Survival ofEscherichia coli andYersinia enterocolitica in stream water: Comparison of field and laboratory exposure.

Experiments were done to compare the influence of three aquatic exposure methods on the behavior of pathogenic and nonpathogenic enteric bacteria (Yersinia enterocolitica andEscherichia coli). Bacterial suspensions were exposed to stream water in membrane diffusion chambers in situ as well as in the laboratory using a large vessel of stream water and in enclosed bottles. The persistence of culturability of the bacterial suspensions was dependent upon the method of aquatic exposure. This difference was most apparent during the initial six days of each experiment. A steady decline in colony forming units was seen after a short stationary period in chambers in situ, while there was an abrupt increase in bacteria within chambers exposed in the laboratory. A rapid initial decrease was observed in the experimental variation using bottles, accompanied by higher levels of injury inE. coli and reduced expression of plasmid-borne virulence phenotypes inY. enterocolitica. However, there were no changes in the plasmid profiles of either organism throughout the 21-day duration of the experiments. In addition, the survival and injury of pathogenic and nonpathogenic strains of both test bacteria was very similar with aquatic exposure. These results suggest that the response of enteric bacteria in aquatic environments is influenced by experimental design as well as other factors and that the comparison of survival data should only be attempted when similar methods are used.