Commonly used indicator bacteria include total coliforms, or a subset of this group, fecal coliforms, which are found in the intestinal tracts of warm blooded animals. Total coliforms were used as fecal indicators by public agencies in the US as early as the 1920s.  These organisms can be identified based on the fact that they all metabolize the sugar lactose, producing both acid and gas as byproducts.  Fecal coliforms are more useful as indicators in recreational waters than total coliforms which include species that are naturally found in plants and soil; however, there are even some species of fecal coliforms that do not have a fecal origin, such as Klebsiella pneumoniae.  Perhaps the biggest drawback to using coliforms as indicators is that they can grow in water under certain conditions.
Escherichia coli (E. coli) and enterococci are also used as indicators.