Haemophilus haemolyticus resides in the upper respiratory tract of humans. This species is so closely related to nontypeable Haemophilus influenzae (NTHi) that molecular differentiation is required for accurate species identification. Whilst NTHi is associated with otitis media, sinusitis, pneumonia and bacteraemia, H. haemolyticus is considered to be a commensal with limited reports of invasive disease. We believe that understanding how H. haemolyticus interacts with the host will improve our understanding of NTHi pathogenesis.
In this study we have compared the ability of clinical H. haemolyticus isolates and reference strains Hh33390 and NTHi86-028NP to associate with and invade nasopharyngeal (D562) and bronchoalveolar (A549) epithelial cell lines. This revealed that nasopharyngeal H. haemolyticus strains were significantly less able to colonise and invade epithelial cells when compared with NTHi (P<0.001), whereas an invasive H. haemolyticus isolate (HI2028) from a child with bacteremia behaved like NTHi with high levels of attachment and invasion. Measurement of inflammatory cytokines in the cell-culture supernatant at 3h post-challenge revealed that commensal H. haemolyticus strains elicited high levels of pro-inflammatory cytokines IL6 and IL8 whereas detection of these cytokines was negligible in supernatant from cells challenged with NTHi and the invasive H. haemolyticus strain. Studies on gut commensals indicate that they elicit a local inflammatory response to regulate colonisation and this may be occurring with H. haemolyticus in the respiratory tract. NTHi is known to evade the host immune response and it appears that the invasive H. haemolyticus strain also has this property. This suggests that H. haemolyticus HI2028 has NTHi-like virulence factors that contribute to the invasive phenotype. Whole genome sequencing of commensal and invasive H. haemolyticus strains is underway in our laboratory. Comparison with NTHi sequences will reveal genetic requirements for disease and possibly identify targets for development of therapies to prevent NTHi disease.