chlorobium, Ancalochloris, Chloroflexus, Chloronema).

Energy Metabolism in most species proceeds under illumination (photosynthesis), but in some species, it can occur in the dark or in the presence of oxygen (oxidation, Thiocapsa), or under anaerobic conditions during the enzymatic transformation of organic compounds (some purple bacteria).

Group 11. Oxygenic phototrophic bacteria. This group is represented by cyanobacteria and prochlorophytes.

Cyanobacteria (formerly known as blue-green Algae) are spherical, rod-shaped, curved unicellular, colonial, and multicellular (filamentous), Gram-negative organisms possessing a rigid cell wall covered by a capsule and exhibiting gliding motility (Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, Stigonematales). They reproduce by binary fission, budding, hormogonia, spores, akinetes, or trichome fragments. Thylakoids contain a complex of photosynthetic pigments - chlorophyll a, allophycocyanin, phycocyanin, and phycoerythrins.

The order Prochlorales (prochlorophytes) unites unicellular or filamentous (branched or unbranched) prokaryotes that resemble cyanobacteria, but unlike the latter, contain chlorophylls a and b and lack accessory red or blue bilin pigments. They are found exclusively as extracellular symbionts in associations with colonial ascidians in tropical or subtropical marine environments.

Group 12. Aerobic chemolithotrophic bacteria and related organisms. This prokaryotic group is divided into three subgroups: 1) colorless sulfur-oxidizing bacteria; 2) iron- and manganese-oxidizing and/or iron- and manganese-depositing bacteria; 3) nitrifying bacteria.

Colorless sulfur-oxidizing bacteria encompass all known morphological forms. They are capable of growth at a pH ranging from 1.0 to 10.5. Gram-negative chemolithotrophs, they are typically found in environments containing reduced sulfur compounds: soils, the oxic-anoxic interface in water bodies, and hydrothermal discharge channels (Macromonas, Thiobacterium, Thiobacillus).

Iron- and manganese-oxidizing and/or iron- and manganese-depositing bacteria represent a heterogeneous group of microorganisms that frequently form specific aggregates. Such aggregates contain structures stained with iron or manganese oxides. The cell morphology varies from rod-shaped (Thiobacillus) and Curved Rods (Leptospirillum) to irregular cocci (Sulfolobus). Cells may bear appendages in the form of prosthecae (prosthecate bacteria - Hyphomicrobium) or stalks (stalked bacteria - Gallionella). Filamentous bacteria with sheaths encrusted with oxides (Leptothrix) are also found. They test Gram-positive, Gram-negative, or lack a cell wall entirely. These are microaerophiles, although some species can grow under standard O₂ partial pressures.

Depending on the substrate oxidized, nitrifying bacteria are divided into two sections: Section A comprises nitrite-oxidizing bacteria (Nitrobacter, Nitrospina, Nitrococcus, Nitrospira), and Section B comprises ammonia-oxidizing bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira,

Nitrosolobus, Nitrosovibrio).

Group 13. Budding and/or appendaged bacteria.

This group includes morphologically unusual bacteria characterized by cellular appendages and complex life cycles (Fig. 7.8).

Fig. 7.8. Stages of the Hyphomicrobium life cycle

Three subgroups are distinguished within this group.

Subgroup 1. Prosthecate bacteria are unicellular microorganisms that form one or more prosthecae per cell. Reproduction occurs via budding or binary transverse fission. Cells lacking prosthecae are rod-shaped, vibrioid, coccoid, or tetrahedral. Prosthecae can give cells a club-like, stellate, or spiral morphology. They are heterotrophs; most representatives are aerobes (Caulobacter, Dichotomicrobium, Hyphomicrobium, Pedomicrobium, Prosthecomicrobium, Stella), while some are facultative anaerobes (Ancalomicrobium). Oligocarbophiles, they grow well on dilute media. Certain species exhibit enhanced growth in the presence of Yeast extract, whereas others have specific Vitamin Requirements. They are found in aquatic environments and soils.

Subgroup 2. Order Planctomycetales consists of unicellular bacteria that form rosettes or filaments. The cells are coccoid, oval, or pyriform. They reproduce by budding and may develop appendage-like structures known as stalks. They inhabit freshwater, marine, and other saline aquatic environments (Planctomyces).

