Sarasota County Water Atlas


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Marine Indicator Bacteria

What does this mean?

Coliform bacteria occur naturally in animal feces (e.g., humans, fish, and livestock), as well as stormwater runoff, soils and vegetation. By themselves, these microscopic organisms are considered harmless and often aid in digestion and vitamin synthesis in the body.

However, high numbers of coliform bacteria in the water may indicate the presence of potentially harmful microorganisms, including Salmonella, Shigella and Vibrio cholera, viruses such as infectious hepatitis, and others that may lead to outbreaks of diarrhea and gastro-urinary infections.

While there is increasing controversy about coliform as an accurate indicator of health concerns, it remains the most common gauge used to indicate whether a water body is safe for swimming. The state's saltwater beaches are sampled weekly for fecal coliform (a sub-group of total coliform, indicating human or animal fecal contamination) and enterococci (present in the intestines of all warm-blooded animals), a more reliable measure of water quality because it survives longer in water. County health departments issue health advisories or warnings based on these standards, although the link between exposure to these organisms and public health risk remains unclear.

The threat of bacterial contamination is one of the reasons some coastal areas are closed to shellfish harvesting, but closures are based on state presumption of contamination rather than actual monitoring. For additional information on shellfish harvesting, visit the Florida Department of Agriculture and Consumer Services webpage devoted to Shellfish Harvesting Management.


How are the data collected? (Methods)

Florida's Healthy Beaches program requires counties to monitor saltwater beaches weekly for fecal coliform (a sub-group of total coliform, indicating human or other animal fecal contamination) and enterococci (present in the intestines of all warm-blooded animals), a more reliable measure of water quality because it survives longer in water. County health departments issue health advisories or warnings based on these standards, although the link between exposure to these organisms and public health risk remains unclear. Additionally, many local governments monitor other swimming sites within their jurisdiction, although monitoring standards may vary.

Samples are collected in sterile 100 ml containers and transported on ice to a certified laboratory. The samples must be cultured on plates within 6 hours of the time they are collected. After a 24-hour incubation period, the number of colonies on the culture plate are counted and reported as the number of colony-forming units per 100 milliliters of sampled water, or CFU/100 ml.

During sample collection, environmental conditions are also observed and recorded, along with recent rainfall amounts.


Calculations

The maximum allowable level, or "action-level" of fecal coliform is 400 CFU/100 ml. The action level for enterococcus is 105 CFU/100mls for a single water sample. In addition to a single sample measure, the geometric mean for enterococci is calculated using the 5 most recent test results. This will reflect the average water quality over the past month. The action level for a geometric mean is 36 CFU/100 ml, as opposed to a single sample action level of 105, signifying a trend towards higher than normal bacterial levels in the water for this particular period. If a sample meets or exceeds any of these limits, the beach must be resampled and an advisory may be posted.


Caveats and Limitations

The use of bacterial indicators is limited because a very small sample (100 ml, about the size of a juice glass) is used to represent a very large body of water. Environmental contamination can also be very localized or patchy, especially if the source of contamination is wildlife. Poor sample results may reflect that particular sample area, but not necessary represent the water quality for the entire beach area. Conversely, waters with indicators exceeding certain levels may be considered a potential health risk but levels within acceptable ranges are not necessarily free of risk.


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