The single sample value is the number of colony forming units (CFUs) or Most Probable Number (MPN) per 100 ml of sample water. The geometric mean value is a statistical tool that pro- vides a normalized average of individual sample results, based on multiple samples collected within a 30-day period.
Table 2. The US EPA bacteria criteria for marine and freshwater recreational areas.13
Background on Microorganisms and Fecal Indicator Bacteria
Enterococci bacteria increases, so does the risk of contract-
ing GI illness (a dose-response relationship).7, 9, 11, 12, 13
of the studies used to determine this safety level define “swimming” as submersion of the head in water.9 The risk of getting sick increases with prolonged exposure or with an increase in the volume and frequency of water swallowed.
During the summer months, the MHB Program routinely monitors coastal beaches for Enterococci, a US EPA-approved indicator of fecal contamination for marine recreational waters. Maine has adopted the EPA criterion
which considers a single sample bacteria level of greater than 104 bacteria per 100 ml of water unsafe for swim- ming (Table 2). States may also manage a beach accord- ing to the geometric mean value based on five or more samples collected within a 30-day period. Bacteria levels over the safety limit increase a swimmer’s likelihood of contracting a water-related illness. Maine uses the Enterolert® methodology of analysis, and results are available 24-28 hours after sample collection.