Objectives: To investigate the phenotypic effect of expression of selected acquired macrolide resistance genes (AMRGs) in non-typeable Haemophilus influenzae (NTHi).

Methods: The AMRGs erm(A), erm(B) and erm(C) were cloned into Escherichia coli JM109 using the shuttle vector pLS88; constructed plasmids extracted from suitable clones were used to transform H. influenzae Rd by electroporation. Erythromycin and azithromycin MICs for suitable transformants were determined by broth microdilution. AMRG expression was determined using quantitative PCR on transformant cDNA with locked nucleic acid dual-labelled hydrolysis probes.

Results: Expression of all AMRGs was observed in the transformants. Some variation in expression between the AMRGs was apparent, but expression of all genes was associated with a notable increase in erythromycin and azithromycin MICs compared with untransformed H. influenzae Rd.

Conclusions: While the establishment of erm genes within WT populations of NTHi remains contentious, H. influenzae is capable of expression of erm. Expression may be associated with a subsequent decreased susceptibility to macrolides in isolates and future monitoring of these genes in NTHi isolates is warranted.