A simple assay for toxic chemicals using a bacterial indicator

A simple, inexpensive and rapid technique to measure toxicity has been developed using Rhizobium meliloti as the indicator organism and its rapid reduction of the tetrazolium dye MTT (3-[4,5-dimethyl-thiazol-2-yl]-2,5-diphenyltetrazolium bromide). Toxic chemicals inhibited the reduction in this bacterium but not in others. Nearly 50 organic chemicals and 14 minerals were tested: inhibition was proportional to the concentration of the toxic chemical. The mechanism to account for both the reduction of the dye and the inhibition of reduction is uncertain. This method provides a simple and inexpensive way to determine the critical concentration of toxic compounds. The assay provides values comparable to those provided by the fat head minnow (Pimephales promelas). Results are also comparable to those obtained with the Microtox and Polytox assays, two commercial assays that use bacteria as indicator organisms.