Effect of R Factors on Rifampicm Resistance in E. coli

THE bactericidal effect of rifampicin, a semi-synthetic rifamycin, is due to its action on DNA-dependent RNA polymerase1 and all rifampicin-resistant mutants of Escherichia coli contain an altered RNA polymerase with an increased resistance to rifampicin in vitro2–4. While studying a possible curing effect of rifampicin on E. coli R factors, we observed that R+ recombinants of some rif-r mutants are more sensitive to rifampicin (Table 1). Of the cells harbouring certain R factors, less than 1% are able to form colonies on rifampicin-supplemented agar, while with certain others there is no detectable effect.