Bacteria are the minute single cell organisms. They are produced by binary fission and may have various shapes. They are so small, required to be examined under microscope. They are present in raw or contaminated water.

Some varieties are harmless and are known as non-pathogenic or non-pathogens. Some varieties are deadly harmful causing water borne diseases. They are known as pathogenic bacteria or pathogens. Most of the varieties of bacteria require oxygen for their survival. These bacteria consume oxygen which is dissolved in water. Depending upon the way of satisfaction of oxygen, they are classified as

(a) Aerobic Bacteria : They survive by using dissolved oxygen in water.

(b) Anaerobic Bacteria : They are not dependent on dissolved oxygen. In the absence of dissolved oxygen they derive their oxygen requirement by decomposing organic matter and extract chemically bound oxygen.

(c) Facultative Bacteria : They can survive with or without free oxygen.

Pathogenic organisms cause water borne diseases and many non-pathogenic bacteria like E. coil, a member of coliform group, also live in the intestine of warm blooded animals and human beings. It is difficult to conduct specific tests for the identification of specific pathogens. Hence, the coliform group of bacteria can serve as indicators of the contamination of water with domestic and industrial waste. The 'coliform group density' which is a criteria of the degree of pollution, is used as the basis for standards of bacterial quality of water supplies.