Plasmid-mediated chloramphenicol resistance in Haemophilus influenzae

A clinical isolate of Haemophilus influenzae HC234 was found to be resistant to chloramphenicol and tetracycline. It was shown that both resistance markers are tranferable as one unit to other Haemophilus influenzae strains and also to Escherichia coli. Data are presented which indicate that conjugation is the most likely mechanism of resistance transfer. HC234 was shown to carry a single plasmid species with a molecular weight of 38 x 10(6).