Fermentation is the process of conversion of carbohydrates to alcohol and CO₂ by some microorganisms in the absence of O_2.  Microbes via fermentation are utilised for the synthesis of a number of produce valuable for human beings.
Some of the applications of microbes are:
(i) Production of bread using baker's yeast.
(ii) Microbes are used to ferment fish, soybean bamboo shoots, etc.
(iii) Different varieties of cheese are produced by fermentation via microbes.
(iv) Wine, beer and other alcoholic drinks are also produced by fermentation.
(v) Vinegar is also produced by fermentation.

1.Several microbes are used in preparing medicines.
2.The most important medicines for which microbes are used are antibiotics.  These are chemical compounds produced by some microbes and are used to kill or to subside the growth of other disease-causing microbes. Streptomycin, tetracycline and penicillin are some of the commonly used antibiotics. These antibiotics have significantly contributed towards the welfare of the human society.

The three microbes that can be chosen for organic farming are:
(i) Rhizobium The nodules on the roots of leguminous plants are formed by the symbiotic association of Rhizobium bacteria.These bacteria fix atmospheric nitrogen into organic forms, which is used by the plants as nutrient.
(ii) Mycorrhiza Fungi from symbiotic association with plants called mycorrhiza. The fungal symbiont absorbs phosphrous from soil and passes it to the plant. Such plant shows resistance to root-borne pathogens, tolerance to salinity and drought and overall increase in plant growth and development.
(iii) Anabaena It is a cyanobacteria that is used as a biofertiliser. It fixes atmospheric nitrogen.

BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The microbes biodegrade the major part of the organic matter in the effluent by using the dissolved oxygen. It reduces the BOD of the effluent.
When the BOD of sewage is reduced significantly, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment forming the activated sludge. Hence, higher the value of BOD, more is the water polluted and more is the amount of biodegradable matter in it.

Microbes are found everywhere, i.e. in soil, water, in/on plants, animals, human, etc. They play very important role in human life in many ways. for example,
(i) in human life products like baking bread, idli, cheese, beer, wine, vinegar, etc.
(ii) enzymes, amino acids, vitamins, antibiotics, organic acids and alcohols are commercially manufactured from the microorganisms.
(iii) live attenuated virus, bacteria and protozoans have been used to develop vaccines for protection from dysentery, typhoid, cholera, etc.
(iv) microbes play an important role in agriculture and recycling of important elements like oxygen, carbon, nitrogen, sulphur and phosphorus.
(v) microbes are used as biocontrol agents in place of pesticides, herbicides, etc.