Bacteria Types: 3 Important Types of Bacteria

The following points highlight the three important types of bacteria. The types are: 1. The Ancient Bacteria 2. Gram-Positive Bacteria 3. Gram-Negative Bacteria.

Type # 1. The Ancient Bacteria:

Most of the phylogenetically primitive bacteria are thermophilic and Gram-negative (except Deinococcus). Some of them have ether-linked membrane lipids like those of archaebacteria. A few representative genera are briefly mentioned here.

Aquifex and related genera are Gram-negative, microaerophilic, hyper-thermophilic, chemo-lithotrophic organisms. A. pyrophilus can grow auto-trophically using H₂, S° or thiosulfate as, inorganic oxidisable energy source at a temperature up to 95°C. A related genus Hydrogenobacter can oxidize H₂ for autotrophic growth. Aquifex has unusual fatty acids and ether-linked lipids.

Thermotoga maritima is also a Gram-negative thermophilic, anaerobic rod-shaped organism (maximum temperature 90°C). The bacteria are enveloped by a sheath-like loose envelop. They are chemoorganotrophic. Like Aquifex, they have also unusual fatty acids and ether-linked lipids. Some related genera are Geotoga, Petrotoga and Flavidobacterium.

Deinococcus represents a group of Gram-positive, aerobic cocci, often occurring in pairs or tetrads. Though the bacteria are Gram-positive, an unusual feature is the presence of an outer membrane like that of Gram-negative bacteria.

Also, their cell wall lacks teichoic acids which are characteristically present in Gram-positive bacteria. Deinococcus radiodurans is highly resistant to radiations, a property which has been attributed to their extraordinary ability to repair damaged DNA with the help of an unusually active RecA protein. Deinococci have a high content of carotenoids and unusual lipids with high amount of palmitic acid.

Thermus includes Gram-negative, thermophilic, rod-shaped organisms which form filaments at higher temperature. They grow in hot springs (optimum temperature 70°C) forming bright orange tufts composed of intertwined filaments. Thermus aquaticus is the source of a DNA-polymerase, known as Taq-polymerase which is used in PCR (Polymerase Chain Reaction) for amplification of DNA.

Chloroflexus species represent the green, filamentous, non-sulfur photosynthetic bacteria. The organisms are Gram-negative and thermophilic. They exhibit a gliding movement like that of Oscillatoria (a cyanobacterial genus) and grow naturally in hot springs in association with thermophilic cyanobacteria and Thermus spp.

The bacteria are microaerophilic, but can grow also anaerobically carrying out anoxygenic photosynthesis with the help of bacteriochlorophyll a and c located in chlorosomes. The organisms are photo-organotrophic and can also grow as chemo-organotrophs under aerobic conditions. A related genus, Heterosiphon is non-photosynthetic and aerobic, but their filaments also show a gliding movement.

Morphology of some of the above-mentioned ancient bacteria is presented in Fig. 4.7:

Type # 2. Gram-Positive Bacteria:

In the second edition of the Manual, all Gram-positive bacteria have been classified into two broad groups. One group containing G + C content of DNA 50 moles % or above has been designated as high GC group, and a second group having lower G + C values, i.e. 50 moles % or less, as low GC group.

A. High GC Group:

This group of Gram-positive bacteria comprises a morphologically diverse assemblage including cocci, straight rods, rods with rudimentary branching and elaborate mycelial forms. All these morphological types are placed in a single Phylum Actinobacteria having only one Class—also called Actinobacteria.

There are many genera, some of which include important pathogens, while many others are of economic importance. As for example, genera like Actinomyces, Nocardia, Mycobacterium, Corynebacterium have pathogenic species. On the other hand, many species of Streptomyces provide clinically useful antibiotics and are of great economic importance in microbial industry.

Also, Brevibacterium and Propiniobacterium are important in dairy industry. Furthermore, actinomycetes in general, as well as Arthrobacter, serve as important soil organisms carrying out mineralization of various organic compounds. Another important genus Frankia contains species that enter into symbiotic association with many species of non-leguminous trees and fix molecular nitrogen.

Some important genera of high GC Gram-positive bacteria are briefly discussed:

(i) Actinomyces:

Actinomyces comprises facultative to strictly anaerobic, straight, curved or slender branched mycelial organisms. The genus includes pathogenic species, like A. israelii causing chronic, destructive abscesses of connective tissues in man, and A. bovis causing lump jaw in cattle.

(ii) Actinoplanes:

Actinoplanes is a fully mycelial actinomycete producing extensive substrate hyphae and rudimentary aerial mycelium. It produces conidia within sporangia which, after being released, become motile.

(iii) Streptomyces:

Streptomyces is the largest genus of actinomycetes having about 500 species. The organisms produce extensive aerial and substrate mycelia. The aerial hyphae form profuse conidia in chains. The conidia are generally smooth-walled, but may be spiny in some species.

The conidial chains may be straight, flexuous, spiral or coiled. The organisms may have different shades of pigments. Streptomycetes occur extensively in soil where they play an important role in the process of mineralization. But they are best known as producer of antibiotics. Thousands of antibiotic compounds have been reported, but very few of them have been found suitable for clinical use. Some of them are streptomycin, chloramphenicol, tetracyclines, erythromycins, rifampicins, neomycin, nystatin, cefamycins, kanamycin etc. Many Streptomyces’s have the ability to attack complex molecules, like lignin, chitin, keratin and aromatic compounds. Keratinase, an enzyme produced by certain Streptomyces’s, is commercially used in tanning industry for removing hairs from hides.

A few species have also pathogenic property e.g. S. somaliensis cause a human infection, called actinomycetoma producing subcutaneous swelling and lesions. S. scabies is the causal organism of potato scab disease. The characteristic odour emitted by dry soil after a shower of rain has been found to be due to a volatile compound, geosmin, produced by streptomycetes.

(iv) Nocardia:

Nocardia is another related genus which produces mainly substrate mycelium, though some species produce also aerial hyphae and conidia. In the majority of Nocardia species, the substrate hyphae break up into rod-like elements. The organisms contain mycolic acid in their cell wall and are weakly acid-fast. Nocardia asteroids is a human pathogen causing nocardiosis. The organism is pathogenic to other animals also. Rhodococcus, a related genus, occur extensively in soil and they are commonly known as nocardioform bacteria. They share the weakly acid-fast character with Nocardia.

(v) Mycobacterium:

The genus Mycobacterium includes two very important human pathogens — M. tuberculosis, the cause of human tuberculosis and M. leprae, the causal organism of leprosy. M. bovis is another species, causing tubercular diseases of cattle. M. avis causes tubercular infection in birds and M. lepraemurium causes leprosy of rat.

M. bovis also infects humans and the infection is spread through drinking of non-pasteurized milk of infected cows. In man M. bovis infects the bones and joints including vertebrae. Besides these pathogenic representatives, the genus includes saprophytic species, many of which can attack such recalcitrant compounds, like paraffin’s, aromatic hydrocarbons, cresol and petroleum products.

