Quantitative determination of E. coli, and fecal coliforms in water using a chromogenic medium

Abstract

A new medium, Chromocult Coliform® Agar (CC agar) developed by E. Merck AG (Darmstadt, Germany) was compared with the Standard Methods, membrane filtration fecal coliform (mFC) medium for fecal coliform detection and enumeration. In the CC agar, non‐E. coli, fecal coliforms (Klebsiella, Enterobacter, and Citrobacter,), (KEC) were identified by the production of a salmon to red colour from p‐galactosidase (LAC) cleavage of the substrate Salmon‐GAL, while E. coli, colonies were detected by the blue colour, produced by the cleavage of X‐glucuronide by β‐ glucuronidase (GUS). Statistically, there was no significant differences between fecal coliform counts obtained with the two media (CC agar and mFC agar) and two incubation procedures (2h‐37°C plus 22h‐44.5°, and 44.5°C) as determined by variance analysis. In our study K coli, represented, on average 70.5–92.5% of the fecal coliform population. A high incidence of false negative KEC (19.5%) and E. coli, (29.6%) colonies was detected at 44.5°C. Two K coli, GUS negative phenotype upon reinoculation into CC agar were GUS+. A total of 31 KEC LAC colonies were streaked onto CC, agar and incubated at 37°C, 29 KEC strains that failed to produce β‐galactosidase at 44.5°C were able to produce the enzyme at 37°C. In our opinion the physiological condition of the fecal coliform isolates could be responsible for the non‐expression of P‐galactosidase and P‐glucuronidase activities at 44.5°C.