Molecular basis of invasive Haemophilus influenzae type b disease.

The molecular basis of central nervous system invasiveness by Haemophilus influenzae has been studied by using genetically defined mutants and in vivo and in vitro model systems. Capsular polysaccharide and lipopolysaccharide are important microbial determinants of the ability of H. influenzae to traverse the nasopharynx and localize in the cerebrospinal fluid and meninges after bacteremia. The genes for type b capsule confer greater invasive potential than do those for other capsular polysaccharides, although the molecular basis for this is not understood. Mutants have also indicated the role of lipopolysaccharide in enhancing the efficiency of bacterial translocation from the nose to the blood and in facilitating intravascular survival. Organisms that localize successfully in the blood and central nervous system are the progeny of a small fraction of the original challenge inoculum, often a single bacterium.