Morphologically, mycobacteria are aerobic slender rods which may be bent or curved, generally of uniform diameter, but sometimes beaded and having a tendency to form rudimentary branches. A distinctive property of the cell wall of these bacteria, not found in other Gram-positive bacteria is the presence of a high content of lipids which may account for as much as 60% of the dry weight of the cell wall. The huge amount of lipids in the wall makes the bacteria highly hydrophobic.

This causes slow permeation of nutrients into the cells with resultant slow growth rate. The lipids contain a variety of fatty acids including a unique component, known as mycotic acids. Besides mycobacteria, the cell-wall of Corynebacterium and Nocardia also contains mycolic acids. The presence of mycolic acid and other lipids in the cell wall makes these organisms acid-fast.

Acid-fastness can be determined by a staining procedure e.g. Ziehl-Neelsen method in which a heat-fixed bacterial smear is covered with carbol-fuchsin solution and steamed over flame for 2-3 min. The smear is then washed with 95% ethanol containing 3% HCI.

Washing with acid-alcohol removes the stain of non-acid-fast organisms, but acid-fast ones retain the stain. Extraction of wall lipids by treatment with alkaline ethanol removes the acid-fast property of these organisms. Mycolic acids are large, saturated branched P-hydroxylated fatty acid molecules having the general structure shown in Fig. 4.8.

(vi) Corynebacterium:

Corynebacterium includes mostly harmless saprophytic soil bacteria, except one — C. diphtheriae which causes diphtheria in humans. Corynebacteria are aerobic non-motile, rod shaped organisms, 2 to 6 μm in length and 0.5 to 1.0 μm in breadth.

The rods tend to be club-shaped and tapered at the opposite ends. Hence, the name, coryne (= club) bacteria. The daughter cells remain generally attached to each other at sharp angles before separation. Sometimes they are arranged side by side like the matchsticks in a box.

C. diphtheriae produces an exotoxin which is responsible for the pathological changes in the affected patient. The diphtheria exotoxin is a protein having a molecular weight of 62,000 Daltons. It is lethal at a dose of 0.1 (μg/kg body weight. On treating the toxin with dilute formaldehyde at pH 8.0 (37°C), it is converted to a non-toxic derivative—called a toxoid— which is used for immunization against diphtheria.

Interestingly, the gene encoding the diphtheria toxin protein is located in a lysogenic bacteriophage (P-phage). Only those C. diphtheriae bacteria possessing the phage are pathogenic. The mechanism of action of the diphtherial exotoxin has been elucidated. It has been found to inhibit the step of polypeptide chain elongation during protein synthesis in the host.

The exotoxin blocks amino acid incorporation through inactivation of the elongation factor EF-2. The inactivation is due to transfer of an ADP-ribosyl group from NAD (nicotinamide adenine dinucleotide) to EF-2 which is a protein factor needed for amino acid incorporation into the elongating polypeptide chain.

Inactivation of EF-2 by diphtheria exotoxin occurs in eukaryotic cells and in archaebacteria, but not in eubacteria. Some species of Corynebacterium, like C. michiganense, C. poinsettiae and C. fasciens cause diseases of different plants, C. ovis causes pseudo tuberculosis in sheep, cattle and horses. C. mediolanum has been used for biological transformation of steroids.

(vii) Frankia:

Frankia includes organisms which infect some 178 species of non-leguminous trees and shrubs to produce actively nitrogen-fixing root nodules. The genera of plant include species of Alnus, Casuarina, Ceanothus, Myrica, Coriaria, Elaeagnus etc. The bacteria belong to the actinomycetes and produce mycelia and conidia within multilocular sporangia. The organisms are extremely slow-growing in artificial cultures and take 6 to 8 weeks to produce visible colonies.

Frankia spp. infect the host cells either through root hair, like rhizobia, or by intracellular penetration. After entering the root tissues, they induce the root meristem to proliferate to produce root nodules. The branching hyphae produce globular vesicles in the root tissue. These vesicles are believed to be the sites of nitrogen-fixation.

The organisms do not form bacteroids as the rhizobia do, but they possess nitrogenase, the N₂-fixing enzyme, containing molybdenum, as well as, in some cases, cobalt. The enzyme is oxygen-sensitive and is protected by high levels of triterpene hopanoids.

(viii) Arthrobacter:

Arthrobacter includes species inhabiting mostly in soil where they perform important ecological functions related to mineralization of complex organic molecules including some herbicides and pesticides. The organisms are aerobic and pleomorphic. During their life cycle, they change their shape from cocci to rods to irregularly branched rods and back to cocci. In the stationary phase, most cells are coccoid.

The changes in shape during the life- cycle of arthrobacters are depicted in Fig. 4.9:

(ix) Brevibacterium:

Brevibacterium is a genus comprising coryneform rod-shaped bacteria. Some members have strong proteolytic activity e.g. B. linens which takes part in maturation of cheese.

(x) Propionibacterium:

Propionibacterium is another genus of coryneform rod-shaped bacteria. In contrast to others, they are mainly anaerobic, though they can tolerate some amount of oxygen and are catalase-positive. They produce propionic acid from a variety of sugars like glucose, lactose, pentose’s and also from organic acids like lactate, malate etc. They contribute substantially to the production of Swiss cheese.

The bacteria are typically rod-shaped, but under stress conditions they become irregular in shape forming beaded club-shaped cells (coryne form). The first heterotrophic CO₂-fixing reaction was discovered by Wood and Werkman (1936) in Propionibacterium acidipropionici while studying propionic acid formation by this organism. A species, P. acne has been found to be associated with acne formation in skin (acne vulgaris).

(xi) Bifidobacterium:

Bifidobacterium includes a group of irregular rods, often slightly curved, or club-shaped organisms with a tendency to form rudimentary branches. The organisms are strictly anaerobic. They occur usually as commensal organisms in the oral cavity, urogenital tracts and the intestine. B. bifidum is present as an intestinal organism typically in breast-fed babies. Bifidobacteria are lactic acid fermenters producing lactic acid from sugars by the heterofermentative pathway. They are usually non-pathogenic.

Morphological features of some of the high G + C Gram-positive bacteria are shown in Fig. 4.10:

B. Low G + C Group:

In the second edition of the Manual, the low G + C Gram-positive bacteria have been placed in the Phylum Firmicutes under which there are three classes, — ‘Clostridia’ Mollicutes and ‘Bacilli’. The inclusion of Mollicutes comprising the cell wall-less eubacteria (the mycoplasmas) may appear surprising, but their 16S r-RNA resembles more closely those of Clostridia than those of any other group of bacteria. So, on the basis of r-RNA homology, they have been included into the low G + C group of Gram-positive bacteria, though mycoplasmas do not respond to Gram-stain (i.e. they are neither Gram- positive nor Gram-negative).

A notable deviation from the earlier edition of the Manual is that the actinomycetes have been separated from the Firmicutes on the basis of their high G + C content of DNA. In actinomycetes, the G + C content varies from 59 to 79 moles %. Therefore, they have been placed in the Phylum Actinobacteria.

