New type of R factors incapable of inactivating chloramphenicol.

Four R factors conferring chloramphenicol (CM) resistance were isolated from Escherichia coli strains of clinical origin. Strains carrying the factors were found to be incapable of inactivating the drug in the presence of acetyl coenzyme A. E. coli W3630 carrying R(70), one of these factors, became sensitive to CM after treatment with glycine, indicating that the spheroplasts of W3630 R(70) (+) were sensitive to the drug and suggesting that the cell membrane is important for CM resistance. The observation that cell-free protein synthesis in W3630 R(70) (+) was inhibited by CM is also compatible with a decrease in permeability. CM resistance in W3630 R(70) (+) appeared to be inducible, because (i) preincubation with subinhibitory concentrations of CM prevented the prolonged lag noted for growth in the presence of 25 mug of CM per ml, and (ii) the preincubation effect was lost after overnight growth in CM-free medium. By contrast, E. coli W3630 cml(+), in which the resistance determinant is integrated into the chromosome, was capable of rapid inactivation of CM. E. coli W3630 cml(+) R(70) (+), which contains the proposed permeability determinant (episomal) as well as levels of the inactivating enzyme (chromosomal) that are comparable with W3630 cml(+), was capable of brief inactivation of CM when inoculated into drug-containing medium. The absence of continued inactivation on more prolonged incubation favors the hypothesis that the R(70) factor inhibited further penetration of CM and that this property possesses the characteristics of induction.