Comparison of Petrifilm and Colilert methods for E. colienumeration in recreational water

Abstract

With a growing interest in recreational water quality, there is a need for more affordable methods for detecting fecal indicator organisms, including Escherichia coli (E. coli). Defined substrate technology (specifically Colilert) is a US Environmental Protection Agency approved method for the detection of E. coli in beach water, but it requires relatively expensive equipment for enumeration of microbes. The 3M Petrifilm product is not currently approved for analysis of recreational water but has been approved for microbial analysis in the food industry. Both methods are simple to use, but Petrifilm requires less equipment and is less expensive than Colilert and thus has been used as a screening tool by citizens’ groups for enumeration of E. coli in beach water. In some cases, however, the prescribed incubation at 35 C has been ignored. The objectives of this study were to compare E. coli concentrations in beach water when enumerated by Colilert (defined substrate) and the Petrifilm E. coli test systems, and to evaluate the effects of incubation temperatures on Petrifilm results. Beach water collected from 20 beaches along lakes Michigan and Superior indicated that 19 beaches had mean E. coli concentrations orders of magnitude higher when Petrifilm was compared to Colilert (4406 vs. 98 mean E.coli/100 mL). In most cases, use of Petrifilm would result in unnecessary advisories and closures at beaches. Incubation of Petrifilm at 25 or 30 C (below manufacturer's recommendation) required at least 2 days for colony counts to match concentrations enumerated by the recommended method. Overall, the Petrifilm method was not as reliable as currently accepted methods for measurement of E. coli concentrations in beach water.