Subgroup 3. Budding and/or appendaged bacteria. For instance, members of the genus Nevskia feature long (1.0-6.0 × 3.0-12.0 µm), rod-shaped cells with transparent stalks. These stalks consist of a capsular substance secreted from one side of the cell. Following binary transverse fission, daughter cells remain attached via their stalks, forming Y-shaped branches. In natural habitats, they occur as cauliflower-like aggregates (N. ramosa).

Group 14. Sheathed bacteria. They grow as cellular chains or filaments with a width of 0.4-7.0 µm and are Gram-negative. The filaments grow within tubular structures formed by extracellular material classified as a sheath. Occasionally, the sheath is so delicate that it is difficult to detect using Phase-contrast Microscopy. Sheaths may become pigmented As a result of iron and manganese oxide deposition. Motile bacteria within this group possess flagella. They are aerobes and chemoorganoheterotrophs. Some species deposit iron and manganese oxides within or on the surface of their sheath and are capable of oxidizing Fe(II) and Mn(II). They are typically found in aquatic environments. Representatives of the genus Crenothrix possess cylindrical to disk-shaped cells, divide by cross-septum formation, and form filaments up to 1 cm long within sheaths that may attach to solid substrates. The sheath can be colorless at the apex or encrusted with iron or manganese oxides at the base. Within the filaments, an increase in transverse septation can be observed at one or both ends, yielding spherical reproductive cells known as "macroconidia".

Group 15. Non-photosynthetic sliding bacteria that do not form fruiting bodies. This morphologically heterogeneous group unites bacteria in the form of cocci, spirals, long flexible rods, sheathed or unsheathed filaments, flexible trichomes, and rosettes capable of gliding motility. Some representatives may possess gas vacuoles, while others contain sulfur inclusions. Bacteria of the genus Beggiatoa grow chemoautotrophically, all others are chemoheterotrophs, and some are capable of mixotrophic growth. The group includes unicellular rods (Cytophaga, Flexibacter, Sporocytophaga), flat filamentous forms (Alysiella), and sulfur-oxidizing gliding bacteria (Achromatium, Beggiatoa, Thiothrix).

Group 16. Fruiting-body-forming gliding bacteria: myxobacteria. Rod-shaped, Gram-negative bacteria capable of gliding motility that produce fruiting bodies and desiccation-resistant myxospores. The cells (0.6-1.2 × 2.0-10.0 µm) belong to two morphological types: a) slender, flexible rods with tapered ends; b) relatively thick, cylindrical rods with rounded ends. Through cellular gliding motility, myxobacterial colonies spread across the substrate surface, hence their designation as swarms. Within the swarm, cells are distributed unevenly, concentrating in radial strands or massive folds along the periphery of the swarm.

Under starvation conditions, cells cluster or aggregate in specific Regions of the swarm, forming large globular or crest-like masses containing 10⁴-10⁶ cells. These masses differentiate into fruiting bodies, whose shape and Structure are species-dependent (Fig. 7.9). Fruiting bodies may be simple, lacking sporangioles (Myxococcus, Angiococcus, Archangium), or complex, consisting of multiple units (Cystobacter, Stigmatella, Polyangium, Chondromyces). Fruiting body size ranges from 100 to 600 µm. During fruiting body maturation, vegetative cells transform into myxospores, which are resting stages (formerly called microcysts).

Fig. 7.9. Vegetative cells (a) and fruiting body (b) of Chondromyces crocatus

Regarding their mode of Nutrition, myxobacteria are chemoorganotrophs capable of degrading various Biomacromolecules. They are found in soils, wood, the bark of living and dead trees, and water.

Group 17. Gram-positive cocci. This is a numerous and heterogeneous group of microorganisms united under common traits such as a spherical cell shape and a positive Gram-staining reaction. It includes aerobes (Micrococcus, Planococcus, Deinococcus), facultative anaerobes (Staphylococcus, Stomatococcus, Streptococcus, Leuconostoc, Pediococcus, Aerococcus), and anaerobic genera (Peptococcus, Peptostreptococcus, Ruminococcus, Coprococcus, Sarcina).