On the same ground, a thermophilic endospore-forming actinomycete, Thermoactinomyces having a comparatively low G + C content (52-55 moles %) has been brought under the low GC group, though the organism clearly resembles actinomycetes in having a mycelial morphology.

Besides Mollicutes, the low GC group includes two main types, the aerobic bacilli and the anaerobic Clostridia. Both these major types include cocci and rod-shaped bacteria which may or may not form endospores.

The characteristics of the three classes:

i. Mollicutes,

ii. Clostridia, and

iii. Bacilli.

(i) The Mollicutes:

The most important character of the Mollicutes is that their cells are not surrounded by a cell wall — a character shared also by some archaebacteria like Thermo plasma. These bacteria are unable to synthesise the precursors of peptidoglycans, the main component of bacterial wall. Due to the absence of a rigid wall, mollicutes are highly plastic and pleomorphic.

Many diverse morphological forms, like cocci, rods, ring-shaped or filamentous cells, or cells without any particular shape are encountered in a single pure culture. The diameter of mycoplasma cells varies between 0.15 and 0.3 μm which is considerably smaller than most other eubacteria.

They are considered as the smallest free-living bacteria. The cells are surrounded by a triple-layered double membrane containing sterols. Mollicutes have also small genomes which are 1/5 to 1/3 in size of other bacteria.

The smallest genome is found in a human pathogen, Mycoplasma genitalium which is only 600 kb long having a molecular weight of about 400 mega Daltons. Other members of Mollicutes have genome size ranging from 400 and 1,000 mega Daltons. G + C content of DNA varies between 23 and 40 moles %.

Mycoplasmal cells multiply by binary fission like most other bacteria, but genomic replication is not synchronized with cytoplasmic division which lags behind. Consequently, multinucleate filamentous structures are common.

These structures subsequently produce chains of coccoidal cells which eventually fragment into individual cells. Mycoplasma are devoid of flagella and they are generally non-motile. But few species as Mycoplasma gallisepticum and M. pneumoniae exhibit a gliding movement, and Spiro plasma shows a flexuous motility.

Most mycoplasmas require a rich medium for growth in artificial culture. Many species have an obligate requirement for sterols and many require also fatty acids, vitamins, amino acids, purines and pyrimidine’s. Such complex nutritional requirement reflects limited biosynthetic ability of these organisms. A small genome might be responsible for these biochemical deficiencies.

The growth of mycoplasmas is slow. On solid media, they form small microscopic colonies, about 10 to 600 µm in diameter. The colonies are transparent with a granular surface and a darker elevated central region giving a typical “fried-egg” appearance.

The class Mollicutes has been divided into four orders — Mycoplasmatales, Acholeplasmatales, Entomoplasmatales, and Anaeroplasmatales. The main genera of these orders are Mycoplasma, Spiroplasma, Acholeplasma and Anaeroplasma. 5S and 16S r-RNA sequence comparisons suggest that mycoplasmas evolved from eubacteria having walled cells through a series of regressive changes. The r-RNA comparisons also indicate that their possible progenitors belonged to the low G + C Gram- positive bacteria like Clostridia. They are phylogenetically closely related to Clostridium ramosum and C. innocuum.

Mycoplasmas cause several diseases of man, animals, insects and plants. In fact, mycoplasmas were first recognized in 1898 as an agent causing bovine pleuropneumonia. The organisms were then called pleuropneumonia like organism (PPLO). Later, this organism was identified as Mycoplasma mycoides.

Other pathogenic mycoplasmas causing diseases in man, animals and insects include M. pneumoniae which causes primary atypical pneumonia in man, M. bovigenitallum„ causing mastitis in cattle, M. agalactia, causing mastitis and contagious agalactia in sheep and goats, and M. gallisepticum, causing chronic respiratory diseases in fowl. Other pathogenic mycoplasmas include Spiroplasma melliferum and S.apis which infect honey-bees. Some mycoplasmas also infect plants.

The plant diseases believed to be due to mycoplasmal infection are small leaf of brinjal, yellow dwarf of rice, sandal spike, die-back of citrus etc. Spiroplasma citri causing stub-born disease of citrus is a helical mycoplasma, 800-1500 nm long and 200-300 nm wide which appears to block the sieve tubes of the phloem tissue. S. kenketii has been identified as the cause of corn-stunt disease.

Members of Acholeplasma are mainly soil-inhabiting saprophytes. In contrast to Mycoplasma, these bacteria do not require sterols for growth. Also, in Acholeplasma the NADPH₂-oxidase is located in the cell membrane and not in the cytoplasm as in Mycoplasma. The genome size of Acholeplasma is larger than that of Mycoplasma.

Anaeroplasma is an anaerobic mycoplasmal organism, inhabiting the rumen of cattle and sheep. All mycoplasmas are resistant to sulfonamides and penicillins, but they are sensitive to tetracycline and erythromycin.

Many plant diseases which were formerly thought to be caused by viruses were later found to be caused by mycoplasmas. Application of tetracyclines resulting in recovery of diseased plants is a suitable indicator for determination of whether a disease is caused by virus or mycoplasma, because viruses do not respond to antibiotics.

Morphology of mycoplasmal cells growing in a liquid submerged culture is shown in Fig 4.11:

(ii) Class: Clostridia:

In the second edition of the manual, the class clostridia has been divided in three orders:

Clostridiales,

Thermoanaerobacteriales, and

Haloanaerobiales.

Of these three, Clostridiales is the largest order containing 8 families and numerous genera. The other two classes include 3 families and some 16 genera.

Brief accounts of some selected genera of this class are given below:

(a) Clostridium:

Clostridium is the largest genus including more than 100 species with diverse properties. Clostridia are obligate anaerobes, endospore-forming and rod-shaped. Most species are motile with peritrichous flagella. Some Clostridia produce capsules, e.g. C. pectinovorans and C. perfringens. Majority of Clostridia are saprophytic, soil-inhabiting organisms. Some of them cause serious human diseases, like tetanus (C. tetani), botulism (C. botulinum) and gas gangrene (C. perfringens). The endospore-forming cells are often bulged giving a drum-stick or spindle shape.

As Clostridia are strictly anaerobic, they do not possess the cytochromes which transfer electrons to oxygen in aerobic organisms. Also, they do not have catalase and peroxidases which destroy hydrogen peroxide produced by the flavoproteins present in Clostridia.

As a consequence, oxygen is toxic to them. Some species like C. pectinovorans and C. histolyticum are partially oxygen-tolerant. In absence of aerobic respiration, Clostridia are dependent on fermentation for energy production. Among the useful fermentation products of Clostridia are butyric acid, n-butanol, acetic acid, acetone and isopropanol.

The species involved in these fermentations are C. butyricum (butyric acid and acetic acid), C. butylicum (butyric acid, butanol, isopropanol and acetic acid), C. acetobutylicum (butyric acid, butanol, acetone and acetic acid), C. aceticum (acetic acid), C. propionicum (propionic and acetic acids) etc.

