Influence of Inorganic Contaminants On Escherichia coli and Its Predator: Implications to the Long-Term Survival of E. coli in Water.

Escherichia coli (E. coli) data from many freshwater locations suggests long-term survival of the bacteria is occurring.  This situation is juxtaposed against “conventional wisdom” that suggests enteric bacteria (e.g., E. coli) should die-off upon reaching the environment. Our hypothesis is that the increased stability of the bacterial populations in water reflects the loss of the bacteria’s natural predators (i.e., protozoa). The loss of the predator is the direct consequence of the pollutant. In this study we examined the interaction between E. coli and protozoa in the presence of heavy metals at different concentrations to evaluate the differential impact of the heavy metals on the two organisms and its impact on E. coli predation by protozoa. E. coli O157:H7, which is marked with the lux gene cassette and Tetrahymena pyriformis (T. pyriformis) were used as model organisms. The toxicity of four metals (Zinc, Zn; Copper, Cu; Nickel, Ni and Chromium, Cr) towards E. coli, T. pyriformis and the interaction between the two were assayed in a 96 well microtiter plates. In general, the toxicity effect was more sever on T. pyriformis than E. coli. EC50 values of Zn for E. coli and T. pyriformis were, for example, 39.9 and 37.4 ppm, respectively. This indicates that one needs a smaller doze of Zn for T. pyriformis than E. coli to see a similar level of toxicity. The results from the interaction studies based on bioluminescence signal from the E. coli strain and use of two differentially lysing enzymes, indicated that the ingestion of E. coli by T. pyriformis was largely limited due to the toxicity of the metals. Over all, our results suggest that contaminants in the environment might decrease the predation stress on E. coli, which might prove to be beneficial for a long term survival of E. coli in the environment.