Mechanisms of antibiotic resistance determined by resistance-transfer factors

Unowsky, Joel (Northwestern University Medical School, Chicago, Ill.), and Martin Rachmeler. Mechanisms of antibiotic resistance determined by resistance-transfer factors. J. Bacteriol. 92:358-365. 1966.-This study was concerned with the mechanism of expression of drug resistance carried by resistance-transfer (R) factors of two types: fi(-) (negative fertility inhibition) and fi(+) (positive fertility inhibition). The levels of drug resistance determined by R factors used in this study were similar to those reported by other investigators. A new finding was that Escherichia coli carrying the fi(-) episome was resistant to 150 to 200 mug/ml of streptomycin. The growth kinetics of R factor-containing cells were similar in the presence or absence of streptomycin, chloramphenicol, and tetracycline, but a period of adaptation was necessary before cells began exponential growth in the presence of tetracycline. By use of radioactive antibiotics, it was shown that cells containing the fi(-) episome were impermeable to tetracycline and streptomycin, whereas cells containing the fi(+) episome were impermeable only to chloramphenicol. Cell-free extracts from fi(+) and fi(-) cells were sensitive to the antibiotics tested in the polyuridylic acid-stimulated incorporation of phenylalanine into protein.