Characterization of mechanisms involved in adherence of Haemophilus ducreyi to eukaryotic cells

The adherence of Haemophilus ducreyi to eukaryotic cells of various origins was investigated by means of a microassay using radiolabelled bacteria. The influence of physicochemical conditions and of different inhibitors on adherence to HEp-2 cells and human fibroblasts was examined. H. ducreyi strains manifested substantial adherence capacity (range, 11-38% of inoculum) to different cells, not discriminating between human and animal origin. The level of adherence was temperature-dependent, being substantially decreased by incubation at 4 degrees C, but was unaffected in the pH range 4-10. The adherence level was significantly reduced in the presence of sodium chloride or tetramethylurea (a hydrophobic bond-breaking agent). In addition, H. ducreyi bacteria manifested a pronounced capacity for binding Congo red to the surface, in comparison with the low binding ability of H. influenzae type b. This further indicates hydrophobic domains to be accessible on the surface of H. ducreyi. Inhibition studies with bacterial EDTA extract, sialic acid, heparin and heparan sulphate resulted in a significant reduction in adherent bacteria. However, adherence was not inhibited with crude 24 kDa pili material, LOS of H. ducreyi or fibronectin. Neither crude nor purified 24 kDa protein of H. ducreyi bacteria showed any capacity to bind monolayers of HEp-2, HeLa or human fibroblasts cells, as tested by immunoblot using specific polyclonal antibodies. The overall results suggest that adherence of H. ducreyi to eukaryotic cells is not specific to a particular cell type, human or animal. Adherence to HEp-2 cells involves a multiplicity of factors such as ionic and hydrophobic forces, and can be mediated by tissue heparin/heparan sulfate proteoglycans. However, specific binding to HEp-2 cells does not seem to be mediated by the 24 kDa pili protein of H. ducreyi.