The US Environmental Protection Agency (EPA) lists the following criteria for an organism to be an ideal indicator of fecal contamination:
# The organism should be present whenever enteric pathogens are present
# The organism should be useful for all types of water
# The organism should have a longer survival time than the hardiest enteric pathogen
# The organism should not grow in water
# The organism should be found in warm-blooded animals' intestines.
None of the types of indicator organisms that are currently in use fit all of these criteria perfectly, however, when cost is considered, use of indicators becomes necessary.