Transferrin-binding ability of invasive and commensal isolates of Haemophilus spp

Haemophilus influenzae type b expresses an inducible siderophore-independent iron-acquisition system that depends on a direct interaction between human transferrin and specific iron-regulated transferrin-binding outer-membrane proteins. To evaluate the importance of this iron-acquisition system amongst haemophili, 156 isolates of Haemophilus spp. (78 commensal isolates and 78 isolates from invasive infections) were examined for their ability to bind transferrin. Of the 78 invasive isolates, all of which were H. influenzae type b, 71 (91%) were capable of binding transferrin, with 57 (73%) binding transferrin constitutively (i.e., even when grown in an iron-sufficient medium). In contrast, only 11 (14%) of the commensal isolates bound transferrin constitutively, with a further 16 (21%) binding transferrin only after growth in an iron-deficient medium. Of the 27 commensal strains that were capable of binding transferrin, 12 were H. parainfluenzae biotype III, 14 were non-typable H. influenzae, and one was H. parahaemolyticus. None of the H. influenzae type b invasive or commensal isolates showed evidence of siderophore production, but 50 (66%) of the remaining 76 commensal isolates appeared to produce an iron chelator. Thus, while not a universal characteristic, detectable transferrin-binding was associated strongly with H. influenzae type b isolates from invasive infections, and was also recognised for the first time in isolates of H. parainfluenzae and H. parahaemolyticus.