ISTA13--international interlaboratory comparative evaluation of microbial assay for risk assessment (MARA).

The microbial assay for risk assessment (MARA) is an innovative system based on an array of 11 different microbial species freeze-dried in a 96-well micro-titer plate. Developed for testing the toxicity of chemicals, mixtures, and environmental samples, the assay employs species of a taxonomically diverse range. In addition to 10 prokaryotic species, a eukaryote (yeast) is included in the range. The MARA's innate scope of a multi-dimensional test allows determination of toxicity based on a unique assay fingerprint or index, numerically expressed as the mean microbial toxic concentration (MTC). The most significant potential of the test is in the additional inference that can be conveyed to the toxicity evaluation because of the presence of each of the constituent species. The performance of MARA was evaluated to ascertain its capability and potential scope in an intralaboratory trial. Sensitivity to toxicants and different environmental samples was assessed. Evaluation included comparison with other tests; namely Microtox, invertebrate (Daphnia magna and Thamnocephalus platyurus) microbiotests, and respiration- and nitrification-inhibition tests. MARA's performance was further assessed with the implementation of an international interlaboratory trial. This involved the participation of 13 laboratories ranging from academic establishments to regulatory agencies. The results of the testing will be presented with assessment of the extent of variability and specific assay components. The trial evaluation indicated that performance of the assay was satisfactory and the results were within the acceptable range. MARA is a robust multispecies assay offering scope for toxicity assessment of a diverse range of samples.