Interlaboratory evaluation of MI agar and the US Environmental Protection Agency-approved membrane filter method for the recovery of total coliforms and Escherichia coli from drinking water

A new membrane filter (MF) medium, MI agar, recently validated for use in recovering chlorine-damaged total coliforms (TC) and Escherichia coli from drinking water, was compared to the US Environmental Protection Agency (EPA)-approved MF method (mEndo agar and nutrient agar supplemented with 4-methylumbelliferyl-beta-D-glucuronide) in a collaborative study. Six wastewater-spiked Cincinnati tap water samples, containing three different concentrations of E. coli (less than or equal to 10, 11-30, and >30 E. coli/100 ml), were analyzed concurrently by 19 geographically dispersed laboratories. Results showed that although the overall recoveries of TC and E. coli with MI agar were 12% and 26% greater, respectively, than those of the approved method, the differences were not statistically significant (alpha=0.05). However, the overall recovery of background/non-coliforms by the new method, 5% of the mEndo agar recovery, was significantly lower (P=0.0009). Since the results of this study show MI agar to be equal to or better than the current method, it meets the criteria for routine compliance monitoring of drinking water.