Aims:  Testing for β‐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 β‐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 β‐d‐glucuronidase activity and hence in detecting E. coli. Methods and Results:  The ability of membrane lactose glucuronide agar (MLGA), Colilert‐18®, MI agar, Colitag® and Chromocult agar to detect β‐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. Conclusions:  MLGA had very poor sensitivity for the detection of β‐d‐glucuronidase activity in strains of E. coli isolated from naturally contaminated water. This is probably because of the fact that β‐d‐glucuronidase activity is pH‐sensitive and that acid is formed by E. coli during fermentation of lactose in MLGA. Significance and Impact of the Study:  The detection of E. coli in drinking water is the primary test used to establish faecal contamination. The poor sensitivity of MLGA in detecting β‐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.

Aims:  Methods for the detection of coliforms in water have changed significantly in recent years with procedures incorporating substrates for the detection of β‐d‐galactosidase becoming more widely used. This study was undertaken to determine the range of coliform genera detected with methods that rely on lactose fermentation and compare them to those recovered using methods based upon β‐d‐galactosidase. Methods and Results:  Coliform isolates were recovered from sewage‐polluted water using m‐endo, membrane lauryl sulfate broth, tergitol TTC agar, Colilert‐18®, ChromoCult® and ColiScan® for primary isolation. Organisms were grouped according to whether they had been isolated based upon lactose fermentation or β‐d‐galactosidase production. Conclusions:  A wide range of coliform genera were detected using both types of methods. There was considerable overlap between the two groups, and whilst differences were seen between the genera isolated with the two method types, no clear pattern emerged. Substantial numbers of ‘new’ coliforms (e.g. Raoutella spp.) were recovered using both types of methods. Significance and Impact of the Study:  The results presented here confirm that both methods based on lactose fermentation or detection of β‐d‐galactosidase activity recover a range of coliform organisms. Any suggestion that only methods which are based upon fermentation of lactose recover organisms of public health or regulatory significance cannot be substantiated. Furthermore, the higher recovery of coliform organisms from sewage‐polluted water using methods utilizing β‐d‐galactosidase‐based methods does not appear to be because of the recovery of substantially more ‘new’ coliforms.

Aims:  To explain the basis for false negative β‐glucuronidase reactions seen with culture media containing lactose as a carbon and energy source. Methods and Results:  Escherichia coli strains were assessed for their reactions in culture media containing a β‐d‐glucuronidase substrate either with or without lactose. An assay was developed to test for the expression of β‐d‐glucuronidase at pH 5·0 and pH 7·2. Strains of E. coli that gave false negative glucuronidase reactions on media containing lactose generally expressed lower concentrations of the enzyme β‐d‐glucuronidase than strains that gave positive results, although the difference was by no means consistent. Most strains that were negative on lactose‐containing media expressed virtually no β‐d‐glucuronidase activity at pH 5·0. Examination of colonies on Membrane lactose glucuronide agar (MLGA) from lightly polluted water showed that c. 10% of the E. coli present failed to yield green colonies on MLGA. Conclusions:  E. coli that failed to produce green colonies on MLGA produced lower levels of β‐d‐glucuronidase than did strains that formed green colonies, the difference being greater at pH 5·0 than pH 7·2. The false negative rate for E. coli 10% which is similar to that experienced in the study that originally described MLGA. Significance and Impact of the Study:  Strains of E. coli that fail to produce typical colonies on MLGA might produce lower concentrations of the enzyme β‐d‐glucuronidase. Whilst the enzyme activity is sufficient to be detected at pH 7·2, fermentation of lactose significantly lowers the pH of the medium and can result in reduced enzyme activity and therefore lack of detection. The false negative rate of c. 10% would be difficult to detect in routine laboratories as it would represent 1% or less of yellow colonies being identified as E. coli (assuming E. coli accounts for 10% of the total coliform population in drinking water).

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The ability of Slanetz and Bartley medium to recover chlorine-stressed enterococci has been studied. Results showed that chlorine injury significantly affected the ability of Slanetz and Bartley medium to recover enterococci while lower concentrations of sodium azide in the same basal medium allowed their recovery. However, reducing the concentration of sodium azide considerably reduced the specificity making it unsuitable for use in the routine examination of water. A non-azide-containing medium, Enterolert(®)-DW appeared to be able to recover injured and non-injured enterococci with similar efficiency. The data presented here suggest that further work is required to improve the recovery of chlorine-injured enterococci by Slanetz and Bartley medium.