Human Serum Bactericidal Activity against Haemophilus influenzae Type b

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Abstract
Bactericidal and opsonizing activity of pooled adult immune serum against H. influenzae type b was examined with and without the addition of phagocytes. Four type b strains from CSF and 3 such strains from the nasopharynx (NP) of healthy children were examined. Duplicate reaction mixtures contained organisms in exponential (E) or stationary phase (S) of growth, serum, a complement source (human agammaglobulinemic serum) and culture medium (bactericidal assay); separate assays contained the above components and polymorphonuclear leukocytes (opsonization system). A decrease in bacterial density of .gtoreq. 1 log was considered significant. All 4 S-CSF strains, 3 of 4 E-CSF strains and 1 of 3 S-NP strains were sensitive to the bactericidal activity of pooled serum. The other E-CSF strain, 2 S-NP strains and all 3 E-NP strains were resistant to the bactericidal activity of pooled serum. Two of 3 E-NP strains were opsonized by pooled serum; the other strains resistant to the bactericidal activity of pooled serum were also resistant to opsonization. Bactericidal and opsonizing activity of serum from an immunized adult was greater than or equal to that of pooled serum against each strain. Assuming normal adults are immune to invasive H. influenzae type b infection, an experimental test reflecting this immunity is the bactericidal activity against CSF isolates tested in stationary phase. Protection against invasive disease due to H. influenzae type b appears more complex than the presence of bactericidal and opsonizing activity in serum.