An international collaborative study of 'genetic drift' in Salmonella typhimurium strains used in the Ames test.

An international collaborative study of the response of 5 Salmonella typhimurium strains (TA1535, TA1537, TA1538, TA98 and TA100) to 4-nitroquinoline-N-oxide was performed. A laboratory's 'in-house' stock of these strains was compared with a set of reference strains, using a standardized protocol. The prime objective of this study was to investigate whether the ability of these strains to produce spontaneous or induced mutants had changed during their prolonged cultivation in different laboratories, i.e. to investigate their 'genetic drift'. Any observed change in mutability might contribute to the variations between laboratories in the results of the Ames test. A second objective was to obtain information on the extent of intra- and inter-laboratory variation when a standardized protocol was used. 38 laboratories participated in this study. The data were analysed statistically by 3 groups using different models and the same conclusion was reached: genetic drift is found in some strains in some laboratories, but does not contribute significantly to inter-laboratory variation in the Ames test. When the inter-laboratory variation was analysed there was considerable correlation between results for the 'in-house' and the reference strain, and between results for different strains in the same laboratory (Margolin et al., accompanying paper).