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 facility 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 less than 0.0001). Nasal washings, blood, and CSF obtained from animals inoculated with the rifampin-resistant isolate were divided and plated on media containing rifampin (1 microgram/ml) or without rifampin. Except for those from one animal, organisms isolated from blood and CSF grew only on medium lacking rifampin, whereas H. influenzae type b growing from nasal washings was frequently found on both media. We conclude that mutation of H. influenzae to rifampin resistance is a hazard of rifampin chemoprophylaxis. Rifampin-resistant isolates have the potential to cause disease in patients and experimental animals, although they may be relatively less pathogenic than the parent, susceptible organism.