A coliform count is the general indication of safety of drinking water supplies in an area, explains the New York Department of Health. It measures the levels of fecal bacteria and E. coli in a water supply, giving an indication of which pathogens are potentially in the water.
Fecal contamination of a water supply is a serious concern, notes the New York Department of Health. Water supplies may be contaminated with a number of different pathogens, but it is not feasible to test for each pathogen individually. A coliform count specifically tests for levels of fecal coliforms, to be used as an indicator of the presence of other possible pathogens that are harmful to the population.