The main strain of bacteria that is tested for is E. coli. In regard to testing, freshwater E. coli can’t exceed 235 colonies per 100 ml.

Escherichia coli (E. coli) are bacteria found in the environment, foods, and intestines of people and animals. They are a large and diverse group of bacteria. Strains that are harmless, and others that can make you sick. Harmful kinds of E. coli can cause diarrhea, while others cause urinary tract infections, respiratory illness, pneumonia, and other illnesses.
For more information, see E. coli CDC.