Haemophilus influenzae bacteremia and meningitis resulting from survival of a single organism.

Infant rats were infected intranasally with mixtures of streptomycin-sensitive and streptomycin-resistant strains of Haemophilus influenzae type b and cultures of nasopharyngeal washings, blood, and cerebrospinal fluid were obtained. If the infecting organisms cooperated with each other during the establishment of infection, nasopharyngeal, blood, and cerebrospinal fluid cultures should have contained mixtures of the variants. If each organism acted independently, then with small infecting inocula all the organisms in nasopharynx, blood, or cerebrospinal fluid should be descended from a single bacterium. Cultures should then contain only one of the variants. Single variant nasopharyngeal cultures were obtained from 8 out of 19 (42%) rats when the intranasal inoculum was <100 organisms. As the inoculum was increased, single variant cultures were less frequently observed. When the inoculum was >/=10(5) organisms, nasopharyngeal cultures were always mixtures. Single variant blood cultures were obtained in 46 of 67 (68.7%) episodes of bacteremia when rats were inoculated intranasally with 10(8) organisms. Single variants were isolated from the cerebrospinal fluid of 13 of 19 (68.4%) rats with meningitis whose blood contained both streptomycin-sensitive and streptomycin-resistant variants. When the blood contained a single variant, this same variant was cultured from the cerebrospinal fluid on 39 of 40 (97.5%) occasions. These studies demonstrated that invasive. H. influenzae b infections of infant rats resulted from independent action, as opposed to cooperative interaction of intransally inoculated organisms. The results also suggested that the meninges were invaded by the hematogenous route.