Pathogenicity of a rifampin-resistant cerebrospinal fluid isolate of Haemophilus influenzae type b

Two children in a day care facility developed Haemophilus influenzae type b meningitis. The second child was enrolled in the facillty after rifampin had been administered to the other attendees. The isolate from the first child was susceptible to rifampin, but the isolate from the second was resistant. Both isolates had identical outer membrane protein PAGE profiles. To investigate the virulence of these isolates, we inoculated infant rats intranasally with either the rifampin-resistant or rifampin-susceptible CSF isolate. The rates of nasal colonization (14 of 20 and eight of eight animals inoculated with the rifampin-resistant and rifampin-susceptible isolates, respectively) did not differ significantly. However, bacteremia occurred less frequently in pups inoculated with the rifampin-resistant strain than in animals inoculated with the susceptible strain (four of 20 vs eight of eight, P