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

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Abstract
To determine the importance of specific capsule type in the pathogenesis of invasive H. influenzae disease, the virulence of type b and type d strains isolated from different children was compared with the virulence of transformation-derived type b and type d organisms. The unencapsulated derivative of these strains was also examined. Virulence was assessed by determining the ability of the strains to produce bacteremia in rats with intranasal or s.c. 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 (42-62%) when 107 colony-forming units (CFU) were given intranasally. S.c. inoculation of 103 CFU of strains with the type b capsule produced bacteremia at a greater frequency than did the strains with the type d capsule (P < 0.002). The type d isolate was more virulent than a mutagenized derivative of the strain. The type b strains evidently are more virulent than type d when inoculated s.c.