The group comprises two families (Micrococcaceae and Deinococcaceae) as well as several genera of independent taxonomic position.

The family Micrococcaceae includes organisms with quite diverse nutritional requirements. They grow in media containing 5% NaCl, and certain strains are capable of developing in the presence of 10% NaCl. Some species are pathogenic to humans and animals. This family encompasses the genera Micrococcus (M. lylae, M. roseus, M. agilis, M. kristinae), Stomatococcus (S. mucilaginosus), Planococcus (P. citreus), and Staphylococcus (S. aureus, S. epidermidis, S. haemolyticus, S. saprophyticus, S. warneri).

The genus Staphylococcus includes spherical bacteria with a cell diameter of 0.5-1.5 µm. Microscopic preparations reveal single cells, sometimes arranged in pairs or forming characteristic clusters (grape-like bunches) resulting from cell division

in more than one plane. Both peptidoglycan and Teichoic Acids are identified in their cell walls. The diamino acid of the peptidoglycan is L-Lysine. The optimal growth temperature is 35-40 ⁰C, with a growth temperature range of 15-45 ⁰C. The pH optimum is 7.0-7.5, with pH limits spanning from 4.2 to 9.3. Representatives of certain species (S. aureus, S. cohnii, S. intermedius, S. epidermidis) exhibit hemolytic activity.

The family Deinococcaceae includes a single genus, Deinococcus (D. radiodurans), whose cells are spherical (0.5-3.5 µm in diameter) and occur in pairs or, more frequently, form tetrads. They are non-motile chemoorganotrophs with a respiratory type of metabolism.

Based on such characteristics as the ability to grow at 10 ⁰C and 45 ⁰C in a medium containing 6.5% NaCl, at pH 9.6 in a medium containing 40% Bile, the capacity to induce α- or β-hemolysis, hydrolyze Arginine, starch, and hippurate, and grow under anaerobic conditions, representatives of the genus Streptococcus are divided into several groups:

- pyogenic streptococci (S. pyogenes, S. agalactiae, S. equi, S. pneumoniae);

- oral streptococci (S. salivarius, S. sanguis, S. mutans, S. sobrinus);

- enterococci (S. faecalis, S. faecium);

- lactic acid streptococci (S. lactis, S. raffinolactis);

- anaerobic streptococci (S. hansenii, S. pleomorphus);

- other streptococci (S. thermophillus, S. bovis).

It is worth noting that members of such physiological groups as enterococci and lactic acid streptococci have been assigned the corresponding generic names Enterococcus and Lactococcus and are considered in parallel within their respective taxonomic groups. In other words, one should keep in mind that such taxa as Streptococcus lactis and Enterococcus lactis are identical.

Members of the genus Leuconostoc (L. mesenteroides, L. paramesenteroides, L. lactis) feature lenticular cells that occur in pairs or chains; they are non-motile chemoorganotrophs that require rich nutrient media. The optimal growth temperature is 20-30 ⁰C, though they can grow within a range of 5-30 ⁰C. In media containing sucrose (α-glucosyl-β-fructose), they produce a significant amount of slime, causing substantial economic losses in sugar production ("frog spawn disease").

The genus Sarcina comprises organisms with nearly spherical cells (1.8-3.0 µm) that form characteristic packets consisting of 8, 16, 32... cells. They are non-motile and do not synthesize pigments. Their metabolism is fermentative, and they are obligate anaerobes. S. ventriculi

is isolated from soil, the gastrointestinal tracts of patients with Stomach disorders, and the surface of cereal seeds. Its cells possess an additional outer cellulosic layer. S. maxima is isolated from cereal seeds and lacks this external layer.

Group 18. Endospore-forming Gram-positive rods and cocci. The taxonomic relationships within this group are not yet fully understood. Most members have rod-shaped cells, with the exception of strains assigned to the genus Sporosarcina, which are coccoid and capable of forming tetrads or packets. The cell width of most species ranges from 0.3 to 2.0 µm. An exception is the genus Oscillospira, whose cell diameter reaches 3.0-6.0 µm, with cells being rod-shaped or forming filaments divided by septa into disk-shaped cells.