Clostridia are also of much importance due to their pathogenic properties. C. tetani, a normal soil inhabitant, may enter through wounds and produce a highly potent neurotoxin (tetanospasmin) which produces the symptoms of tetanus. C. perfringens, C. histolyticum etc. produce a serious necrotic disease, known as gas gangrene.

These bacteria in addition to toxin produce a variety of tissue destroying enzymes, like collagenase, proteinases, hyaluronidase etc. C. botulinum is the cause of a serious type of food-poisoning due to a highly lethal exotoxin elaborated by the organism. The toxin, botulin poison, being a protein is heat-labile and is completely destroyed by heating at 100°C for 10 minutes. The toxin is produced in imperfectly sterilized canned meat or other proteinaceous food materials.

(b) Epulopiscium:

An interesting organism, a giant bacterium, named Epulopiscium fishelsoni, isolated from the intestinal tract of brown surgeon fish has been found on the basis of r-RNA homology to be closely related to Clostridium. It is a motile, cigar-shaped enormously large organism, measuring 80 µm x 200 to 500 µm. It is nearly a million times larger in volume than an average E. coli cell. It possesses peritrichous flagella.

The order clostridiales also includes some other unusual genera. Although they are all anaerobic, they differ morphologically as well as in several other characteristics from typical Clostridia. Many of them do not form endospores.

Some of these genera are mentioned below:

(c) Sarcina:

Sarcina includes large cocci (up to 4 µm) in cell packets containing up to 64 cells held together by cellulose. S. ventriculii is normally a soil organism, but it has been found in the gastric content of patients suffering from gastric ulcer. The organism has a remarkable ability to grow over a broad pH range, from pH 0.8 to pH 9.8.

(d) Desulfotomaculum:

Desulfotomaculum spp. are anaerobic, rod-shaped, endospore-forming is peritrichously flagellate motile bacteria. Although the cell wall is of Gram-positive type, the bacteria stain as Gram-negative. An important feature of these bacteria is that they have the ability to reduce sulfate to sulfide by the dissimilatory sulphate reduction pathway carrying out so-called sulphate respiration. A similar process occurs in the species of Desulfovibrio which are Gram-negative organisms. Desulfotomaculum ruminis is involved in H₂S production in the rumen of herbivorous animals.

(e) Heliobacter:

Heliobacter and the related genera Heliophilum and Heliobacillus, placed in the family Heliobacteriaceae under the order Clostridiales are anaerobic photosynthetic bacteria carrying out anoxygenic photosynthesis. They have an unusual chlorophyll, bacteriochlorophyll g.

An atypical feature of these bacteria is that unlike other photosynthetic bacteria, the photosynthetic pigments are localized in the cytoplasmic membrane itself and not in chromatophores or chlorosomes. A character shared by these bacteria with Desulfotomaculum is that they have a Gram-positive cell wall, but the bacteria stain are Gram-negative. All other anoxygenic photosynthetic bacteria are truly Gram-negative.

(f) Veillonella:

Veillonella spp. are anaerobic, Gram-negative, heterotrophic cocci measuring 0.3 to 2.5 p.m in diameter, occurring in pairs, chains or clusters or also as single cells. V. alcalescens occurs in saliva as also in rumen of herbivores. The organisms have a fermentative metabolism, but are unable to utilize glucose or other sugars. V. alcalescens ferments organic acids, particularly lactic acid, to produce propionic acid.

Morphology of some of the bacteria included in the class ‘Clostridia’ is shown in Fig. 4.12:

(iii) Class ‘Bacilli’:

The class ‘Bacilli’ has been divided into two orders, Bacillales and Lactobacillales. There are 9 families under Bacillales and 6 under Lactobacillales. The members of the order Bacillales are diverse, both morphologically as well as physiologically.

The genera include rods, cocci, mycelial forms and trichome-forming types. Endospores may or may not be present. They may be motile or non-motile; when motile they are usually peritrichously flagellate. They are usually aerobic to facultatively anaerobic, usually catalase positive and have a chemoorganotrophic nutrition. The bacteria are mostly saprophytic, but a few are pathogenic. Representative genera include Bacillus, Staphylococcus, Thermoactinomyces and Caryophanon.

The second order, Lactobacillales, is represented by non-sporing rods and cocci, usually catalase negative, non-motile, fermentative and facultatively anaerobic organisms. There are some pathogenic species. Representative genera of the order include Lactobacillus, Lactococcus, Leuconostoc, Streptococcus and Enterococcus.

The characteristics of some of the genera belonging to the two orders are briefly mentioned here:

(a) Bacillus:

Bacillus is the largest genus of the order Bacillales. The members are all aerobic, endo-spore forming rods, catalase positive having a chemoorganotrophic metabolism. They are generally motile and provided with peritrichous flagella. They are widespread in soil.

Bacillus anthracis is the cause of anthrax disease, commonly occurring in cattle, but can also infect man. B. cereus sometimes causes food poisoning. B. thuringiensis and B. sphaericus possess a parasporal crystalline protein having insecticidal properties. This protein is commercially used as insecticide.

(b) Staphylococcus:

Staphylococcus spp. are aerobic to facultatively anaerobic, non-motile, non-spore forming cocci (0.9 to 1.3 µm in diameter), occurring singly or more often in bunches. Staphylococci are generally non-pathogenic, but also include pathogenic strains or opportunistic pathogens. S. aureus causes pyogenic infections in man, causing boils, abscesses, impetigo and carbuncle.

It is toxicogenic and coagulase positive. Coagulase forms a wall of fibrin around the infected portion (lesion) which prevents entry of leucocytes and thereby also preventing phagocytosis of the bacteria. Coagulase negative staphylococci are non-pathogenic. The opportunistic species include S. epidermidis and S. saprophytics.

(c) Caryophanon:

Caryophanon spp. are normal constituents of cow dung. They are aerobic, catalase positive, Gram-positive trichome forming organisms. C. latum forms multicellular trichomes, 1.5-2.0 x 15-20 µm in size, motile, with peritrichous flagella. Individual cells of trichomes are disc-shaped, 1.5-2.0 x 0.5-1.0 µm in dimension which are stacked one upon the other.

(d) Thermoactinomyces:

Thermoactinomyces includes thermophilic, aerobic, branched, septate mycelium-forming actinomycetes producing true single endospores within tips of hyphae. The inclusion of the genus in the Order Bacillales and its separation from other actinomycetes are justified by a comparatively low G + C content of DNA (52 to 54.8 moles %), r-RNA homology with members of the order Bacillales and the ability to form true heat-resistant endospores containing Ca-dipicolinate.

The organisms grow naturally in decaying hay-stacks, compost piles and grain storage silos. The endospores have an outer and an inner spore-coat, a cortex and a central core like other typical endospores. A common species is T. vulgaris.

