NEW MEDIUM FOR THE SIMULTANEOUS DETECTION OF TOTAL COLIFORMS AND ESCHERICHIA-COLI IN WATER

A new membrane filter agar medium (MI agar) containing a chromogen, in doXyl-beta-D-glucuronide, and a fluorogen, 4-methylumbelliferyl-beta-D -galactopyranoside, was developed to simultaneously detect and enumera te Escherichia coli and total coliforms (TC) in water samples on the b asis of their enzyme activities. TC produced beta-galactosidase, which cleaved 4-methylumbelliferyl-beta-D-galactopyranoside to form 4-methy lumbelliferone, a compound that fluoresced under longwave UV light (36 6 nm), while E. coli produced beta-glucuronidase, which cleaved indoxy l-beta-D-glucuronide to form a blue color. The new medium TC and E. co li recoveries were compared with those of mEndo agar and two E. coli m edia, mTEC agar and nutrient agar supplemented with 4-methylumbellifer yl-beta-D-glucuronide, using natural water samples and spiked drinking water samples. On average, the new medium recovered 1.8 times as many TC as mEndo agar, with greatly reduced background counts (less-than-o r-equal-to 7%). These differences were statistically significant (sign ificance level, 0.05). Although the overall analysis revealed no stati stically significant difference between the E. coli recoveries on MI a pr and mTEC agar, the new medium recovered more E. coli in 16 of 23 sa mples (69.6%). Both MI agar and mTEC agar recovered significantly more E. coli than nutrient apr supplemented with 4-methylumbelliferyl-beta -D-glucuronide. Specificities for E. coli, TC, and noncoliforms on MI agar were 95.7% (66 of 69 samples), 93.1% (161 of 173 samples), and 93 .8% (61 of 65 samples), respectively. The E. coli false-positive and f alse-negative rates were both 4.3%. This selective and specific medium , which employs familiar membrane filter technology is used to analyze several types of water samples, is less expensive than the liquid chr omogen and fluorogen media and may be useful for compliance monitoring of drinking water.