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The USGS Water Science School

Bacteria in Water

Pictures of fecal coliform and e. coli plates growing bacteria.

Bacteria are common single-celled organisms and are a natural component of lakes, rivers, and streams. Most of these bacteria are harmless to humans; however, certain bacteria, some of which normally inhabit the intestinal tract of warm-blooded animals, have the potential to cause sickness and disease in humans. High numbers of these harmless bacteria often indicate high numbers of harmful bacteria as well as other disease-causing organisms such as viruses and protozoans.

One method of determining bacteria counts is to count the number of bacteria colonies that grow on a prepared medium.

Coliform bacteria

The Chattahoochee Bacteria Monitoring Network is sampling for total coliform and E. coli bacteria.

Diagram showing how e. coli and fecal coliform make up total coliforms

Total coliform

Total coliforms are gram-negative, aerobic or faculative anaerobic, nonspore forming rods. These bacteria were originally believed to indicate the presence of fecal contamination, however total coliforms have been found to be widely distributed in nature and not always associated with the gastrointestinal tract of warm blooded animals. The number of total coliform bacteria in the environment is still widely used as an indicator for potable water in the U.S.

Fecal coliform

Fecal coliform bacteria are a subgroup of coliform bacteria that were used to establish the first microbial water quality criteria. The ability to grow at an elevated temperature (44.5 C) separate this bacteria from the total coliforms and make it a more accurate indicator of fecal contamination by warm-blooded animals. Fecal- coliform bacteria are detected by counting the dark-blue to blue-grey colonies that grow on a 0.65 micron filters placed on mFC agar incubated in a 44.5 C oven for 22-24 hours. The presence of fecal coliforms in water indicates that fecal contamination of the water by a warm-blooded animal has occurred, however, recent studies have found no statistical relationship between fecal coliform concentrations and swimmer-associated sickness.

E. coli

Escherichia coli (E. coli) is a rod-shaped bacteria commonly found in the gastrointestinal tract and feces of warm-blooded animals. It is a member of the fecal coliform group of bacteria and is distinguished by its inability to break down urease. E. coli numbers in freshwater are determined by counting the number of yellow and yellow brown colonies growing on a 0.45 micron filter placed on m-TEC media and incubated at 35.0 C for 22-24 hours. The addition of urea substrate confirms that colonies are E. coli. This bacteria is a preferred indicator for freshwater recreation and its presence provides direct evidence of fecal contamination from warm-blooded animals. Although usually harmless, E. coli can cause illnesses such as meningitis, septicemia, urinary tract, and intestinal infections. A recently discovered strain of E. coli (E. coli 0157:H7) can cause severe disease and may be fatal in small children and the elderly.

The relation between bacteria counts and sickness

Consumption of or contact with water contaminated with feces of warm-blooded animals can cause a variety of illnesses. Minor gastrointestinal discomfort is probably the most common symptom; however, pathogens that may cause only minor sickness in some people may cause serious conditions or death in others, especially in the very young, old, or those with weakened immunological systems.

Chart showing positive relation between E.coli counts and sickness

As these charts show, studies have shown that there is not a positive relation between either total coliform or fecal coliform counts and sickness. But, studies have shown a positive relation between E. coli and sickness -- as E. coli counts go up, the occurrences of sicknesses go up. That is why this project is focusing on E. coli as the main indicator of potential risk.

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Page Last Modified: Monday, 17-Mar-2014 11:03:09 EDT