Correcting for toxic inhibition in quantification of genotoxic response in the umuC test

An improved procedure for quantification of results from the umuC tests for genotoxicity is presented. The calculation method better separates toxic growth inhibition (cytotoxicity) from genotoxic effects than currently used methods and therefore, greatly extends the applicability of genotoxicity tests on environmental samples. The basic principle is to normalize the genotoxic response compensating for both decreasing biomass and growth rate reduction that results from cytotoxicity. The improved method and the currently used method was compared for umuC tests on the pure compounds: methylmethanesulfonate (MMS), N-methyl-N'-nitro-N-nitroguanidine (MNNG), sodium azide (NaN3), and 4-nitroquinoline-1-oxide (4-NQO). For compounds with no or low cytotoxicity, the two calculation methods gave practically identical results, while for highly cytotoxic compounds, the traditional method overestimated genotoxicity. umuC tests were also carried out on leachate polluted groundwater sampled downgradient of a landfill (Grindsted, Denmark). All polluted samples showed high cytotoxicity concomitant with high genotoxicity when the results were quantified in the traditional way. The new method showed that these results were in fact false positive, as the apparent genotoxicity was a result of cytotoxicity. Based on the mathematical analysis leading to the improved procedure for correction for cytotoxicity, it is suggested to alter the present test design of the umuC test in order to obtain well-defined exposure concentrations as well as mathematical consistency in the quantification of genotoxicity.