Most species stain Gram-positive, though they may be Gram-variable or Gram-negative. As a rule, they are motile with peritrichous flagellation. The metabolism and ecology of various representatives vary considerably. This group includes a number of genera that hold an independent systematic position: Amphibacillus, Bacillus, Clostridium, Desulfotomaculum, Oscillospira, Sporohalactobacter,

Sporolactobacillus, Sporosarcina, and Sulfobacillus. Regarding their oxygen requirements, endospore-forming bacteria encompass all types: obligate aerobes (Sporosarcina), aerobes and facultative anaerobes (Bacillus), facultative anaerobes (Sporolactobacillus), and obligate anaerobes (Clostridium, Desulfotomaculum, Oscillospira). This group includes agents of butyric acid (Clostridium butyricum) and acetone-butanol fermentations (C. acetobutylicum), urolytic bacteria (Sporosarcina ureae, Bacillus pasteurii), free-living molecular nitrogen fixers (C. pasteurianum), and producers of antibiotics such as gramicidin C (B. brevis var. G.-B.), polymyxin (B. polymyxa), and subtilisin (B. subtilis), as well as enzymes like proteases (B. subtilis) and amylases (B. licheniformis), amino acids such as L-glutamate (B. megaterium) and L-Tryptophan (B. subtilis), entomocidal toxins (B. thuringiensis), and other biologically active substances.

Ecologically, these are cosmopolitans found in soil, mud, freshwater and marine habitats, animal and plant remains, and within the bodies of insects, animals, and humans. Pathogenic human species include B. anthracis, the causative agent of anthrax; C. botulinum, the agent of botulism; C. perfringens, the agent of gas gangrene; and C. tetani, the agent of tetanus. Certain species of the genus Bacillus formerly considered Saprophytes are now classified as opportunistic pathogens, for example, B. cereus. In recent years, a number of Infections caused by B. subtilis have also been described.

Group 19. Regular, non-spore-forming Gram-positive rods. Bacteria in this group are rod-shaped, ranging from short rods (nearly coccoid) to long rods, filaments, or trichomes, but they maintain a regular shape with little pleomorphism. The cells are Gram-positive, non-spore-forming, and rarely pigmented. Organisms in this group are mesophiles, facultative or obligate anaerobes that carry out homo- or Heterofermentative lactic acid fermentation and require complex nutrient media for growth. They are typically found in association with plants or animals or in decomposing organic matter. Pathogenic species are present. Representatives of the genera Lactobacillus and Listeria are of particular interest. Bacteria of the genus Lactobacillus are found in milk, rinses from dairy-Processing equipment, cereal and meat products, wine, fruit juices, and the human Oral Cavity and intestinal tract. Their cell morphology ranges from long to short rods (coccobacilli), often forming chains. They are non-motile, or when motile, possess peritrichous flagellation. They are Gram-positive, though aging cultures and increasing medium acidity may render them Gram-negative. During sugar fermentation, they produce lactate (up to 50%), acetate, formate, succinate, ethanol, and CO₂. The type species of the genus is L. delbrueckii.

The type species of the genus Listeria is L. monocytogenes, the causative agent of Listeriosis—an acute infectious disease characterized by enlargement of the retropharyngeal and other glands, a mononuclear WHITE BLOOD CELL response, and frequently, septicemia.

Group 20. Irregular, non-spore-forming Gram-positive rods. Most members of this group are irregular Gram-positive rods (Acetobacterium, Actinomyces, Arthrobacter, Bifidobacterium, Brevibacterium, Corynebacterium, Propionibacterium), including aerobes (Arthrobacter, Brevibacterium), some of which are motile. Others are facultative anaerobes (Actinomyces, Corynebacterium, Propionibacterium), obligate anaerobes (Acetobacterium, Acetogenium, Bifidobacterium), or Gram-negative (Acetogenium).

Group 21. Mycobacteria. This group comprises the genus Mycobacterium. The cells are rod-shaped, acid-fast, aerobic, slow-growing, and either free-living or vertebrate parasites. Acid-fastness is due to the presence of Waxes in the cell wall, which is particularly crucial for identifying mycobacteria. Mycobacterial cells may form mycelium-like filaments that easily break down into rods or cocci. They stain with difficulty, but are considered Gram-positive. They are non-motile, aerobic chemoorganotrophs. The type species is M. tuberculosis.

