Transferable plasmid-linked chloramphenicol acetyltransferase conferring high-level resistance in Bacteroides uniformis.

Bacteroides uniformis RYC3373 resistant to 64 micrograms of chloramphenicol per ml was isolated from a peritoneal pelvic abscess of a patient not previously treated with this drug. Chloramphenicol resistance was transferable at low frequency to a suitable Bacteroides fragilis recipient. The acquisition of resistance was linked to the presence of a 39.5-kilobase plasmid (pRYC3373), which was subsequently transferred to a secondary recipient. The transfer of Cm resistance occurred by a conjugation-like process. Donor and transconjugant strains produced chloramphenicol acetyltransferase constitutively. The Km for chloramphenicol was 40 microM, and its inactivation by 5-5'-dithiobis(2-nitrobenzoic acid) suggested its similarity to the type II enterobacterial enzymes encoded by different conjugative plasmids and also to a previously described enzyme of B. fragilis F47 and F48. The specific activity and the resistance level in pRYC3373-bearing strains were more than 10-fold higher than in the case of the enzyme from B. fragilis strains F47 and F48. The genetic basis of chloramphenicol acetyltransferase synthesis in Bacteroides spp. had not been previously established.