USE OF -BROMO-4-CHLORO-3-INDOLYL-BETA-D-GALACTOPYRANOSIDE FOR THE ISOLATION OF BETA-GALACTOSIDASE-POSITIVE BACTERIA FROM MUNICIPAL WATER-SUPPLIES

A new medium, mX-Gal, has been developed for the membrane filter enume ration of beta-galactosidase-positive bacteria in municipal water supp lies. mX-Gal medium contains the chromogenic beta-galactosidase substr ate -bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). All Ae romonas, Citrobacter, and Enterobacter strains isolated from raw water on mX-Gal medium were beta-galactosidase positive. In contrast, only 10 to 20% of these strains produced a red colony with a metallic sheen on m-Endo agar LES medium. Of 674 chlorinated water samples analyzed for total coliforms on m-Endo agar LES medium and for beta-galactosida se-positive bacteria on mX-Gal medium, 18 that were negative for colif orms on m-Endo agar LES showed beta-galactosidase-positive bacteria on mX-Gal. Of a total of 50 beta-galactosidase-positive bacteria isolate d from these samples, 76% were identified as Aeromonas hydrophila.