Surface determinants of Haemophilus influenzae pathogenicity: comparative virulence of capsular transformants in normal and complement-depleted rats.

In an assessment of the contribution of different capsular polysaccharides to the pathogenicity of Haemophilus influenzae, the virulence of H influenzae strain Rd and of a series of capsular transformants (types b, c, d, and f) of strain Rd was compared and in normal and complement-depleted rats. After intraperitoneal inoculation, the encapsulated transformants were strikingly more virulent than strain Rd, although their pathogenicity varied; type b was the most virulent, type c the next most virulent, type f less virulent, and type d the least virulent. C3 depletion enhanced the susceptibility of rats to systemic infection but did not influence the relative virulence of the transformants. Comparative studies of clearance showed efficient intravascular survival of type b, c, and f transformants but not of the type d transformant or strain Rd; C3 depletion enhanced the intravascular survival of type d. Further analysis of the capsular transformants revealed differences in the electrophoretic characteristics of their lipopolysaccharides. These studies indicate that elaboration of a unique capsular polysaccharide may not be a sufficient explanation for the greater virulence of H influenzae type b and that lipopolysaccharide may contribute to the mediation of the differential pathogenicity of the various serotypes.