COMPARATIVE ENZYMOLOGY OF CHLORAMPHENICOL RESISTANCE *

The concept of drug inactivation as a possible mechanism of bacterial resistance to antibiotics is well established and best illustrated by the P-lactamase group of enzymes that have been isolated from penicillin-resistant species of bacilli, staphylococci, and a host of gram-negative microorganisms.1 Although chloramphenicol has been in clinical use for more than two decades and the emergence of resistant bacteria in a number of genera has been well documented, there has been little information available until quite recently on the biochemical details of the specific mechanisms responsible for the resistance of clinically important species. The first observations that led to our present understanding of chloramphenicol resistance due to antibiotic inactivation were those of Dunsmoor and collaborators,2 and Miyamura3 only six years ago. Although such studies clearly established the fact that residual antibiotic activity was lacking in culture Altrates of resistant bacteria grown in the presence of chloramphenicol, it remained for Okamoto and Suzuki4 to show that specific inactivating enzymes could be obtained from cell-free extracts of Escherichia coli, which harbored a transferable R factor for chloramphenicol resistance. The importance of this observation cannot be underestimated since it marked the convergence of two independent approaches to antibiotic resistance, namely, the biochemical method and the recently discovered genetic concepts of transferable extrachromosomal resistance as exemplified by the Resistance Transfer Factors (RTF)5 and the staphylococcal plasmids.6 The chloramphenicol resistance system appeared to be an ideal model, since resistant staphylococci and enteric bacteria were important clinical pathogens and synthetic routes were available for the preparation of a wide variety of analogues and isomers of chl~ramphenicol.~ The commercial availability of radioactive ( I4C) chloramphenicol also provided a powerful tool for studying the metabolic fate of the antibiotic. Within a relatively short period of time, the R factor type of chloramphenicol resistance was shown to be due to acetylation of the antibiotic in the presence of acetyl-Coenzyme A and a specific R-factor mediated The products of chloramphenicol inactivation were shown to be the 3-acetyl and 1,3-diacetyl esters of the antibiotic, and the overall stoichiometry was as follows: