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 (Bitton and Dutka 1986;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.