(a) Heterotrophic bacteria

(1) They act as decomposers and help in the formation of humus.

(2) They help in the production of curd from milk.

(3) Many antibiotics are obtained from some species of bacteria.

(4) Many soil bacteria help in fixation of atmospheric nitrogen.

(b) Archaebacteria

(1) Methane gas is produced from the dung of ruminants by the methanogens.

(2) Methanogens are also involved in the formation of biogas and sewage treatment.