Abstract.A rapid, quantitative microbial assay, which is specific for heavy metal toxicity, has been developed. The assay (MetPLATE TM) is in a 96-well microtitration plate format and is suitable for determining toxicity characteristics such as median inhibitory concentrations. The sensitivity of MetPLATE TM to heavy metals [Cu, Zn, Cd, Pb, Hg, Cr(III)] was generally higher than Microtox and was of the same order as or better than Daphnia and fish bioassay. MetPLATE was insensitive to organic compounds at concentrations higher than those found in the environment. Six out of 10 industrial wastewaters or process waters surveyed were toxic. Heavy metal analysis of these waters confirmed the presence of heavy metals in the toxic samples. MetPLATE can be run concurrently with other assays for general toxicity to help determine the nature of chemicals causing toxicity.The deleterious effect of chemicals in the environment has traditionally been assessed with indicator organisms such as fish and invertebrates. Due to the large inventory of chemicals and the time-consuming nature of the traditional bioassays, microbioassays based on protozoa, microalgae, bacteria, and enzymes have become increasingly popular for the screening of environmental toxicants Dutka and Bitton, 1986). The trend in microbioassays is to package the assays in test kits, which facilitates application of toxicityscreening assays in the field or at smaller laboratories (Bitton and Koopman 1992).A limited number of test kits are presently available. The ECHA Biocide Monitor is a qualitative test based on inhibition of dehydrogenase activity and requires 18-24 h for completion (Dutka and Gorrie 1989). The Polytox kit includes a mixture of bacterial cultures, isolated from wastewater, which are used in respirometric tests (Elnabarawy et al. 1988). The Toxi-chromo Test is based on the inhibition of the biosynthesis of 13-galactosidase in Escherichia coli (Reinhartz et al. 1987). Both the Polytox and Toxi-chromo tests are quantitative tests, because they enable the determination of ECsos.Correspondence to: G. Bitton A common feature of currently available microbioassay kits is that they test for general toxicity, i.e., they respond to both heavy metal and organic toxicants. Apparently, hydrolases, particularly [3-galactosidase, are not sensitive to toxic organic compounds. In fact, researchers are still assaying for [3-galactosidase activity in the presence of sodium dodecyl sulfate, an organic chemical that is highly toxic to many of the test organisms used in toxicity testing protocols. [3-Galactosidase is also stable in pure acetone (Bitton, Kolar, and Koopman, unpublished data). This prompted investigation of the sensitivity of this enzyme to organic and inorganic toxicants. The results show that, indeed, [3-galactosidase is insensitive to all of the organic toxicants tested, while being sensitive to heavy metals (Dutton et al. 1988;Bitton et al. 1992aBitton et al. , 1992b.A qualitative assay that is specific for heavy metal toxicity and is not affect...

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Chromogenic substrates have been generally used in enzymatic assays for the specific determination of heavy metal toxicity. A toxicity assay based on the specific inhibition of β‐galactosidase by heavy metals and using a fluorogenic substrate was evaluated for its sensitivity to heavy metals and organic toxicants. The toxicity assay, FluoroMetPLATE, was specific for heavy metals and was more sensitive than the widely used Microtox assay. Except for lead, our assay displayed sensitivity similar to that of the 48‐h acute Ceriodaphnia dubia assay. Monitoring of industrial samples showed that the FluoroMetPLATE assay gave similar results as the daphnid toxicity assay for 22 of 29 samples. For the remaining samples, for which there was no agreement between the two tests, it was found that the toxicity was mostly due to organic toxicants, confirming the specificity of FluoroMetPLATE for heavy metal toxicity.