Enumeration of heterotrophs, fecal coliforms and Escherichia coli in water: comparison of 3M™ Petrifilm™ plates with standard plating procedures

A total of 177 naturally contaminated water samples were analyzed by membrane filtration according to the Standard Methods for the Examination of Water and Wastewater published by the American Public Health Association. Filters were incubated in parallel on mHPC-agar and 3M™ Petrifilm™ Aerobic Count Plates (Petrifilm™ AC plates) for heterotrophic counts. Fecal coliforms and Escherichia coli were enumerated on mFC-agar and 3M™ Petrifilm™ E. coli/Coliform Count Plates (Petrifilm™ EC plates). Typical colonies on each media type were confirmed following standard procedures. Heterotrophic counts were between 103 and 104 CFU/mL and the average log10 counts obtained on Petrifilm™ AC plates were about two-fold lower than on mHPC-agar. Counts for fecal coliforms and E. coli were between 102 and 103 CFU/mL. Average log10 counts for confirmed fecal coliforms obtained on Petrifilm™ EC plates were slightly lower than on mFC agar with a correlation coefficient of 0.949. The average log10 counts for confirmed E. coli on Petrifilm™ EC plates and on mFC agar were statistically not different (P=0.126) with a correlation coefficient of 0.879. Specificity of Petrifilm™ EC plates and mFC agar was evaluated by comparing typical colony counts with confirmed counts. On mFC agar, counts for typical colonies were by 2 log10 CFU higher than the actual confirmed counts. In contrast, on Petrifilm™ EC plates typical colony counts were almost identical to confirmed colony counts for both fecal coliforms and E. coli. This comparison illustrates the high specificity of Petrifilm™ EC plates for enumeration of both fecal coliforms and E. coli in water.