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Chapter: 11th 12th standard bio zoology Human Body higher secondary school

Beneficial Activities of Bacteria

Sewage disposal, Decomposition of plant and animal remains, Soil fertility, Recycling of matter, Role of Bacteria in Industry : Dairy Industry, Vinegar, Alcohols and Acetone, Curing of tobacco, tea and coffee Retting of fibres Role of bacteria in medicine, Role of bacteria in genetic engineering, Role of bacteria in biological control

Beneficial Activities of Bacteria

         Sewage disposal : Organic matter of the sewage is decomposed by saprotrophic bacteria.

         Decomposition of plant and animal remains: Saprotrophic bacteria cause decay and decomposition of dead bodies of plants and animals. They release gases and salts to atmosphere and soil. Hence these bacteria are known as nature's scavengers.

Soil fertility :

       The ammonifying bacteria like Bacillus ramosus and B. mycoides convert complex proteins in the dead bodies of plants and animals into ammonia which is later converted into ammonium salts.
         The nitrifying bacteria such as Nitrobacter, Nitrosomonas convert ammonium salts into nitrites and nitrates.
         Nitrogen fixing bacteria such as Azotobacter and Clostridium and
Rhizobium (a symbiotic bacterium) are capable of converting atmospheric nitrogen into organic nitrogen. All these activities of bacteria increase soil fertility.

Recycling of matter

Bacteria play a major role in cycling of elements like carbon, oxygen, Nitrogen and sulphur. Thus they help in maintaining environmental balance. As biological scavengers they oxidize the organic compounds and set free the locked up carbon as CO2. The nitrogenous organic compounds are decomposed to form ammonia which is oxidized to nitrite and nitrate ions by the action of nitrifying bacteria. These ions are used by higher plants to synthesize nitrogenous organic compounds. The nitrogenous compounds are also oxidized to nitrogen by denitrifying bacteria.

Role of Bacteria in Industry

1. Dairy Industry

Lactic acid bacteria e.g.(Streptococcus lactis) are employed to convert milk sugar lactose into lactic acid.
C(12) + H(22)O(11)+H(2O)=4C(3)H(6)O(3)+Energy
Lactose       Lactic Acid
Different strains of lactic acid bacteria are used to convert milk into curd, yoghurt(Lactobacillus bulgaricus) and cheese(Lactobacillus acidophobus).

2. Vinegar

Vinegar (Acetic acid) is obtained by the activity of acetic acid bacteria (Acetobactor aceti). This bacterium oxidizes ethyl alcohol obtained from molasses by fermentation to acetic acid or vinegar.

3. Alcohols and Acetone

Butyl alcohol, methyl alcohol and acetone are prepared from molasses by the fermentation activity of the anaerobic bacteriumClostridium acetobutylicum.

Curing of tobacco,tea and coffee

The leaves of tea, tobacco and beans of coffee are fermented by the activity of certain bacteria to impart the characteristic flavour. This is called curing of tea, tobacco and coffee.

Retting of fibres

The fibres from the fibre yielding plants are separated by the action of bacteria like Clostridium species. This is called retting of fibres.

Role of bacteria in medicine

1. Antibiotics: Antibiotics such as bacitracin ( Bacillus subtilis), polymyxin(Bacillus polymyxa), Streptomycin(Streptomyces griseus) are obtained from bacterial sources.
2. Vitamins: Escherichia coli living in the intestine of human beings produce large quantities of vitamin K and vitamin B complex. Vitamin B2 is prepared by the fermentation of sugar by the action of clostridium species.

Role of bacteria in genetic engineering

Most of our knowledge in genetics and molecular biology during 20th century has been due to research work on micro-organisms, especially bacteria such as E.coli. One success has been the transfer of human insulin genes into bacteria and commercial production of insulin has already commenced.

Role of bacteria in biological control


Certain Bacillus species such as B.thuringiensis infect and kill the caterpillars of some butterflies and related insects. Since the bacteria do not affect other animals or plants they provide an ideal means of controlling many serious crop pests.

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