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 beta-D-glucuronide was the only carbon source. Inhibition produced by one of five heavy-metal salts (Cd2+, Cr6+, Hg2+, Pb2+ or Zn2+) was the end-point and comparison standard to determine the EC50 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 Cr6+ and Hg2+ at two concentrations (0.25 and 0.5 mg l-1) almost completely inhibited this E. coli strain. All toxicant concentrations tested produced at least partial inhibitions of growth; Cr6+, Hg2+ and Cd2+, in that order, were most toxic, and Pb2+ the least. Zn2+ gave higher EC50 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.