The world of bacteria II

The world of bacteria II

Like volume 1 of Bergey’s or manual, volume 2 includes many of the familiar or ordinary chemoheterotrophic eubacteria; however, the organisms are gram-positive rather than gram-negative. Most have a simple morphology and none from the prosthesis, sheaths, or cysts. Some Genera forms heat-resistant endospores. Although most of the organisms have a regular, uniform shape, some are pleomorphic. Most of the organisms occur singly, in pairs, or in chains but some are arranged in trichomes and others form branching hyphae. Motility, if present, is by means of flagella. In general, reproduction occurs mainly by transverse binary fission; however, multiplication by fragmentation or by conidiospore production can occur in several genera. Saprophytes and parasites are included; some of the parasitic organisms can be highly pathogenic for humans, animals, or plants.

Bergey’s manual of systematic bacteriology

The arrangement of the bacteria continues to be based primarily on practical considerations rather than on phylogenetic relationships. The organisms are divided into major sections, each bearing a descriptive common name. Formal taxonomic names are emphasized mainly at the family, genus, and species level of classification.

Gram-positive cocci

All the cocci in this group have the following features

1. They possess cytochromes.
2. They are able to respire with oxygen i.e. have an oxidative type of metabolism.
3. Some can also obtain energy under anaerobic conditions by fermentation (have a fermentative type of metabolism)

The organisms have been placed in two families. Members of the family Deinococcaceae exhibit the following characteristics:

1. The cocci mainly in tetrads or cubical packets.
2. The organisms have an usually high resistance to gamma and ultraviolet radiation.

The family contains a single genus, Deinococcus, which forms red colonies. The radiation resistance of the genus is reflected by the name of one of the species, D. radio trans. The organisms can often be isolated as spoilage agents from foods preserved by treatment with ionizing radiation.

The family Micrococcaceae exhibits the following features:

1. The cocci occurs mainly in clusters, tetrads, or cubical packets of eight cells.
2. The cells do not exhibit any unusual resistance to gamma and ultraviolet radiation.

Three of the genera include in the family are described below:

Micrococcus

These non-motile cocci are aerobic, oxidative, catalase-positive. Their colonies may be red, orange, yellow, or not pigmented. Micrococci are harmless saprophytes occurring in soil and freshwater, but they can also be found on the skin of humans and animals.

Planococcus

These organisms are also aerobic, oxidative, catalase-positive cocci; however, the cells are motile and possess one to three flagella. The colonies are yellow-brown. Planococci is harmless saprophytes that occur in marine environments.

Staphylococcus

Staphylococcus are non-motile cocci that rare catalase-positive and facultatively anaerobic, having both an oxidative and a fermentative type of metabolism. They are parasites, occurring on the skin and mucous membranes of humans and warm-blooded animals. The major pathogenic species is S.aureus, which can cause boils, abscesses, wound infections, postoperative infections, toxic shock syndrome, and food poisoning in humans, and infections in animals, such as mastitis in cattle. In the laboratory, S.aureus produces white to golden-colored colonies and is positive for the coagulase test (a test for the ability of bacteria to cause blood plasma to clot). S. epidermis and S. saprophytic, which are coagulase-negative, can cause wound infections, endocarditis, and urinary tract infections.

Aerotolerant fermentative cocci

These cocci have the following characteristics:

1. They do not possess cytochrome.
2. They have only a fermentative type of metabolism and do not respire, yet they can grow anaerobically.
3. The cells are arranged in pairs, chains, or tetrapods.

Some representative genera are described below.

Streptococcus

This genus has the following features:

1. The cells are arranged in pairs or chains.
2. They are catalase-negative.
3. They organisms are homofermentative, i.e., the predominant end product of sugar fermentation is lactic acid. (in the case of streptococcus, it is the L(+) optical isomer of lactic acid.)

Although the genus is usually considered aerotolerant, some strains can tolerate only how low levels of oxygen and some are anaerobic. Nutritional requirements are complex, including several amino acids and vitamins. The streptococci are divided into categories known as the Lancefield groups based on differences in their cell wall polysaccharides.

Some streptococci are - hemolytic on blood agar: the colonies are surrounded by a clear, colorless zone that indicates complete lysis of the erythrocytes. Other streptococci are the -hemolytic (colonies surrounded by the cloudy colorless or greenish zone of partially lysed erythrocytes)

Or are nonhemolytic.Most streptococci are parasites of humans and animals and several species are pathogenic. There are many species of streptococci; a few examples follow. S. Pyogenes (-hemolytic; Lancefield group A) is the most clinical important species. It causes Spectrococcal a sore throat, scarlet fever, erysipelas, acute glomerulonephritis, rheumatic fever and other human infections. S. Mutans (nonhemolytic; not placed in any Lancefield group) inhibits the human oral cavity and is the major causative agent of dental caries.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.