(a) Haterotrophic bacteria : These bacteria are natural scavengers because they decompose the organic matter. This activity is economically used in sewage disposal and in improving fertility of soil by formation of humus.
(b) Archebacteria : These bacteria live as symbionts in the rumen of herbivorous animals that chew their cud (cow, buffalo) and help in fermentation of cellulose. These bacteria are used in the production of methane (biogas) from the dung.
