Characterization of chloramphenicol-resistant Haemophilus influenzae

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Abstract
Nine chloramphenicol-resistant (minimal inhibitory concentration, .gtoreq. 15 .mu.g/ml) H. influenzae strains isolated in various parts of the world were analyzed to characterize the genetic and biochemical bases of the resistance; 4 were type b. All 9 contained conjugative plasmids, ranging in MW from 34 .times. 106 to 46 .times. 106, which encoded for resistance to chloramphenicol and tetracycline or chloramphenicol, tetracycline and ampicillin. DNA homology studies showed that these plasmids were closely related to a previously described ampicillin-resistant plasmid, RSF007, and to each other. All 9 isolates and their chloramphenicol-resistant transconjugants produced chloramphenicol acetyltransferase. Chloramphenicol resistance in these strains of H. influenzae is apparently via plasmid-mediated production of chloramphenicol acetyltransferase.