Microbes are heterogeneous groups with distinct classes of living organisms. Originally they were classified under plant and animal kingdom. But this classification does not satisfy many researchers and hence a third kingdom called “Protista” was formed for them. On basis of the difference in cellular organization and biochemistry, microbes are classified into two groups, namely: Prokaryotes and Eukaryotes. Bacteria and blue - green algae fall under prokaryotes whereas fungi, algae, protozoa and slime moulds fall under the category of Eukaryotes. Bacteria do not contain chlorophyll. They are unicellular and do not contain branching structure except in so called “Higher Bacteria” (Actinomycets).