AN INTERLABORATORY COMPARISON OF RAPID METHODS FOR CONFIRMING THE IDENTITY OF COLIFORM 0RGANISMS
Report No DWI0673

Feb 1989

SUMMARY

I OBJECTIVES

To ascertain if tests for -galactosidase and -glucuronidase can be used as alternatives to existing standard methods for confirming the identity of coliform organisms or E coli.

II REASONS

A survey of water industry microbiologists carried out by the Rapid Methods Panel of the Standing Committee of Analysts Working Group 2 has shown that there is a real need for coliform confirmation techniques which are more rapid than those currently recommended.

III CONCLUSIONS

  1. The -galactosidase test would appear to be a suitable means of determining rapidly whether an organism has the potential to produce acid from lactose. However, since it gives no information about gas production, the -galactosidase test does little more than confirm that colonies identified as lactose fermenters on primary isolation media are indeed able to utilise this carbohydrate. The value of using this test instead of assuming that all lactose fermenting colonies on selective coliform media are in fact coliform organisms is therefore questionable.

  2. If it is agreed that use of the -galactosidase test has some merit, the RAPIDEC method would appear to be the most efficient in that it gives results in 2 hours with only a 1 per cent chance of failure to alert.

  3. On the basis of the data obtained in this trial, the -glucuronidase test would appear to be less specific for E coli than has been previously reported. However, this may be due to the use of LTMB medium, rather than a more systematic scheme, as the reference method for identifying E coli. In comparison to LTMB, the most sensitive -glucuronidase test, RAPIDEC, failed to identify 11 per cent of the isolates as E coli. Of the isolates not identified as E coli by LTMBs 22 per cent possessed the -glucuronidase enzyme.

  4. On the basis of these results the LTMB and -glucuronidase tests cannot be regarded as equivalent although the rapidity of the -glucuronidase test may make it a useful additional test in certain operational situations.

IV RECOMMENDATIONS

Recommendations will be made once the results of this study have been discussed by the Rapid Methods Panel of the Standing Committee of Analysts Working Group 2.

V RESUME 0F CONTENTS

The results of a trial to compare the validity of using -galactosidase and -glucuronidase tests for confirming the identity of coliform organisms are given. In the trial several commercially available forms of the -galactosidase and -glucuronidase tests were compared with the existing recommended coliform confirmation procedures. A new method developed by Strathclyde Regional Council for detecting indole production and -galactosidase activity was also included. The trial was organised by Strathclyde Regional Council with guidance from the Rapid Methods Panel of the joint DoE/PHLS Standing Committee of Analysts Working Group 2. The study, which involved eleven water microbiology laboratories, was part-funded by the Department of the Environment.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.