Relationships among some R plasmids found in Haemophilus influenzae

Abstract

Tetracycline resistance in a strain of H. influenzae isolated in the U.K. was found to be determined by an apparently non selftransmissible plasmid of 31 x 106 daltons (31 MDal), designated pUB701. Deoxyribonucleic acid hybridization studies indicated that pUB701 shares about 70% base sequence homology with the 30 MDal ampicillin resistance R plasmid RSF007, isolated in the United States from H. influenzae, and 64% sequence homology with the 38 MDal tetracycline and chloramphenicol resistance R plasmid pRI234, isolated in the Netherlands. Heteroduplex studies between RSF007 and pUB701 confirmed the fact that these plasmids were largely homologous, except that pUB701 contained the tetracycline resistance transposon TnD, whereas RSF007 contained the ampicillin resistance transposon TnA. A strain of H. parainfluenzae resistant to both chloramphenicol and tetracycline carried two species of plasmid deoxyribonucleic acid of 2.7 and 0.75 MDal. The authors were unable to prove that either resistance was plasmid borne in this strain. Hybridization studies with a [3H]thymine labeled tetracycline resistance enteric plasmid suggested that the tetracycline transposon was integrated into the chromosome of H. parainfluenzae UB2832. It is concluded either that the strains that were studied received R factors of the same incompatibility group bearing different resistance genes, or that different resistance genes were translocated to a common resident plasmid of H. influenzae.