False-negative β-d-glucuronidase reactions in membrane lactose glucuronide agar medium used for the simultaneous detection of coliforms and Escherichia coli from water

Testing for beta-d-glucuronidase activity has become the basis of many methods for the detection of Escherichia coli in both food and water. Used in combination with tests for the presence of beta-d-glucuronidase, these tests offer a simple method for simultaneously detecting coliforms and E. coli. The purpose of this study was to determine the effectiveness of several different procedures in detecting beta-d-glucuronidase activity and hence in detecting E. coli. The ability of membrane lactose glucuronide agar (MLGA), Colilert-18((R)), MI agar, Colitag((R)) and Chromocult agar to detect beta-d-glucuronidase activity was tested with over 1000 isolates of E. coli recovered from naturally contaminated water samples. Four of the media gave very similar results but MLGA failed to detect glucuronidase activity in 15.6% of the cultures tested. MLGA had very poor sensitivity for the detection of beta-d-glucuronidase activity in strains of E. coli isolated from naturally contaminated water. This is probably because of the fact that beta-d-glucuronidase activity is pH-sensitive and that acid is formed by E. coli during fermentation of lactose in MLGA. The detection of E. coli in drinking water is the primary test used to establish faecal contamination. The poor sensitivity of MLGA in detecting beta-d-glucuronidase activity suggests that this medium and others containing high concentrations of fermentable carbohydrate should not be used for the detection of E. coli.