Molecular properties of transmissible R factors of Haemophilus influenzae determing tetracycline resistance.

The tetracycline-resistant Haemophilus influenzae strains LU121 and FR16017, recently isolated in West Germany, each harbour a plasmid; that of the former (pLU12U) has a mol. wt of 31.5 X 10(6) and that of the latter (pFR16017) has a mol. wt of 33 X 10(6). Conjugation and DNA-DNA hybridization studies have shown that both plasmids are self-transmissible and carry tetracycline-resistance genes. The purified plasmid DNA of H. influenzae strain LU121 transformed a sensitive Escherichia coli strain to tetracycline resistance. The two R factors are closely related to the H. influenzae plasmid specifying ampicillin resistance (pKRE5367). Electron microscope DNA heteroduplex analysis indicated that pLU121 and pFR15017 probably carry the tetracycline-resistance transposon TnD and that pKRE5367 probably carries the ampicillin-resistance transposon TnA. There is more than one integration site for the insertion which probably represents TnD in pFR15017. All three plasmids have a similar plasmid core and could have a common evolutionary origin.