(e) Listeria:

Listeria is another genus under the order Bacillales. The majority of the members occur extensively in soil and on plants, but a few cause listeriosis in man and animals. The organisms are aerobic to microaerophilic, catalase positive, non-acid fast and Gram-positive. They are non-spore forming, motile, rod-shaped bacteria. L. mono-cytogenesis causes meningo-encephalitis or meningitis in man.

(f) Sporolactobacillus:

A spore-forming lactic acid bacterium, Sporolactobacillus has been removed from the rest of the lactobacilli and transferred to the order Bacillales. These organisms resemble other lactobacilli in most characters but they form endospores — a feature unknown in lactobacilli.

The other order under the class ‘Bacilli’ is Lactobacillales which includes both cocci and rod- shaped Gram-positive, aerobic to facultatively anaerobic or micro-aerophilic, non-motile bacteria. The order includes a number of useful as well as pathogenic organisms.

(g) Lactobacillus:

Lactobacillus is a large genus with about 80 species. The bacteria are straight non-motile rods, lacking catalase and cytochromes, facultatively anaerobic to microaerophilic and non-spore forming. They are fermentative and produce lactic acid alone, or together with other products. Depending on the species and nature of fermentation, lactobacilli are homo-fermentative or heterofermentative.

The former group include species like L. lactis, L. helveticus, L. acidophilus, L. bulgaricus etc. The heterofermentative species are L. brevis, L. fermentum, L. viridiscens etc. Most of the members of the genus require complex media for growth.

A favorite medium is milk. Species of the genus often have obligate requirement of vitamins of the B-complex group, like riboflavin, thiamine, nicotinic acid, folic acid, pantothenic acid and biotin, and also of amino acids. Many members of the genus are used for production of fermented milk products, like yogurt, curd, casein, sour milk and fermented vegetables products, like pickles.

(h) Pediococcus:

Pediococcus is another genus included in the family Lactobacillaceae. The organisms, like P. cerevisiae, are catalase negative cocci occurring in tetrads or packets, carrying out homo-fermentation and produce lactic acid.

(i) Streptococcus, Enterococcus, Lactococcus:

The spherical or nearly spherical Gram-positive bacteria belonging to the order Lactobacillales have been divided into the genera Streptococcus, Enterococcus, Lactococcus, and Leuconostoc. The first three genera were previously included in a single complex genus which was named Streptococcus.

Later, the genus has been split into three genera. Even after removal of the members of Enterococcus and Lactococcus, the remaining species of Streptococcus are heterogeneous. There are more than 40 species of Streptococcus which are divided in two main groups — pyogenic streptococci and oral streptococci.

Some major distinguishing features of streptococci, enterococci and lactococci are shown in Table 4.3:

Species of Streptococcus are important human pathogens. S. pyogenes causes acute tonsillitis and pharyngitis, as well as skin infections, like impetigo. Sometimes it can infect bones and joints. Pneumonia (lobar) is caused by S. pneumoniae.

Enterococcus faecalis is an opportunistic pathogen which may cause urinary tract infections and even endocarditis.

Lactococcus lactis and L. cremoris are homo-fermentative lactic acid producers and are used in preparation of various dairy products.

Leuconostoc spp. are relatively large, more or less elliptical, catalase negative, facultatively anaerobic bacteria, occurring in pairs or short chains. They produce lactic acid by the heterofermentative pathway. L. mesenteroides and L. dextranicum produce profuse dextran, a polymer of glucose, when the bacteria grow in sucrose medium. Leuconostoc spp. are used in production of fermented vegetable foods, like sauerkraut (produced from fermented cabbage), pickled cucumber and ‘idli’.

Morphological features of some members of the class ‘Bacilli’ are shown in Fig. 4.13:

Type # 3. Gram-Negative Bacteria:

On the basis of r-RNA relationships, the Gram-negative bacteria are far more diverse than the Gram-positive ones. This has necessitated their classification in widely separated phyla. For example, the ancient bacteria like Aquificae, Thermotogae, Deinococci, Chloroflexi, Thermi etc. are all Gram- negative.

Similarly, the anoxygenic photosynthesizes — Chlorobi and oxygenic photosynthesizes- cyanobacteria are also primitive and Gram-negative. The largest phylum of Gram-negative bacteria is Proteobacteria. It is a highly complex assemblage including more than 1,300 species spread over 332 genera.

It has been divided into five classes, called α, β, g, d and e-proteobacteria. Outside this complex phylum and the phyla of primitive bacteria, several others have been created to accommodate the rest of Gram-negative bacteria. Among these are Planctomycetes, Chlamydiae, Spirochaetes, Bacteroidetes, Fusobacteria, etc.

The phylogenetic diversity of the Gram-negative bacteria is reflected in the variations of their forms and functions. Morphologically, these bacteria not only have the usual coccal, rod-shaped, spiral and curved cells, all having a rigid wall, but also stalked, filamentous and flexible cells.

Variation is also noticed in their mode of multiplication which may be binary fission, budding or other means, like formation of specialized spores e.g. myxospores. The myxobacteria can build micro-fruiting bodies by accumulation of individual bacteria. So far as the mode of nutrition is concerned, the Gram-negative bacteria show even greater diversity.

Besides normal chemo-organotrophy, there are forms showing chemo-lithotrophy, photo-organotrophy and photo-lithotrophy. In addition, diazotrophy, and methyl trophy are also known. All grades of oxygen relationship occur in these bacteria, starting from fully aerobic to obligately anaerobic.

For the sake of better comprehension, the different phyla and their representative types have been discussed here on the basis of their morphological or physiological similarities, rather than on their r-RNA phylogeny. As for example, the photosynthetic bacteria (anoxygenic types) have been discussed together although they belong to different groups of r-RNA homology.

Similarly, the stalked and prosthecate bacteria have been treated together, although phylogenetically they are distant. However, the systematic positions of the selected representative types according to the second edition of the Manual have been mentioned in each case.

(i) Phylum Chlamydiae:

In the second edition of the Manual, the 16^th Phylum Chlamydiae has a single class of the same name. The latter includes a single order Chlamydiales with four families and five genera. Of these, Chlamydia is the most important and well-known.

The bacteria are very small, coccoid (0.2 to 1.5 μ), non-motile, obligately parasitic organisms, causing diseases in man, animals and birds. The cell wall lacks peptidoglycan and muramic acid. The genome size is small (4 to 6 x10⁶ Daltons) and has G + C content ranging between 41 to 44 moles %.

The small genome size is reflected in their limited metabolic capacity. The bacteria are unable to utilize carbohydrates or other substrates to synthesise ATP. As a result, they become absolutely dependent on the host cells for supply of energy. Therefore, chlamydiae are known as energy-parasites. They possess a unique membrane-located translocase for transporting ATP of the host cell into their own cells.

Chlamydiae, being obligate parasites, cannot be grown in culture media and have to be studied from cell cultures. They have a characteristic developmental cycle. Infection of a host cell occurs through an elementary body (EB) which is a small spherical structure (0.3-0.4 µm).

