Biological Classification of Class 11

They are obligate anaerobes occurring in marshy habitats. They are capable of converting CO2 and formic acid into methane and hence the name methanogens. This property is exploited commercially in the production of fuel gas and methane in gobar gas plants; the biogas fermenters. Some of the methanogens live in rumen of herbivorous animals like buffalo, cow etc. (ruminants) that chew their cud. These microorganisms assist in fermentation of cellulose in such animals.


They live in habitats having salinity and high light intensity. The minimum salt concentration required for growth of halophiles is 2-2.5M while optimum is 4-5M. They posses red carotenoids in their cell membrane. They protect them from intense harmful radiations.

They may be cocci (e.g. Halococcus) or bacilli with polar flagellum (e.g. Halobacterium) in shape. The nutrition is chemoheterotrophic. Under anaerobic conditions, Halobacterium develops purple membrane having photoreceptor pigment bacteriorhodopsin. In light, it acts as a proton pump and helps to synthesize ATP. The unique property of halophiles to withstand high salt concentration is due to:

  • Unique composition of cell membrane containing branched chains of lipids,
  • Mucilage covering around the cell wall,
  • Absence of sap vacuoles.

The cell can’t be plasmolysed even in high salt concentration due to absence of sap vacuoles.


These are capable of tolerating high temperature as well as high acidity and hence the name thermoacidophiles. They often live in hot water springs where the temperature is as high as 80°C and the pH as low as 2. They oxidize sulphur to sulphuric acid under aerobic conditions and the energy obtained in these reactions is utilized for synthesis of organic food. The medium becomes highly acidic due to production of sulphuric acid. Under anaerobic conditions sulphur is reduced to H2S. They are facultative anaerobes.

Thermoacidophiles are capable of withstanding extremely low pH and high temperature due to

—The cell membrane containing branched chains of lipids.

—The presence of resistant enzymes that can operate under acidic conditions.

Mycoplasma (Nocard and Roux, 1898)

MLO (mycoplasma-like organisms) or PPLO (pleuropneumonia like organisms) were discovered by Nocard and Roux (1898) in pleural fluid of cattle having bovine pleuropneumonia.

They are the smallest (0.1 - 0.15 µm) and simplest free living Gram negative, monerans or procaryotes.

A cell wall is absent. However, a substantial amount of polysaccharides having even acetyl glucosamine are associated with cell membrane which is rich in cholesterol.

Replicating disc is present at one of its ends.

Reproduction occurs by fission or first forming a branching filament with numerous nuclear bodies followed by constriction in between the nuclear bodies and separation of cells as new individuals.

Mycoplasma produces primary atypical pneumonia and mycoplasmal urethritis in humans, pleuropneumonia in animals, little leaf disease in Brinjal, yellow in Aster, and witches broom in plants.


They are Gram negative obligate pleomorphic but walled intracellular parasites which are resident of or are transmissible from arthropods.

They are intermediate between true bacteria and viruses.

Rickettsiae require exogenous factors for growth.

They often become pathogenic in mammals and humans where they cause typhus group of fevers. spread by droplet method, lice, ticks, fleas, etc., e.g., Coxiella burnetti (Q fever), Rickettsia rickettsiae (Rocky Mountain Spotted Fever spread by ticks), R. typhi (Endemic Typhus), R. prowazeckii (Epidemic Typhus), Rochalimaea quintana (Trench Fever).


They are Gram negative intracellular parassites of about 0.25 µm size, often grouped along rickettsiae.

Reproductive cycle that involves formation of initial or reticulate bodies (RB) and elementary bodies inside host phagosome.

Chlamydiae are thus energy parasites.

Chlamydia trachomatis causes conjunctivitis, nongonococcal urethritis and lymphogranuloma venereum.

C. pneumoniae causes pneumonia and bronchopneumonia.

C. psittaci is bird parasite but contaminated faeces causes psittacosis in humans.

Points to Remember

Louis Pasteur (1822 - 1895) is considered father of microbiology.

Robert Koch (1834-1940) is regarded as father of bacteriology.

The ‘germ theory of diseases’ was given by Robert Koch.

Robert Koch identified the protein tuberculin derived from Mycobacterium tuberculosis.

Mycobacterim leprae (Hensen’s bacillus) causes leprosy and can not be cultured in vitro, therefore, eyes of Armadillo are used to prepare vaccination.

E. coli is the most studied bacterium.

A mycoside (6, 6-dimycolyl trehalose) - also called cord factor is derived from mycolic diseases caused by Mycobacterium diphtheriae and M. tuberculosis.

The non motile bacillus forms are called mycobacteria.

The fragrance, which emanates form the soil after first rain or freshly ploughed soil, is due to growth of bacteria - Streptomyces in the soil. An oil geosmin is responsible for the characteristic odour.

Antibiotic penicillin and cephalosporins inhibit cross-linking of peptidoglycan strands.

Teichoic acid, present in the cell wall of gram (+) bacteria, binds Mg++ and protects bacteria from thermal injuries.

Waksman coined the term antibiotic and discovered streptomycin.

Bdellovibrio bacteriovirus is a very small (0.3 - 0.4 × 0.8 - 1.2 mm) monotrichous predatory bacterium which purifies the water of river Ganges.

The term “Cyanobacteria” was coined by ICBN (1978).

Cyanobacteria associated with protists are called Cyanellae.

Many cyanobacteria show chromatic adaptation (Gaidukov phenomenon), i.e., change in pigmentation with change in wavelength of light, e.g., Oscillatoria.

Maximum protein content (71%) on dry weight basis is found in Spirulina, a cyanobacterium. It is also the only vegetable source of vit B12.

Crown galls (plant cancers) reduce the nutrient flow in the plant.

T. J. Burril was first to find bacterial diseases of plants.

At university of Illinois, Prof. Anand Mohan Chakraborty (an Indian born molecular biologist) and his coworkers (1979) developed a superstrain of Pseudomonas which can degrade oil.

L-forms are wallless bacterial forms which develop in response to penicillin and related antibiotics. They get converted into normal bacteria after the removal of antibiotic. L-forms were discovered in case of Streptobacillus moniliformis and are named after Lister Institute of London.

Aerorhizobium is a bacterium which forms stem nodules in Sesbania.

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