The role of toxins and virulence factors in hetero trophic bacteria present in drinking water supplies

Heterotrophic plate count (HPC) bacteria are ubiquitous in both man‐made and natural water systems and consist of several bacterial species which rely on carbon molecules, generally pre‐formed by other organisms, as a nutrient source. Previous studies have suggested that heterotrophs may be significant to human health if present in drinking water supplies and may be responsible for incidences of gastro intestinal disturbance. Bacteria which are associated with gastrointestinal (GI) illness may be classed as enterotoxigenic (ETEC), entero invasive (EIEC), enteropathogenic (EPEC) and/or entero haemorrhagic (EHEC). An overview of the literature, discussing both the proposed role of heterotrophs in causing GI symptoms and the known factors associated with entero toxicity, e.g. cytotoxic and cytotonic activity, invasiveness and haemolysin activity will be presented. The results of a recent UK study will also be discussed. In this study isolates were obtained from both pre‐flush and post‐flush drinking water samples taken from both houses and multi‐occupancy buildings from several geographically unrelated sites. Studies to determine the occurrence of toxigenic/virulence factors in the heterotrophs isolated were undertaken, by assessing their potential to cause GI upset, by determination of their invasiveness and cytotonic and cytotoxic effects using three different cell lines, together with haemolysin activity. The results of this study concur with others and suggest that the frequency of isolation of heterotrophic species with the capability to cause GI symptoms is too low to be of significant concern to health authorities or water companies. Furthermore, where a positive result was obtained with respect to the possession of virulence characteristics, the bacteria in question usually displayed more than one type of virulence factor.