The elementary body attaches to the host cell surface and enters into it by phagocytosis activity of the host cell. Within the phagosome, the elementary body is transformed into a reticulate body (RB) having a larger size (0.6-1.5 µm), a less dense reticulate nuclear structure and many ribosomes.

The reticulate body then divides to form a large number of similar bodies to fill most of the space of the host cell. Next, these bodies are transformed into elementary bodies. These bodies are eventually released by lysis of the host cells to complete the life-cycle. It takes 48 to 72 hr.

The developmental cycle is diagrammatically represented in Fig. 4.14:

C. trachomatis infects human beings causing an eye-disease known as trachoma. It can also cause urethritis and other diseases. Another pathogenic species C. psittaci is the cause of psittacosis in man. It can also infect many animals causing diseases of intestinal, respiratory and genital tracts, as also of eye, placenta etc. C. pneumoniae is one of the common causes of pneumonia in man.

(ii) Phylum Spirochaetes:

Phylum Spirochaetes includes a single class, a single order and three families. There are several medically important genera, like Spirochaete, Treponema, Borrelia, Leptospira etc. Spirochaetes are remarkable for their flexible spiral cells and their characteristic motility. The cells are very thin, generally 0.1 to 1.0 µm in thickness, and comparatively long (5.0 to 20 µ or even longer).

Some of them are aerobic while others are facultative or anaerobic. G + C content varies widely among different genera between 25 to 65 moles %. They are also ecologically quite diverse. Many of them are aquatic, both in fresh water and marine habitats.

Others live symbiotically in association with various animal organisms, as in the hindgut of termites, the digestive tract of mollusks and other animals. They also occur in the oral cavity of mammals including man. Among the important pathogens are Treponema pallidum causing syphilis and Borrelia burgdorferi, the cause of lyme disease in man.

The characteristic flexuous movement of the spirochaetes is due to the presence of a unique type of flagellar complex known as axial filament which consists of two to many individual prokaryotic flagella extending from the poles and lying inside the outer membrane.

(iii) Phylum Bacteroidetes:

The Phylum Bacteroidetes includes three classes:

Bacteroides,

Flavobacteria and

Sphingobacteria.

Some well-known genera are Bacteroides, Flavobacteria, Cytophaga, Flexibacter, Saprospira and Crenothrix. Of these, the last four belong to Sphingobacteria. Members of Bacteroides are obligate anaerobes, inhabiting oral cavities, intestinal tracts of mammals and the rumen of herbivores. B. ruminicola and B. succinogenes are rumen organisms which are capable of degrading cellulose, pectin and other complex carbohydrates. B. fragilis is pathogenic to man.

Flavobacterium species are mainly soil organisms where they degrade complex compounds including chitin. F. meningosepticum is an opportunistic human pathogen. Cytophaga and the related Sporocytophaga are thin rod-shaped bacteria capable of degrading cellulose. They are normally present in soil, C. columnaris is a fish pathogen causing fin-rot. Flexibacter spp. are long flexuous thread-like bacteria containing a yellow carotenoid pigment, called flexirubin.

(iv) Phylum Planctomycetes:

The Phylum Planctomycetes includes a stalked bacterial genus Planctomyces. The organisms are elliptical and provided with a stalk that is formed of polysaccharide excreted by the bacteria and has a holdfast with which the bacteria can attach to substratum.

The bacterial cells are characterized by crateriform pits on the wall and numerous pili. The organism reproduces by budding. The daughter cells are at first non-stalked and are flagellate motile swarmers. Later they produce stalk and become attached.

An unusual feature of Planctomyces, unknown in prokaryotes, is that their nuclear body is surrounded by a membrane. Also, their cell wall does not contain peptidoglycan, a character they share with chlamydiae. Other genera of the phylum are non-stalked.

Morphological features of Planctomyces are shown in Fig. 4.15:

(v) Phylum Proteobacteria:

Except the phyla discussed above and the cyanobacteria (discussed later), the rest of the Gram- negative bacteria are included in a large and complex Phylum which has been named as Proteobacteria. It constitutes the largest group among all eubacteria comprising 5 classes. These are — Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria and Epsilonproteobacteria. The separation of the phylum into five classes is based primarily on 16S r-RNA sequence studies. Each class is further divided into orders, families, genera and species.

The number of orders and families in the five classes of Proteobacteria is:

Class Alphaproteobacteria — 6 orders and 18 families.

Class Betaproteobacteria — 6 orders and 12 families.

Class Gammaproteobacteria — 12 orders and 20 families.

Class Deltaproteobacteria — 7 orders and 17 families.

Class Epsilonproteobacteria — 1 order and 2 families.

(a) α-Proteobacteria:

On the basis of 16S r-RNA homology, the α -proteobacteria are phylogenetically related to each other. Nevertheless, the bacteria included in this class are morphologically as well as physiologically highly diverse. Morphological diversity is shown by the presence of cocci (Nitrococcus), coccoid rods (Rickettsia), regular rods (Rhizobium), prosthecate rods (Hyphomicrobium), prosthecate vibrios (Caulobacter) and spirilli (Rhodospirillum). Physiologically, the group is even more diverse. There are purely chemo-organotrophs, partly or fully chemo-lithographs, methanotrophs, diazotrophs as well as phototrophs. Some are pathogenic including even intracellular obligate parasites.

The phylogenetic relationships among some selected genera based on 16S r-RNA are shown in Fig. 4.16 and the important physiological groups with some representative genera are given in Table 4.4:

Rickettsia includes several important human pathogens causing typhus and rocky mountain spotted fever. The diseases are spread through arthropod vectors, like fleas, ticks, mites and lice. The organisms are obligate parasites and cannot be grown in artificial cultures. The bacteria enter host cells by phagocytosis and multiply in the cytoplasm. They are released from host cells by bursting the cells, causing tissue damage.

(b) Betaproteobacteria:

The class Betaproteobacteria has also been divided into 6 orders containing bacteria which are morphologically and physiologically diverse. There are cocci (Neisseria), simple rods (Thiobacillus), ellipsoidal cells (Nitrosomonas), cocco-bacilli (Bordetella), spirilli (Spirllum) as well as chains of rod-shaped bacteria covered by a sheath (Sphaerotilus). Flagellation is usually polar in motile species. The betaproteobacteria may be chemoorganotrophic (Aquaspirillum), chemolithotrophic (Nitrosomonas), photoheterotrophic (Rhodocyclus) or methylotrophic (Methylobacillus).

The six orders of the class are named below together with some well-known genera:

The order Burkholderiales includes simple motile rod-shaped bacteria, like Burkholderia, non-motile encapsulated coccoid rods, like Bordetella and long chains of rods within a sheath as in Leptothrix and Spherotilus.

Burkholderia cepasia is well known for its ability of degrading more than 100 different organic compounds and it plays a significant role in biodegradation in soil. It is also plant pathogenic and an opportunistic human pathogen.

