The major mechanism of resistance to chloramphenicol is the production of a chloramphenicol acetyltransferase which converts the drug to either the monoacetate or the diacetate. These derivatives are unable to bind to the bacterial 50S ribosomal subunit and thus cannot inhibit peptidyl transferase activity. The chloramphenicol acetyltransferase (CAT) gene is usually encoded on a plasmid or transposon and may transpose to the chromosome. Surprisingly, in view of the very limited use of chloramphenicol, resistance is not uncommon, even in Esch. coli, although it is most frequently seen in organisms that are multiresistant.
