Use of a simplified fluctuation test to detect low levels of mutagens

As a mutagen screening procedure we have used a modification of the Luria and Delbrück fluctuation test in which the individual tubes are scored by eye for the presence or absence of a mutation. The test is simple and extremely sensitive, detecting concentrations of mutagens up to 100-fold lower than conventional tests. Measuring mutation to tryptophan independence in Escherichia coli strain WP2 we have found that methyl methanesulphonate (0.5 mug/ml), mitomycin C (0.0015 mug/ml), dichlorvos (5 mug/ml, and K2CrO4 (0.5 mug/ml) are all positively mutagenic in the test, whereas NiCl2 is negative. Chronic exposure to low levels of mutagens using this method appears to induce more mutations than might be predicted by extrapolation from short exposure experiments at higher doses. The procedure is applicable to any system which involves mutation to prototrophy from a non-leaky auxotrophic requirement and should prove valuable in detecting and investigating the effects of low doses and chronic exposures.