(a) Bacteria are very important for humans. They causes decay and decomposition of dead bodies of plants and animals, so they help in cleaning the earth.
(b) They fix atmospheric N2 into nitrates, lactic arid bacteria convert milk sugar lactose into lactic arid.
(c) They convert ethyl alcohol into vinegar, produces butyl alcohol and acetone.
(d) A number of antibiotics are obtained from mycelial bacterium like streptomycin antibiotics from streptomyces bacterium.