Use of 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside for the isolation of β-galactosidase-positive bacteria from municipal water supplies

Abstract

A new medium, mX-Gal, has been developed for the membrane filter enumeration of β-galactosidase-positive bacteria in municipal water supplies. mX-Gal medium contains the chromogenic β-galactosidase substrate 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside (X-Gal). All Aeromonas, Citrobacter, and Enterobacter strains isolated from raw water on mX-Gal medium were β-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 β-galactosidase-positive bacteria on mX-Gal medium, 18 that were negative for coliforms on m-Endo agar LES showed β-galactosidase-positive bacteria on mX-Gal. Of a total of 50 β-galactosidase-positive bacteria isolated from these samples, 76% were identified as Aeromonas hydrophila.Key words: β-galactosidase, 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside, total coliforms, Aeromonas.