Microbial Assay For Risk Assessment – A New Innovative Development For Toxicity Testing Of Chemicals And Environmental Samples

Wadhia, K.1, Dando, T.2, and Kuhn, I.3
1ALcontrol Laboratories, 2NCIMB Ltd, 3Karolinska Institute, Sweden


Microbial Assay for Risk Assessment (MARA), an array of eleven genetically diverse microorganisms, freeze-dried in a microtitre plate, was used for the evaluation of toxicity of various environmental samples ranging from complex industrial effluents
to potable water samples. The array was exposed to a concentration gradient of the test sample in the presence of a growth
medium. Growth of the microorganisms in the array at each sample dilution was measured by recording the reduction of the
redox dye tetrazolium red.

After incubation the plates were scanned utilising a flat bed scanner. The image produced from this was analysed using
purpose-built software to provide a unique fingerprint of the sample expressed in terms of sensitivity of the constituent ten
prokaryotic and one eukaryotic (yeast) species. Sample toxicity assessment using MARA is conveyed in terms of a microbial
toxic concentration for each microbial species and the overall assay mean. These values generated by the software are
determinations equivalent to EC50 values.

The system can also be used for initial screening of samples of potentially low (or zero) toxicity. In this instance, the
percentage growth inhibition of each of the eleven microorganism upon exposure to the neat sample is ascertained by
comparison with the test control.

The results of the testing performed with the MARA system were compared with toxicity assessment made using conventional
ecotoxicity methods e.g. Microtox®. The inference of the evaluation implemented concluded that MARA is a useful tool for
ecotoxicity testing.

Bioassays; Biological assessment; Ecotoxicity; Effluent; Leachates; Microbial assay; Soil extracts; Waters

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