Mycobacteria are capable of forming spores. In this process, the cytoplasm breaks down into discrete regions (fragments) that contract and condense (Fig. 7.10). These fragments serve as independent germinative bodies—spores. The entire cell content is consumed in spore formation. During spore formation, the cell wall becomes mucilaginous, less distinct, and nearly disappears as the spores are released. In their mode of formation and biological significance, mycobacterial spores differ significantly from the endospores of endospore-forming bacteria. Rather than a single spore, mycobacterial cells produce multiple spores that are sensitive to unfavorable environmental factors.

Fig. 7.10. Spore formation in mycobacteria:

a — concentration of plasma into discrete fragments; b, c — Condensation of fragments and their conversion into spores; d — dissolution of the cell wall; e — mature spores

Groups 22–29. Actinomycetes. Gram-positive bacteria capable of forming branching filaments 0.5–1.0 µm in diameter. These filaments may break down into elements of various sizes or remain intact and form arthrospores. Spores may be single, in chains of varying length, or enclosed in sporangia; they are typically non-motile, though some genera possess flagellated spores. Actinomycete genera are distinguished by morphological features, such as the shape of sporophores, as well as the presence of marker chemical components in the cell wall, membranes, and whole-cell hydrolysates (Fig. 7.11). In most cases, they are aerobes, but some genera are represented by facultative or obligate anaerobes. They are chemoheterotrophs. Certain species are pathogenic to humans, animals, and plants.

Fig. 7.11. Some types of actinomycete spore-bearers

Nocardioform actinomycetes form filaments that break down into short elements. Some genera form an aerial mycelium with chains of spores. Division into genera is based primarily on the cell wall chemotype, the presence or absence of mycolic acids, and other chemical characteristics. This subgroup is represented by the genera Nocardia and Rhodococcus. The vegetative hyphae of the genus Nocardia range from rudimentary to highly branched, with a diameter of 0.5-1.2 µm, growing on the surface and penetrating into the agar medium. Hyphae often break down into bacteroid elements—ranging from rod-like to coccoid. Aerial hyphae are almost always formed. Spore chains are found on the aerial hyphae and occasionally on the substrate hyphae. Gram-positive or variable. Aerobes, partially acid-fast. Colonies have a membranous or velvety surface; they are brown, reddish-brown, pink, orange, red, purple, gray, or white; and they appear smooth or granular. Representatives of the genus Rhodococcus may form various morphological forms, ranging from rods to a branched vegetative mycelium. The morphogenetic cycle begins with a stage of cocci or short rods, which undergo a successive series of morphological changes completing the life cycle. Thus, cocci can germinate into short rods, form filaments with lateral outgrowths or branching elements, or produce extensively branched hyphae. The next generation of cocci or short rods is formed during the fragmentation of rods, filaments, and hyphae. Gram-positive, partially acid-fast aerobes.

Genera with multilocular sporangia include Dermatophilus, Frankia, and Geodermatophilus. In representatives of the genus Dermatophilus, the aerial mycelium develops in a CO₂-enriched atmosphere. The substrate mycelium consists of long, terminally tapered threads that branch laterally at right angles. Septa formed in the transverse, horizontal, and vertical planes give rise to eight parallel rows of coccoid cells (spores), each motile via a flagellum. Gram-positive aerobes and facultative anaerobes that grow on complex nutrient media. Mammalian parasites (mainly agricultural animals). D. congolensis is the causative agent of exudative dermatitis. Representatives of the genus Frankia do not form an aerial mycelium. Vegetative hyphae branch sparingly or profusely. Multilocular sporangia, spherical or irregular in shape, are formed terminally, laterally, or intercalarily on the vegetative hyphae. Sporangiospores are non-motile, 1.0-5.0 µm in size, and range from colorless to black. Mycolates are not detected. Aerobes or microaerophiles. They grow slowly (generation time 17 days). Most strains are capable of fixing atmospheric gas in vitro and within plants, acting as symbionts of various angiosperms where they induce nodule formation (F. alni).

