Research Product

Direct counts of Hg2+ resistant cells in natural waters were obtained by fluorescence microscopy after incubation for 20 h in the presence of a growth substrate, nalidixic acid, to halt cell division, and inhibiting concentrations of Hg2+. This method discriminated Hg2+ resistant from sensitive Escherichia coli strains. Estuarine samples were used to compare this procedure with two other toxicity measurements that determine the effect of Hg2+ on colony growth and on rates of radioactive thymidine incorporation into cellular material. Toxicity measurements based on direct viable counts and thymidine incorporation rates had comparable sensitivities, and both were 3-4 orders of magnitude more sensitive than the method that utilized colony counts. Thus, the direct enumeration of Hg2+ resistant cells is useful for predicting the potential of aquatic communities to sustain heterotrophic activity, an essential microbial process, in the presence of Hg2+.