Environmental occurrence of the enterococcal surface protein ( esp ) gene is an unreliable indicator of human fecal contamination

Abstract

The enterococcal surface protein ( esp ) gene found in Enterococcus faecalis and E. faecium has recently been explored as a marker of sewage pollution in recreational waters but its occurrence and distribution in environmental enterococci has not been well-documented. If the esp gene is found in environmental samples, there are potential implications for microbial source tracking applications. In the current study, a total of 452 samples (lake water, 100; stream water, 129; nearshore sand, 96; and backshore sand, 71; Cladophora sp. (Chlorophyta), 41; and periphyton (mostly Bacillariophyceae ), 15) collected from the coastal watersheds of southern Lake Michigan were selectively cultured for enterococci and then analyzed for the esp gene by PCR, targeting E. faecalis/ E. faecium ( esp fs/fm ) and E. faecium ( esp fm ). Overall relative frequencies for esp fs/fm and esp fm were 27.4 and 5.1%. Respective percent frequency for the esp fs/fm and esp fm was 36 and 14% in lake water; 38.8 and 2.3% in stream water; 24 and 6.3% in nearshore sand; 0% in backshore sand; 24.4 and 0% in Cladophora sp.; and 33.3 and 0% in periphyton. The overall occurrence of both esp fs/fm and esp fm was significantly related ( χ 2 = 49, P < 0.0001). Post-rain incidence of esp fs/fm increased in lake and stream water and nearshore sand. Further, E. coli and enterococci cell densities were significant predictors for esp fs/fm occurrence in post-rain lake water, but esp fm was not. F + coliphage densities were not significant predictors for esp fm or esp fs/fm gene incidence. In summary, the differential occurrence of the esp gene in the environment suggests that it is not limited to human fecal sources and thus may weaken its use as a reliable tool in discriminating contaminant sources (i.e., human vs nonhuman).