Actinoplanes form a sparse, fine mycelium that branches but does not fragment. They produce pigments of various colors that diffuse readily into the medium. Spherical spores are formed inside sporangia (spore vesicles) and are either sessile or borne on sporangiophores. Upon contact with water, spores are released from the sporangia and swim using polar flagella. Gram-positive, although some hyphae may fail to retain the Gram stain. Found in soil and decomposing plant material—Actinoplanes philippinensis. Representatives of the genus Micromonospora possess a well-developed, branched, and septate mycelium with a diameter of 0.5 µm. Non-motile spores are formed singly on short or long sporophores, which are often arranged in branched clusters (Fig. 7.12). Sporophores are formed monopodially or sympodially. Aerial mycelium is absent. Gram-positive, non-acid-fast chemoorganotrophs. Found in soils, water, marine environments, and mud—M. chalcea.

Fig. 7.12. Spore formation in micromonosporae:

a - Micromonospora - spores sit on the surface as bumps or short chains on short sporophore stalks; b - Microbispora - two-celled spores on short stalks; c - Micropolyspora - spore chains, straight or spirally curved; d - Actinobifida - dichotomous branching of spore-bearing branches, with a single spore at the tip of each branch

Streptomycetes and related genera. Actinomycetes of this group form a branching mycelium (about 1.0 µm in diameter) that tends to fragment. The genus Streptomyces features vegetative hyphae 0.5-2.0 µm in diameter that form an extensively branched mycelium which rarely fragments (S. albus, S. antibioticus, S. diastaticus,

S. albidoflavus, S. gHseoviHdis). When mature, the aerial mycelium bears chains of three or more non-motile spores. Some species can form sclerotio-, pycnidio-, and sporangio-like structures. Colonies are leathery or butter-like. The colony surface is initially smooth, but over time an interwoven aerial mycelium develops, which may be floccose, granular, powdery, or velvety. They produce various pigments that determine the color of the substrate and aerial mycelium, and frequently synthesize pigments that diffuse into the medium. Many strains produce one or more antibiotics. The temperature optimum is 25-35 ⁰С, and the optimal pH growth range is 6.5-8.0. Widely distributed in soil, including composts. Some species are animal and human pathogens, while others are phytopathogens. The genus Streptoverticillium has a branched substrate mycelium 0.8-1.2 µm in diameter. The aerial mycelium consists of long straight hyphae bearing whorls of branchlets. Each branchlet whorl terminates in an umbel consisting of a chain of spherical or ellipsoidal spiny or hairy spores (S. baldaccii, S. griseocameum, S. viridoflavum).

Group 30. Mycoplasmas (Mollicutes): bacteria lacking a cell wall. Mycoplasmas lack a true cell wall and are incapable of synthesizing its precursors, such as muramic and diaminopimelic acids. Cells are extremely small, sometimes ultramicroscopic (~200 nm), and highly pleomorphic. Their morphology ranges from cocci to filaments capable of forming mycelium-like structures. Reproduction occurs via the formation of coccoid structures—elementary bodies—within filaments, alongside binary fission and budding. They are Gram-negative.

Mycoplasmas (phylum Tenericutes) are grouped into the class Mollicutes, which contains the order Mycoplasmatales with three families: Mycoplasmataceae, Acholeplasmataceae, and Spiroplasmataceae. The division of mycoplasmas into families is based on their sterol requirements, resistance to digitonin, presence of Lactate dehydrogenase, Genome Size, and ability to synthesize fatty acids from acetate. For instance, the sterol-dependent mycoplasmas of the family Mycoplasmataceae require sterols, which are a major component of Membrane Lipids in parasitic forms. The sterol-dependent mycoplasmas of the family Spiroplasmataceae, which have a helical cell shape, are capable of rotational and screw-like motility during certain phases (spherical and short helical cells in the lag phase, elongated cells in the stationary phase; flagella and axial fibrils are absent).