Bordetella pertussis is a human pathogen causing whooping cough, particularly in children. The sheath-forming genera, Spherotilus and Leptothrix, grow profusely in polluted waters. They characteristically deposit metal oxides of iron and manganese on their sheath. Spherotilus natans is a common pest of sewage treatment plants.

The sulfur-oxidising chemolithotrophic genus Thiobacillus belongs to the order Hydrogenophilales. Some species of this genus are obligate chemolithotrophs, like T. thiooxidans and T. ferrooxidans. They use S⁰, H₂S, thiosulfate, Fe²⁺etc. as oxidisable substrates for reduction of CO₂ by the Calvin cycle. ATP is generated by oxidative phosphorylation and substrate level phosphorylation of adenosine 5′-phosphosulfate (APS).

Other species of Thiobacillus like T. Novellus and T. intermedius are facultatively chemolithotrophic. All sulfur-oxidizing bacteria produce sulfuric acid as oxidation product and they are highly acid resistant. T. ferrooxidans can also oxidize Fe²⁺ to Fe³⁺ and can transfer electrons to oxidative phosphorylation pathway for ATP production. These bacteria have been utilized for recovery of copper from low-grade ores. Thiobacilli also play an important role in the sulfur-cycle of nature.

They oxidize reduced sulfur-compounds released by protein decomposition, or elemental sulfur produced by photosynthetic bacteria to sulfate which is the main source of sulfur for most microorganisms and plants. Thiobacilli are motile, polar flagellate, aerobic, rod-shaped bacteria having G + C content of 62 to 68 moles %.

The order Neisseriales includes a variety of morphological forms, like aerobic, non-motile, catalase-positive cocci which are often in pairs, spirilli with bipolar tufts of flagella and filamentous organisms e.g. Vitroscilla. Neisseria has two important human pathogens, N. gonorrhoeae and N. meningitides causing gonorrhoea and meningitis, respectively. N. gonorrhoeae has encapsulated cocci in pairs, the capsule consisting of polyphosphates. N. meningitides has a capsule containing N-neuraminic acid, a compound usually present in eukaryotes. Both are aerobic and catalase positive.

Among the other genera included in the order Neisseriales are Aquaspirillum which are motile, aerobic to microaerophilic organisms, with bipolar flagella inhabiting fresh-water ponds and lakes, and Vitreoscilla and Simonsiella which are filamentous aerobic organisms. Vitreoscilla is a colourless, flexible, gliding organism. The filaments tend to break in short segments or hormogonia. The hormogonia are about 1.0 µ in width and vary greatly in length (3 to 70 µ). G + C content is 43.6 moles %. Another genus Simonsiella is also filamentous.

The order Rhodocyclales includes, among others, the genera Rhodocyclus and Zoogloea. Rhodocyclus species are non-sulfur purple photosynthetic bacteria. The cells are strongly curved, sometimes nearly circular. Zoogloea species are aerobic rod-shaped bacteria embedded in copious slime. Z. ramigera cells are embedded in branched slime structures. The species occurs in sewage treatment tanks producing floes.

The representative genera of the two other orders (Nitrosomonadales and Methylophiales) belonging to the class Betaproteobacteria have been treated together with the allied genera of other proteobacteria.

Morphological characteristics of some of the genera of Betaproteobacteria are shown in Fig. 4.17:

(c) Gamma proteobacteria:

It constitutes the largest class in the phylum Proteobacteria. Many organisms of this class are physiologically and pathologically important. Some of the important orders are Chromatiales (the sulfur purple bacteria), Xanthomonadales of which many members are plant pathogenic, Legionellales, Pseudomonadales, Enterobacteriales etc.

Like the two earlier classes of proteobacteria, gammaproteobacteria also are morphologically and physiologically diverse. Members include rods, vibrios and trichome forming types. Among the different physiological types are anoxygenic phototrophs, diazotrophs, simple heterotrophs as well as one-carbon compound metabolizers. There are several important human, animal and plant pathogens in this class.

Enterobacteriales:

The largest and the most important order of gammaproteobacteria is Enterobacteriales which consists of a single family Enterobacteriaceae having 41 genera. Some of these genera are Escherichia, Enterobacter, Salmonella, Shigella, Yersinia, Klebsiella, Proteus, Citrobacter, Serratia etc.

Enterobacteria are generally motile with peritrichous flagella except Shigella, Klebsiella and Yersinia. Members of this order are generally rod-shaped (0.3 to 1.5 µm in width and 3 to 5 µ in length), oxidase negative and capable of growth in simple media using glucose via EMP. Majority of them carry out mixed acid fermentation producing a variety of organic acids, like acetic, lactic, formic and succinic acids. Some members can ferment lactose to produce acid and gas, and they are commonly known as coliform bacteria. Escherichia coli is a typical coliform organism. The others of this group include Salmonella, Shigella, Proteus, Yersinia etc. They are so named because they are present in the large intestine of man and other mammals.

Escherichia coli is the most well-known organism among all bacteria for its widespread use in bacterial genetics, biochemistry and molecular biology. It has served as a model organism in microbiological research for a long time. E. coli is a normal inhabitant of intestine, but specific strains can be opportunistic pathogen causing gastroenteritis. It is also the most important cause of urinary tract infection. E. coli is used as an indicator organism for determining the suitability of water for drinking purpose, because its presence in water indicates contamination with fecal matter.

Enterobacter aerogenes is closely allied to E. coli. It occurs mainly in water, soil and sewage. It is generally non-pathogenic, but sometimes may be an opportunistic pathogen causing urinary tract infection.

Klebsiella pneumoniae is a non-motile capsulated bacterium occurring mostly as a saprophyte in soil, water and plants, as well as a microbial component of the human intestine. Some strains are pathogenic causing severe pneumonia and other ailments like urinary tract infection, septicaemia or diarrhoea. Klebsiella can fix di-nitrogen.

Salmonella represents mainly pathogenic enterobacteria together with Shigella. Species of Salmonella are mostly facultative anaerobes. S. typhi is the cause of typhoid fever and S. paratyphi causes paratyphoid. S. typhimurium and S. enteridis are important agents of gastroenteritis. Some other species are pathogenic to animals.

Shigella dysenteriae is a non-motile, aerobic organism causing bacillary dysentery. The pathogenicity of this bacterium is due to production of endotoxin as well as several types of exotoxins which act on the mucosal membrane of the intestines, as well as on the blood vessels of the central nervous system.

The other genera of Enterobacteriales include Proteus, Serratia, Erwinia, Yersinia etc. Proteus vulgaris vigorously motile, aerobic, urea-hydrolyzing bacterium. Proteus mirabilis is a common pathogen causing urinary tract infection. These species are normal inhabitants of polluted water and -soil. They are also common in the intestines of man and animals. Serratia marcescens is widely distributed in soil and water, but is also an opportunistic human pathogen. It produces a blood-red pigment in medium as well as in contaminated food. The pigment is prodigiosin.

Yersinia pestis is an ovoid, non-motile, aerobic to facultatively anaerobic organism, often occurring in pairs or chains. It is the causal organism of plague. It infects also rodents and is transmitted to man through bites of fleas from the dead infected rats. Y. enterocolitica is a motile bacterium causing gastroenteritis, particularly in children.

