Biological effects of dyes on bacteria. VI. Mutation induction by acridine orange and methylene blue in the dark with special reference to Escherichia coli WP6 (polA1).

Acridine orange (AO) and methylene blue (MB) in the dark were shown to be weak to moderate mutagens (induction of resistance to T5 phage) in repair-deficient strains of Escherichia coli B/r. However, strain WP2 (wild-type) was not mutated by AO in the dark, in confirmation of earlier data. The presence of 2 microM AO reduced by 41% the spontaneous mutation rate in strain WP2, from 4.1 to 2.4 mutants/10(8) cells/generation. In the polymerase I-deficient strain WP6 (polA1), 2 microM AO increased the mutation rate in the dark 14-fold. We propose that both spontaneous and AO-induced mutagenesis in the absence of light occur at the site of semiconservative DNA replication. If the intercalation mechanism for the effects in the absence of light is valid, the wild-type strain (WP2) may be resistant to frameshift mutagenesis induced by intercalated compounds, while the polymerase I-deficient strain (WP6) may be highly suceptible to the presence of an intercalated dye such as AO at the DNA-replication fork. MB and AO likely act through different mechanisms since MB is only a moderate mutagen in strain WP6 and the other repair-deficient strains tested.