The family Mycoplasmataceae is represented by two genera: Mycoplasma and Ureaplasma. The diameter of spherical cells ranges from 125 to 250 nm, while filamentous forms can reach up to 150 µm in length. As cultures age, the filaments break down into chains of coccoid cells. On solid nutrient media, they form small, hemispherical, flattened colonies 10-600 µm in diameter, occasionally reaching 4 mm. Typical colonies are biphasic and resemble a "fried egg". They consist of an opaque, granular central area that grows down into the medium and a flat, translucent peripheral zone. They reproduce by binary fission, budding, or the multiple release of elementary bodies. Elementary bodies are ~100 nm structures found throughout the colony mass. They are believed to result from degenerative processes occurring under extreme conditions that would otherwise prove lethal to the cell. Upon cell death, spore-like particles are released which, upon encountering optimal conditions, initiate a new generation. Mycoplasmas parasitize a wide range of mammalian and avian hosts. They can cause acute and chronic pathological processes that often manifest as latent infections. The pathogenic role of M. pneumoniae in humans is well established. M. hominis is frequently isolated from the urogenital tract and may provoke urethritis and other inflammatory conditions. M. arthritidis is isolated from the synovial fluid of affected joints. M. salivarium and M. orale are oral commensals. The type species of the genus is M. agalactiae. A defining feature of the genus Ureaplasma (U. diversum, U. felinum, U. urealiticum) is the ability to hydrolyze urea.

Acholeplasmas do not require sterols for growth. On solid nutrient media, they form relatively large colonies (~3 mm). They are facultative anaerobes and chemoorganotrophs with a fermentative type of metabolism. They include free-living saprophytes, parasites, and potential mammalian and avian pathogens. The type species is Acholeplasma laidlawii.

The family Spiroplasmataceae was first described by I. Skripal in 1978. The diameter of spherical cells ranges from 0.1 to 0.25 µm. In liquid media, helical cells dominate, measuring ~0.12 µm in diameter and up to 2-4 µm in length. Rounded, non-helical forms develop on solid media. On solid substrates, they form colonies characteristic of mollicutes—fried-egg colonies with distinct margins. On media with a moist surface, granular colonies with diffuse borders appear. "Daughter" colonies emerge at the periphery of the primary colony as a result of individual cells migrating outward. Colony size ranges around 0.2 mm. Facultative anaerobes. Temperature range: 20-37 ⁰С, with an optimum at 32 ⁰С. Isolated from citrus leaves, though disease cannot be reproduced experimentally. The type species is Spiroplasma citri.

According to phylogenetic systematics, the phylum Tenericutes has been abolished, and the class Mollicutes is now placed within the Gram-positive bacteria (Firmicutes). The phylogenetic relationships and evolutionary Pathways of the mollicutes remain a subject of active debate. Specifically, comparative analysis of mycoplasma 16S rRNA with Gram-positive bacteria has confirmed their affinity with members of the genus Clostridium.

Group 31. Methanogens. Gram-negative or Gram-positive bacteria whose cell walls lack both murein and an outer membrane. Their membranes consist of Isoprenoids linked by ether bonds.

Obligate anaerobes. Chemoautotrophs or chemoheterotrophs that invariably produce methane as a metabolic byproduct. No other organisms besides methanogens produce methane. They utilize H₂ + CO₂, formate, acetate, methyl group-containing compounds (methanol, methylamines, methylsulfides), etc., as sources of carbon and energy. Cells can be rod-, lancet-, or coccoid-shaped, assimilating H2 + CO2, formate, or H₂ + methanol. The cell wall contains pseudomurein—Methanobacterium (M. formicicum), Methanobrevibacter (M. ruminantium), Methanosphaera (M. stadtmanae), Methanothermus (M. fervidus). Bacteria with coccoid, rod-, spiral-, or plate-shaped cells that use H₂ + CO₂, formate, or alcohols + CO₂ while lacking pseudomurein in their cell walls are assigned to the genera Methanococcus (M. vannielii), Methanogenium (M. cariaci), Methanomicrobium (M. mobile), and Methanospirillum (M. hungatei). Pseudosarcinae, cocci, or sheathed rods utilizing trimethylamine or acetate as growth substrates constitute a subgroup comprising the genera Methanococcoides (M. methylutens), Methanolobus (M. tindarius), and Methanosarcina (M. barkeri).

