SERUM BACTERICIDAL EFFECT ON CAPSULATED AND NON‐CAPSULATED HAEMOPHILUS INFLUENZAE

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Abstract
In order to test serum bactericidal activity on Haemophilus influenzae, a micromethod was employed. The bactericidal (BC) effect of preimmune and hyperimmune rabbit sera was studied on a type a and a type b strain and their respective non‐capsulated variants. The influence of variation of the experimental conditions on the BC effect was investigated. It was observed that the amount of guinea pig complement added as well as the number of bacteria and reaction time had an influence on the BC effect. The serum dilution showing ≥ 50% killing of the inoculum was chosen as endpoint titre (BC titre). Unheated preimmune rabbit sera, in which no specific antibodies were demonstrable with the complement fixation assay, were bactericidal for the two non‐capsulated strains. Heat treatment to 56°C for 5 min destroyed this effect, which could not be restored by addition of diluted guinea pig serum. Unheated preimmune rabbit sera were not bactericidal for capsulated bacteria. Hyperimmune sera showed heat‐resistant (56°C, 30 min) BC activity on both non‐capsulated and capsulated bacteria, the latter showing quite high BC titres (about 1×105). Serum samples form 30 blood donors had demomstrable, heat‐resistant BC titre against the capsulated bacteria tested, whereas only about one half of the serum samples showed heat‐resistant BC activity against the non‐capsulated bacteria used. Ten non‐capsulated H. influenzae strains isolated from patients with various infections revealed a variable sensitivity to preimmune rabbit sera (normal sera), these strains being more resistant than the two noncapsulated variant strains utilized in the study.

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