No abstract

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.

Studies on the pathogenesis of Haemophilus influenzae b infection have used bacteria grown in vitro which are relatively serum-sensitive (using serum devoid of anticapsular antibody) compared to organisms taken from infected hosts. We compared the virulence of relatively serum-sensitive and serum-factor induced serum-resistant H. influenzae b by inoculating rats with organisms having one or the other phenotype. The serum-resistant phenotype was more virulent following intraperitoneal or intravenous inoculation; however, there was no difference in the incidence of colonization or bacteraemia following intranasal inoculation. Furthermore, organisms colonizing the pharynx of rats had the serum-resistant phenotype. Thus, different phenotypes of the same strain of H. influenzae b differed in virulence following parenteral, but not intranasal, inoculation of bacteria. This could be explained by a change from serum-sensitive to serum-resistant phenotype shortly after entering the nasopharynx. The phenotype of micro-organisms grown in vitro may differ from organisms in infected individuals and these differences may be of critical importance in studies of immunity to infection and the pathogenesis of infection.

We questioned whether strains of ampicillin-resistant, non-beta-lactamase-producing (AmpR NBLP) Haemophilus influenzae with lower affinity penicillin-binding proteins (PBPs) might have altered virulence. The virulence of resistant transformant strains and the susceptible recipient was compared using infant rats. Following intraperitoneal inoculation, there was a significantly lower mortality rate and incidence and magnitude of bacteremia with two of three transformants compared to the recipient strain. Reduced virulence was not associated with greater bactericidal activity of serum or human neutrophils or faster clearance of the transformant following intravenous injection. Heated rat or human plasma supported exponential growth of the recipient, but not the transformant, suggesting deficient in vivo multiplication. We conclude that H. influenzae with altered PBPs are less virulent in an infant rat model which may be related to differences in in vivo growth.