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 c oli from drinking water, was compared to the US Environmental Protecti on Agency (EPA)-approved MF method (mEndo agar and nutrient agar suppl emented with 4-methylumbelliferyl-beta-D-glucuronide) in a collaborati ve study. Six wastewater-spiked Cincinnati tap water samples, containi ng 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 geo graphically dispersed laboratories. Results showed that although the o verall recoveries of TC and E. coli with MI agar were 12% and 26% grea ter, 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 mEnd o 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 dri nking water.