Mechanism of R factor-mediated chloramphenicol resistance

The results of early studies on the mode of expression of the resistance transfer factor (RTF) in enteric bacteria were summarized by T. Watanabe (Bacteriol. Rev. 27:87, 1963). The biochemical mechanism of R factor-mediated resistance to chloramphenicol was attributed to a decreased permeability of resistant cells to the antibiotic in view of (i) inconclusive evidence of antibiotic inactivation, and (ii) the experiments of S. Okamoto and D. Mizuno (J. Gen. Microbiol. 35:125, 1964), which suggested that the cell-free synthesis of polypeptide in R factor strains was fully sensitive to chloramphenicol. The first direct evidence for an apparent permeability barrier to chloramphenicol was the observation (J. Unowsky and M. Rachmeler, J. Bacteriol. 92:358, 1966) that a threeto fourfold greater uptake of 14Cchloramphenicol occurred with chloramphenicolsensitive (R-) Escherichia coli as compared with resistant (R+) cells. Similar findings were obtained (W. V. Shaw, unpublished data) by similar techniques. S. Okamoto and Y. Suzuki (Nature 208:1301, 1965) demonstrated, however, that R+ strains of E. coli with the chloramphenicol resistance determinant were able to inactivate the antibiotic in the presence of acetyl-coenzyme A (acetyl-CoA). Subsequently, it has been shown conclusively that chloramphenicol-resistant R factor strains of E. coli inactivate the antibiotic by enzymatic acetylation to form the 3-acetyl and 1, 3-diacetyl derivatives (W. V. Shaw, J. Biol. Chem. 242:687, 1967; Y. Suzuki and S. Okamoto, J. Biol. Chem 242: 4722, 1967). More recent studies (W. V. Shaw and R. F. Brodsky, Antimicrobial Agents and Chemotherapy-1967, in press) have shown that 19 consecutive clinical isolates of enteric bacteria with an R factor for chloramphenicol resistance contain the enzyme chloramphenicol acetyltransferase. The present collaborative studies were undertaken to determine whether the acetylation mechanism of resistance might account for the ap-