On the basis of mode of nutrition, bacteria are grouped into two broad categories ?autotrophic and heterotrophic. Autotrophic bacteria are able to synthesize their own food from inorganic substances, as green plants do. These are further may be ?photoautotrophic (eg, green sulphur, purple - sulphur and purple non-sulphur bacteria) or chemoautotrophic (eg, sulphur- iron-hydrogen, ammonifying, nitrifying bacteria, etc) Heterotrophic bacteria cannot synthesize their own organic food. These are of three types ? parasites (eg, Salmonella, Xanthomonas etc), saprotrophic. (eg, Pseudomonas) and symbiotic (eg, Rhizobium).