Group 32. Sulfate-reducing archaea. Possess irregularly shaped cells, often triangular, 0.4-1.3 µm in diameter, occurring singly or in pairs, with or without flagella. Gram-negative. When illuminated with 420 nm light, they exhibit blue-green fluorescence. On solid media, they form greenish-black, smooth colonies 1-2 mm in diameter. Obligate anaerobes. Chemolithotrophs, chemoorganotrophs, or chemomixotrophs. Require thiosulfate and H2 for autotrophic growth; growth is weak in the presence of sulfate. Produce H2S. Capable of reducing S⁰. Growth temperature range: 60-95 ⁰С, with an optimum around 83 ⁰С. pH range: 4.5-7.5, with an optimum at 6.0. Isolated from shallow and deep-sea marine hydrothermal ecosystems. This group includes the genus Archaeoglobus (A. fulgidus).

Group 33. Extremely halophilic aerobic archaebacteria (halobacteria). Represented by coccoid organisms (0.8-2.0 µm) or irregular rods (0.3-1.2 × 1.0-15.0 µm). Motile via tufts of polar flagella or non-motile. Gram-negative (rods) or Gram-variable (cocci). Rod-shaped cells undergo lysis when suspended in distilled water. Colonies exhibit various shades of red due to carotenoid pigments. Aerobes, though some strains can grow anaerobically in the presence of nitrate. Chemoheterotrophs requiring at least 1.5 M NaCl for growth; the optimal NaCl concentration for most is 2.0-4.0 M. Found in saline and alkaline lakes, salt evaporation ponds, and saline soils: Halobacterium (H. salinarium), Halococcus (H. morrhuae), Natronobacterium (N. gregoryi), Natronococcus (N. occultus).

Group 34. Wall-less archaebacteria. Represented by the genus Thermoplasma, which comprises pleomorphic cells varying in shape from spherical (0.1-5.0 µm) to filamentous. Lacking a cell wall, these cells are enclosed solely by a cell membrane approximately 7 nm thick. Membrane lipids consist of dibiphytanyl (C40) diglycerol tetraethers. Gram-negative, occasionally motile via flagella. Obligate thermophiles (growth temperature range 33-67 ⁰С) and obligate acidophiles (growing at pH 0.5-4.0). Cell lysis occurs at neutral pH. Facultative anaerobes. Anaerobic growth is stimulated by elemental sulfur, which is reduced to H₂S. Distributed in coal refuse piles. The type species is T. acidophilum.

Group 35. S⁰-metabolizing extreme thermophiles and hyperthermophiles. These are Gram-negative motile or non-motile rods, filaments, cocci, or disc-shaped cells. They include aerobes, facultative anaerobes, and obligate anaerobes. They grow chemoautotrophically or chemoheterotrophically. Under anaerobic conditions, they oxidize H₂S or S⁰ to H₂SO₄. Electron donors include H₂ or organic compounds. Growth temperature range: 45-110 ⁰С, with an optimum of 70-105 ⁰С. Distributed in continental solfataric fields or marine hydrothermal systems. The group is divided into three subgroups. Subgroup 1 unites thermoacidophilic aerobic or facultatively anaerobic coccoid organisms that grow below pH 4.0: genus Acidianus (A. infernus, temperature optimum 88 ⁰С, maximum growth temperature 96 ⁰С; A. brierleyi, temperature optimum 70 ⁰С, maximum 75 ⁰С); genus Sulfolobus (S. acidocaldarius, temperature range 55-87 ⁰С, pH range 1.0-6.0). Subgroup 2 comprises rod- or filamentous obligate anaerobic bacteria growing above pH 4.0. Genus Thermofilum (T. pendens, rod-shaped cells 0.15-0.35 µm in diameter and 1.0 to ~100 µm long; pH range 4.0-6.7; temperature interval 70-95 ⁰С). Subgroup 3 unites obligate anaerobic coccoid or disc-shaped bacteria growing above pH 4.0. Bacteria of the genus Desulfurococcus possess Gram-positive-type coccoid cells, either motile or non-motile. Temperature range: 70-95 ⁰С (optimum 85 ⁰С), pH range: 4.5-7.0 (optimum around 6.0). The type species is D. mucosus.