Cows, Streams, and E. Coli: What everyone needs to know
Michigan State University Extension , 2010

Wolfson, L., Harrigan, T.

Contamination of water by bacteria is one of the leading causes of impairment in U.S. surface waters. While many bacteria occur naturally in the environment and are an important component of many ecosystem processes, some are of concern because they may cause diseases. These bacteria (E.coli 0157:H7, Salmonella, etc.), as well as viruses (enteroviruses, adenoviruses, etc.) and some protozoans (Cryptosporidium, Giardia, etc.), are referred to as pathogens. Most are found in the gastrointestinal tract of humans and other warm-blooded animals and are shed in the feces. One type of bacteria found in the intestines and feces is Escherichia coli. Most people refer to it as E. coli. It is an important type of fecal coliform bacteria that can help prevent the growth of harmful bacteria within the intestines. Although there are some strains of E. coli that cause illness, such as E. coli 0157:H7 (the strain often associated with food poisoning), most E. coli are harmless. The presence of E. coli can be used as an indicator of the presence of fecal contamination. Water quality standards are set within States to limit exposure to recreational waters when pathogens are present. In most states, including Michigan, the organism (indicator) used to determine if the water quality standard for recreation is exceeded is E. coli, even though it is usually other bacteria, viruses and protozoa that cause many of the illnesses associated with swimming in and ingesting polluted water. If E. coli is present in certain quantities, then there is a greater probability that pathogens will be present as well. Why don’t we monitor directly for pathogens? Pathogens are very diverse, frequently mutate, and can be much more difficult and costly to measure. Also, large volumes of water are usually required for any test that may be done in a laboratory. Measuring just one pathogen does not provide information on the others that may also be present. Thus, presence of E. coli bacteria is used as an indication of the presence of pathogens. The EPA recommended standards for E. coli in recreational waters are based on a threshold concentration. When those numbers are exceeded, the health risk from waterborne illness is deemed unacceptably high1.