Surface Factors and Nasopharyngeal Colonization by Hemophilus influenzae B

Summary

Although capable of systemic invasion, Haemophilus influenzae type b (Hib) are found mainly in asymptomatic colonization of the nasopharynx. Hib expresses extracellular factors and cell surface structures with a potential for influencing colonization, but their respective contributions have not been clarified. It has been found to release a factor inhibitory to cultured ciliated epithelium and, like several other meningeal pathogens, to elaborate an IgA₁ protease. The capsule, better known as a determinant of invasiveness, also appears to confer a slight advantage in the initial stages of colonization. Certain of the nonfimbrial outer membrane proteins appear to be surface-exposed in the presence of capsule, but none have yet been identified as adhesins. Cell-surface expression of the lipopolysaccharide is quite limited in Hib as it seems to exist in vivo; however, released fragments of membrane containing LPS have toxic activities that may influence colonization. Hib was long believed to be non-fimbriated but now is known to have a rarely expressed ability to make a pilus that promotes a mannose-resistant binding to human cells. Experiments in infant rats indicate that fimbriation may increase the ability to initiate colonization. In the rat as in man, however, the Hib recovered in culture are consistently non-fimbriated.