Introduction: Specific alterations in the chromosomally-encoded ftsI gene are responsible for the beta-lactamase-negative ampicillin-resistant (gBLNAR) genotype in Haemophilus influenzae. Horizontal gene transfer of the ftsI gene between the commensal species Haemophilus haemolyticus and the pathogenic species Haemophilus influenzae have previously been inferred from mosaic structures of the ftsI gene observed in some clinical isolates.
Results: We successfully transferred segments of the ftsI gene between H. haemolyticus and H. influenzae by electroporation. The transfer of ftsI from a gBLNAR strain of one Haemophilus species conferred a rise in MIC of a sensitive strain of the other Haemophilus species. Sequencing revealed that the rise in MIC was due to homologous recombination of ftsI genes. The frequency of homologous recombination of ftsI between separate species was between 10-6 and 10-7 with H. influenzae strain Rd as a recipient, and between 10-4 and 10-5 with H. heamolyticus strain ATCC 33390T as a recipient. Similar recombination frequencies were observed for intra- and inter-species transformation.
Conclusions: In this study we document that homologous recombination of the ftsI gene can occur in vitro between H. influenzae and H. haemolyticus, and that ftsI from a resistant Haemophilus strain confers an increase in MIC in a sensitive strain of the other species. These results indicate that commensal species of Haemophilus could be potential reservoirs, from which the pathogenic H. influenzae can acquire antibiotic resistance genes.