Rapid enumeration of Escherichia coli in marine bathing waters: potential interference of nontarget bacteria

Aims: To compare the Escherichia coli quantification given by the 'Coliplage (R)' assay, based on the direct measurement of the beta-d-glucuronidase (GLUase) activity and the reference Most Probable Number (MPN) method from seawater sites and investigate the possible interference of non-E. coli strains in the GLUase activity measurement. Methods and Results: Comparison performed from 69 French coastal bathing sites (1401 samples) showed nonconcordance between both methods, only for 8% of samples. Non-E. coli 4-methylumbelliferyl-beta-d-glucuronide (MUG+) were isolated from nonconcordant samples. Phylogenetic analysis showed that Gammaproteobacteria were dominants and mainly represented by Vibrio species, which displayed GLUase activities on the same order of magnitude and sometimes much higher as E. coli reference strains. Conclusions: The'Coliplage (R)' assay is a rapid method for the quantification of E. coli showed few discordances with the standard MPN method. Some Vibrio species could interfere on the direct GLUase activity measurement of E. coli. Significance and Impact of the Study: Data present the first qualitative investigation on disagreement between Coliplage (R) and the MPN results. If the interference of Vibrio species is confirmed in situ, appropriate treatments should be developed to remove the interfering signal.