Macrolides are frequently used to treat respiratory infections and the increasing prevalence of macrolide-resistant species is a significant problem. Macrolide resistance can be caused by various ribosomal mutations or via acquired macrolide resistance genes (AMRGs), including the erm genes that encode binding site-altering ribosomal methylases, and a variety of other genes that encode efflux mechanisms (including the mef and msr genes) or drug-inactivating enzymes (including the mph genes).
Most isolates of non-typeable Haemophilus influenzae (NTHi) carry an intrinsic efflux mechanism rendering them less susceptible to macrolides than other species, but high-level resistance is rare and is usually associated with ribosomal mutations. Recently, a mobile, multiresistance integrative conjugative element (ICE), designated ICEPmu1, has been described in a bovine Pasteurella multocida isolate and was found to carry the AMRGs erm(42), msr(E) and mph(E). Given that previous studies have shown that animal- and human-derived Pasteurella spp. are able to share resistance genes with NTHi via horizontal transfer, there is precedent for the possible transfer of ICEPmu1 and other AMRG-bearing mobile elements from P. multocida to NTHi, thus allowing NTHi to acquire AMRGs.
Our aims were to demonstrate conjugal transfer of ICEPmu1 from P. multocida to NTHi and to investigate the phenotypic effect of erm(42), msr(E) and mph(E) in NTHi. Transfer occurred at a frequency of 5.73 x 10-6 per recipient. PCR analysis revealed that the ICEPmu1 transferred to NTHi was truncated such that mph(E) and msr(E) were deleted; however, erm(42) was transferred successfully and the minimum inhibitory concentrations of erythromycin and clindamycin were increased 15-fold for the transconjugant compared to the recipient. The transfer functions of ICEPmu1 were retained and active in NTHi; intra-species (NTHi to NTHi) conjugal transfer occurred at a frequency of 1.14 x 10-7 per recipient. The nature of the event in which ICEPmu1 was truncated and the chromosomal insertion point of ICEPmu1 in NTHi are currently being determined. Further attempts at transferring the complete ICEPmu1 into NTHi are ongoing.