Evaluation of the toxicity of several heavy metals by a fluorescent bacterial bioassay

This new bioassay determined the toxicity of chemical compounds dissolved in water by measuring the degree of inhibition of the ultraviolet light‐stimulated fluorescence of Escherichia coli in a culture medium in which 4‐methylumbelliferyl β‐D‐glucuronide was the only carbon source. Inhibition produced by one of five heavy‐metal salts (Cd 2+ , Cr 6+ , Hg 2+ , Pb 2+ or Zn 2+ ) was the end‐point and comparison standard to determine the EC 50 and minimum effective concentration (MEC) that produced a decrease of E. coli growth rate, increased doubling time and percentage inhibition and reduced numbers of generations; all these values were derived from the fluorescence signals. Only Cr 6+ and Hg 2+ at two concentrations (0.25 and 0.5 mg I −1 ) almost completely inhibited this E. coli strain. All toxicant concentrations tested produced at least partial inhibitions of growth; Cr 6+ , Hg 2+ and Cd 2+ , in that order, were most toxic, and Pb 2+ the least. Zn 2+ gave higher EC 50 values at 3 h of incubation than at 4 h. The method was simple, rapid and inexpensive and would permit a large number of samples to be tested quickly.