Genotoxicity studies in semiconductor industry. 1. In vitro mutagenicity and genotoxicity studies of waste samples resulting from plasma etching

A semi-automated, microplate version of the SOS Chromotest for the analysis of complex environmental extracts

Environmental monitoring for genotoxicity requires that a large number of measurements be made across space and time. This requirement demands a rapid and efficient bioassay system. The SOS Chromotest is a rapid, efficient bacterial system for the detection of DNA damaging agents. Over 100 publications have described its use on a variety of samples. Relatively few studies have used the test to examine complex mixtures. Effective testing of complex samples poses a variety of problems. Although solutions have been proposed, few have validated the resulting protocol. In this work we present a semi-automated microplate version of the SOS Chromotest for the examination of complex mixtures. Experiments were conducted to determine the optimal cell concentration, exposure time, substrate conversion time and S9 enzyme concentration. The performance of the method was evaluated using 6 reference genotoxins and 3 complex mixtures. The complex mixtures examined are extracts of diesel particulate matter, urban dust and coal tar. The results obtained indicate that optimal responses often require fewer cells (approximately equal to 5-10 x 10(6) CFU/ml) and a longer exposure (3 h) than that recommended in the original protocol. Interfering effects of colored and turbid samples are removed using centrifugation and initial optical density readings taken 60 min after cell resuspension and lysis. The performance of the established protocol was evaluated using mitomycin C and benzo[a]pyrene results for 207 microplates and solvent control results for 293 microplates. The results indicate that the established method is accurate, sensitive and precise. Coefficient of variation on mean SOSIP values for mitomycin C and benzo[a]pyrene are < 5%. Solvent control data indicate that the standard threshold for determination of a positive response (induction factor > 1.5) is excessively conservative. All liquid transfers were automated using the Biomek automated laboratory workstation. Automation permits a throughput of up to 72 samples per day and maintains excellent precision and accuracy.

SOS chromotest results in a broader context: empirical relationships between genotoxic potency, mutagenic potency, and carcinogenic potency

Environmental monitoring requires that large numbers of samples be processed in a relatively short period of time. While microbioassays facilitate rapid testing, the results are often difficult to interpret in the broader context of human or animal health. Determining the consequences of exposure to genotoxic substances will ultimately require in situ monitoring of exposed organisms. However, it is immediately possible to construct a broad empirical framework within which available microbioassay results can be interpreted. To do this for SOS Chromotest results, we investigated the empirical relationships between SOS genotoxic potency and mutagenic potency (as measured with the Salmonella/microsome assay), as well as between genotoxic potency and carcinogenic potency (as measured using standard, chronic animal bioassays). Strong relationships were identified between; 1) genotoxic potency and mutagenic potency for 268 direct-acting substances (r2=0.76) and 2) genotoxic potency and mutagenic potency for 126 S9-activated substances (r2=0.65). Ordinary least squares regression analyses of the SOS genotoxicity-Salmonella mutagenicity relationship revealed a significant effect of SOS genotoxicity as well as differences in mutagenic potency that can be attributed to the Salmonella strain used to measure mutagenic potency. Analyses of S9-activated substances revealed a significant interaction between the SOS genotoxic potency (SOSIP) effect and the Salmonella strain effect. Two regression models relating SOS genotoxicity and Salmonella mutagenicity were used to predict the mutagenic potency of several industrial effluent extracts previously analyzed for SOS genotoxicity by White et al. [(1996): Environ Mol Mutagen 27:116-139]. Predictions are consistent with published mutagenic potency values for similar industrial waste materials. A consistent relationship was also identified between genotoxic potency and carcinogenic potency for 51 substances. Linear regression analyses revealed an effect of SOS genotoxic potency as well as differences in carcinogenic potency that may be attributable to experimental animal and route of exposure. The correlation between genotoxicity and carcinogenicity was fairly weak (maximum r value = 0.51). Previous studies revealed similar strength of association between Ames mutagenicity and carcinogenicity. Predicted carcinogenic potencies of previously examined genotoxic, industrial effluent extracts are generally low compared to the pure substances included in the data set.