Complement-mediated serum activities against genetically defined capsular transformants of Haemophilus influenzae

Although there are six different capsular serotypes of Haemophilus influenzae (a-f), only type b strains commonly cause systemic infections in man. The present study was performed to determine whether the propensity of the type b organism to cause invasive infections is due to a unique ability to evade complement-mediated host defenses. The ability of genetically defined capsular transformants (a-f) of an unencapsulated H. influenzae to resist the bactericidal and opsonic activities of serum was examined. The unencapsulated organism and the type f transformants were relatively susceptible to serum bactericidal activity in both adult and infant serum pools, the type a and e transformants were relatively resistant, and the types b, c and d transformants were intermediate. With respect to serum opsonic activity in both adult and infant serum pools, the unencapsulated organism and the type f transformant were relatively susceptible, the type a, b and e transformants were relatively resistant and the type c and d transformants were intermediate. Thus, although the type b capsule endows the organism with the ability to resist the bactericidal and opsonic effects of complement, this property is not unique to type b.