Selective enrichment of R⁻ segregants as the main mechanism of ‘curing’ of the R factor by acridine dyes

Summary

By the use of appropriate strains of Escherichia coli, Shigella flexneri and Salmonella typhimurium with and without an R factor, R₁₀₀, the mechanism of ‘curing’ of R factor by acridine dyes was examined. This R factor was shown to confer increased sensitivity to acriflavine upon the host cells. E. coli strain W-3630, once infected with R₁₀₀, has never been observed to segregate R⁻ cells. When mixtures of R⁺ and R⁻ cells of this strain were grown in acriflavine broth, the proportion of R⁻ cells increased and was also correlated with the proportion in the initial inoculum. Other bacterial strains carrying R₁₀₀ segregate R~ cells spontaneously. Growth tests starting with varying proportion of R⁺ and R⁻ cells of these strains in acriflavine broth also gave a marked correlation between the initial and final proportions of R⁻ cells, and indicated that the main cause of ‘curing’ the R factor was the selective enrichment of R⁻ segregants present in the initial inocula or arising spontaneously during growth of the R⁺ culture. These results suggest that the mechanisms underlying the ‘curing’ of F and R factors are different. Tests with several acridine dyes gave results similar to those with acriflavine.