Assessing enterococci as an alternative fecal indicator for irrigation water quality

Potential health risks associated with water used to irrigate fresh produce are increasingly recognized worldwide, leading to the implementation of water quality standards as part of many food safety programs. These standards are generally based on the presence of the fecal indicator bacterium (FIB), generic Escherichia coli; however, the persistence and potential growth of this species in the environment suggests current standards may not accurately predict health risks associated with irrigation water. In this context, we evaluated enterococci as an alternate FIB for assessing agricultural water quality under baseflow conditions, which typically coincide with peak demands for supplemental irrigation in the study area. We compared culture-based concentrations of Enterococcus to E. coli over two growing seasons at four different irrigation canals in Alberta, Canada, that historically exceed the E. coli irrigation guideline on a frequent basis. We also examined enterococci species distributions among irrigation water, animal feces, and human sewage. Escherichia coli and enterococci were significantly correlated at three of the four canals, although on average, more guideline exceedances and higher mean concentrations of enterococci were reported. Specific differences in FIB dynamics were observed between canals. Most notably, enterococci concentrations were significantly higher than E. coli at one canal, while E. coli concentrations were significantly higher than enterococci at a different canal. Enterococci species distributions were markedly different between irrigation water and animal feces. Enterococci species reportedly associated with plants predominated in irrigation water, whereas these species were absent or present in very low proportions in human sewage and animal feces. Results of this study suggest the limitations of using enterococci for measuring microbial quality of irrigation water are similar to E. coli, in that risk analyses with respect to food safety may be confounded by the presence of environment-associated species and/or strains.