Comparison of the beta-glucuronidase assay and the conventional method for identification of Escherichia coli on eosin-methylene blue agar

The purpose of this study was to compare the IMViC (indole, methyl red, Voges-Proskauer and citrate utilization) tests with the beta-glucuronidase (GUD) assay far the identification of suspect Escherichia coli on Levine's eosin-methylene blue (EMB) agar. After testing 258 suspect E. coli colonies from raw meat and meat products, 163 and 44 were found to be E. coli and non-E. coli, respectively, by both methods. Nine isolates were IMViC positive (i.e., ++--or -+--) but GUD negative; among these isolates, six were confirmed to be E. coli by API 20E (biomerieux, Marcy-l'Etoile, France) with the remaining three being non-E. coli. There were 42 isolates that were IMViC negative but GUD positive; among these isolates, seven were pure E. coli cultures, 33 were mixed cultures containing E. coli, and the remaining two were Proteus spp. The sensitivities for the identification of E. coli on EMB were 80.9% (169/209) and 97.1% (203/209), respectively by the IMViC tests and GUD assay; whereas the specificities were 93.9% (46/49) and 95.9% (47/49), respectively, by the IMViC tests and GUD assay. It is proposed that the GUD assay can be an effective alternative to the conventional IMViC tests for the identification of suspect E. coli on EMB.