Another order of Gammaproteobacteria is Pasteurellales, having two important genera, Pasteurella and Haemophilus. P. multocida, normally present in the respiratory tract of animals causes haemorrhagic diseases in cattle and fowl cholera. Haemophilus influenzae is an important human pathogen causing mainly meningitis. The encapsulated bacteria are highly pleomorphic.

The order Legionellales has also two important genera, Legionella and Coxiella. Coxiella burnettii is the cause of Q-fever. The organisms are related to Rickettsia belonging to the alphaproteobacteria. Legionella pneumophila is a human pathogen causing a type of pneumonia.

The organisms are slender rods, aerobic, motile with polar to sub-polar flagella and mainly aquatic. Both Coxiella and Legionella can be grown in artificial culture, but species of Rickettsia cannot be artificially cultivated, though they grow in cell monolayers.

(d) Delta proteobacteria:

This class of Gram-negative bacteria consists of 7 orders and 17 families. More than half of these families include the anaerobic sulfate reducing bacteria. The rest includes, among others, the myxobacteria which form characteristic micro-fruiting bodies and the genus Bdellovibrio which parasitizes other eubacteria.

The anaerobic sulphate reducing bacteria have been placed in the first four orders of the class Deltaproteobacteria. The well-known genera of these bacteria are Desulfovibrio. Desulfuromonas and De-sulfur-ococcus. Another physiologically allied genus, Desulfotomaculum is a Gram-positive bacterium which has been placed under the order Clostridiales.

The sulfate reducing bacteria thrive at the bottom of aquatic bodies rich in decomposing organic matter. They reduce sulfate or rarely elemental sulfur to sulfide. The sulfide produced by the activities of these bacteria reacts with metal ions of iron, manganese etc. to yield black sulfides.

The bacteria possess the ability to transfer electrons from the cytochromes to sulfate acting as the terminal electron acceptor in place of oxygen which is absent under anaerobic conditions. This process is known as dissimilarity sulfate reduction. These bacteria thus carry out a type of anaerobic respiration, known as sulfate respiration. They generate ATP by electron transport system, but use sulfate in place of oxygen.

The small order Bdellovibrionales includes the interesting genus Bdellovibrio which is parasitic on other Gram-negative bacteria. The bacteria are aerobic curved rods, highly motile (100 cell lengths/sec) with a single comparatively thick polar flagellum (~ 50 nm thick) covered with a sheath which is continuous with the outer membrane of the Gram-negative cell.

The presence of a sheathed flagellum is an unusual character for prokaryotes. Most strains are obligately parasitic on other bacteria. Like bacteriophages, bdellovibrios form plaques on a lawn of host bacteria which are generally pseudomonads and enterobacteria. The bacteria are aquatic, and occur both in fresh-water and marine habitats. G + C content of DNA varies from 33 to 38 moles % in marine species and about 50 moles % in fresh-water isolates.

Bdellovibrios have a life cycle. The rapidly swimming cell collides with a target cell and gets attached with the non-flagellated end. It then rotates rapidly at a speed of about 100 revolutions per second to bore a hole in the cell wall of the target cell through which it enters into the periplasmic space.

The parasite remains restricted in this space and never enters into the cytoplasm of the host cell. Soon after entry, the host cell increases greatly in size and is transformed into a more or less globular bdelloplast. The peptidoglycan undergoes change due to an enzyme produced by the parasite resulting in the formation of the bdelloplast.

Other important changes include degradation of host cell DNA, RNA and 70S ribosomes. The invading parasite then develops into an elongated coiled structure using the nucleoside monophosphates and other products derived from degradation of the host cell DNA, RNA, fatty acids, amino acids etc.

The developing parasite now occupies most of the space of the host cell and the host cell cytoplasm retracts to an insignificant part of the greatly enlarged bdelloplast. The coiled structure of the parasite next breaks up into individual Bdellovibrio cells which develop flagella and are released by lysis of the host cell with the help of an enzyme which is active on the bdelloplast wall. The number of progeny bdellovibrios per host cell varies between 3 to 5 at 30°C. It takes about 3.5 hr to complete the life cycle.

Fig. 4.18 shows a Bdellovibrio and the life-cycle:.

The order Myxococcales of deltaproteobacteria encompasses myxobacteria which are characterized by formation of micro-fruiting bodies. They are unique among prokaryotic organisms and are comparable to eukaryotic Acrasiales and slime molds.

Myxobacteria are aerobic, rod-shaped, flexible organisms, mostly with tapering or blunt ends and capable of a gliding movement. They occur abundantly on surface soil, compost, bark of trees, decomposing plant parts and specially in animal dung. They grow as yellow to orange swarms of vegetative cells, leaving behind a trail of slime as the swarm glides along. G + C content varies from 67 to 70 moles %.

The most characteristic feature of myxobacteria is their fruit-bodies. These are brightly coloured, may be simple or elaborate and small, measuring a few millimeters. Within the fruit bodies individual bacteria are transformed into resting cells, called myxospores. Myxospores are resistant to desiccation, osmotic shock and partly to UV-light, but not to heat. The important genera are Myxococcus, Archangium, Cystobacter, Stigmatella, Polyangium and Chondromyces.

The fruit bodies of some representative genera are shown in Fig. 4.19:

Fruit-body formation by myxobacteria represents an extraordinary example of colonial morphogenesis in which many thousands of individual vegetative cells forming a ‘swarm’ cooperate and behave almost like a multicellular organism. The individual cells of a swarm aggregate to form a mound which grows in size and is differentiated into a specific way depending on the organism to build a fruit-body.

During differentiation most of the cells are sacrificed and comparatively few are used to form the myxospores. A special advantage derived from fruit-body formation is that the simultaneous germination of large number of myxospores can produce a swarm of vegetative cells which can migrate as a body in search of food.

Fruit-body formation is initiated by shortage of food which possibly sends a chemical signal that the life of the swarm is endangered. Most species of myxobacteria can digest food materials, including other bacteria, by secretion of various hydrolytic enzymes.

(e) Epsilon proteobacteria:

It is the smallest class of proteobacteria comprising of a single order Campylobacteriales. There are two well-known genera — Campylobacter and Helicobacter, both being of medical importance. Campylobacter species are microaerophilic, highly motile vibrios with a single polar flagellum. The bacteria are oxidase and catalase positive. They grow best at 42°C and in simple media at a reduced oxygen pressure (~ 7% by volume). C. jejuni is an important causal agent of diarrhoeal diseases in man. C. fetus is known to cause abortion in cattle.

Helicobacter pylori is also a microaerophilic bacterium, having a vibroid to spiral cell. The bacteria are motile with 3 to 5 sheathed flagella. They are found in the human gastric mucosa and have been implicated as a cause of duodenal and gastric ulcer. They produce abundant urease. The ammonia released by the enzymatic action might damage the mucous membrane of the stomach.