Contribution of Haemophilus influenzae type b lipopolysaccharide to pathogenesis of infection

The mechanism(s) by which the lipopolysaccharide (LPS) of Haemophilus influenzae type b may contribute to the virulence of this organism is unclear. Purified LPS of Haemophilus influenzae type b or phosphate buffered saline was administered intranasally to infant rats prior to the intranasal instillation of approximately 2-20 × 10⁶ cfu of Hib two or three times per day for three consecutive days. The preadministration of 2.0 μg Hib LPS resulted in a significantly greater incidence of bacteremia (P = 0.0006) than PBS 30 min after the completion of the intranasal inoculations. Four days following completion of intranasal Hib inoculation the incidence of bacteremia was greater (P = 0.017) in the animals pretreated with LPS at 2.0 μg compared to the PBS pretreated animals. Preadministration of 0.2 μg LPS had no effect on the incidence of bacteremia or meningitis. There were no differences in the histology of the nasal cavities or turbinates of infant rats inoculated intranasally only with LPS or PBS. There were no differences in the frequency or density of bacteremia following intranasal administration of LPS from either Hib or E. coli. Although the mechanism is unknown, our findings suggest that the LPS of Hib may contribute to the ability of H. influenzae type b to invade the nasal mucosa in this infant rat model.