Introduction: Non-typeable Haemophilus influenzae (NTHi) is a commensal of the upper respiratory tract and an important opportunistic pathogen, causing both acute and chronic respiratory infections. Many isolates of NTHi have been shown to be able to invade respiratory epithelial cells in vitro, and it has been suggested that this may be a virulence attribute, particularly in persistent infections where the intracellular environment would protect the bacterial cells from both the host immune response and antibiotic exposure. H. haemolyticus is very closely related to NTHi and shares the same upper respiratory tract habitat but is not an opportunistic respiratory pathogen. Little information is available on the ability of H. haemolyticus to invade epithelial cells in vitro.
Objectives: To compare the ability of NTHi and H. haemolyticus to invade respiratory epithelial cells in vitro. The hypothesis is that if invasion is a virulence attribute for NTHi, then isolates of H. haemolyticus should be comparatively non-invasive.
Methods: A collection of NTHi (n=16) from different clinical samples (4 each from sputum, ears and eyes of infected patients and throats from healthy volunteers) and H. haemolyticus (n=20) all from the throats of healthy volunteers was established, and molecular methods used for identification. A gentamicin survival assay was used to determine the ability of each isolate to invade immortal bronchial epithelial cells (BEAS-2B) in vitro, with results expressed as a percentage of bacterial cells from the challenge inoculum that were recovered from the BEAS-2B intracellular space after 4 hrs co-incubation.
Results: The isolates of NTHi showed variable ability to invade the BEAS-2B cells (range: 0.03 to 43.5%, median: 3.2%) and the isolates of H. haemolyticus were uniformly non-invasive (range: 0.0 to 0.37, median: 0.0) with the difference being statistically significant (p=0.003).
Conclusions: The inability of the isolates of H. haemolyticus to invade BEAS-2B cells in vitro, supports a role for respiratory cell invasion in the pathogenesis of some NTHi infections.