Role of intravascular replication in the pathogenesis of experimental bacteremia due to Haemophilus influenzae type b.

The potential role of extravascular and intravascular replication was studied in initiation of sustained bacteremia in experimental infection due to Haemophilus influenzae type b. When organs and fluid from rats were cultured after intranasal inoculation of the rats with H. influenzae type b, the organism was not recovered from any putative extravascular focus before development of bacteremia. To evaluate the potential contribution of intravascular replication in initiation of bacteremia due to H. influenzae type b, we obtained serial blood cultures after intravenous or intranasal inoculation. Bacterial counts increased exponentially immediately after intravenous and 12-18 hr after intranasal inoculation. Using the same model system, we observed bacteremia due to Streptococcus pneumoniae after intraperitoneal but not intravenous inoculation. After intraperitoneal inoculation, the magnitude of bacteremia in individual rats did not regularly increase exponentially over time. These findings are consistent with extravascular replication leading to bacteremia due to S. pneumoniae and efficient intravascular replication leading to sustained bacteremia due to H. influenzae type b.