Comparative virulence of Haemophilus influenzae with a type b or type d capsule.

To determine the importance of specific capsule type in the pathogenesis of invasive Haemophilus influenzae disease, we compared the virulence of type b and type d strains isolated from different children with the virulence of transformation-derived type b and type d organisms. In addition, the unencapsulated derivative of these strains was also examined. Virulence was assessed by determining the ability of the strains to produce bacteremia with intranasal or subcutaneous inoculation. Unencapsulated derivatives were unable to cause bacteremia by any route; all type b strains (whether natural or derived by transformation), a natural type d, and a type d derived by transformation were able to produce bacteremia with similar frequency (42 to 62%) when 10(7) colony-forming units was given intranasally. Subcutaneous inoculation of 10(3) colony-forming units of strains with the type b capsule produced bacteremia at a greater frequency than did the strains with the type d capsule (P less than 0.002). The type d isolate was more virulent than a mutagenized derivative of the strain. We conclude that the type b strains are more virulent than type d when inoculated subcutaneously.