Microscopic organisms, which may be a single cell or multicellular organism. Microorganisms are diverse and include bacteria, protozoa, fungi, algae, microscopic plants and animals (rotifers and planarians), and viruses. Examples of microbial contamination of drinking water include viruses such as hepatitis, protozoa such as Giardia, and bacteria such as coliform. Coliform is a group of bacteria found in the digestive tracts of mammals and in their wastes. Coliform in water can indicate fecal pollution, which can be a serious problem due to the potential for contracting diseases from pathogens. Pathogenic organisms are a principal contaminant of concern because they pose an acute health risk. Drinking only a single glass of water containing pathogens can cause illness. E. coli is one type of coliform bacteria, is the major species in the fecal coliform group, and is a good indicator of fecal pollution and the possible presence of pathogens.

For more information on microorganisms, see information on EPA’s National Primary Drinking Water Regulations.