Human serum bactericidal activity against Haemophilus influenzae type b

Area of Science:

• Immunology
• Microbiology
• Infectious Diseases

Background:

• Haemophilus influenzae type b (Hib) is a significant cause of invasive bacterial disease, particularly in children.
• Immunity to Hib is thought to involve serum bactericidal and opsonizing activities.

Purpose of the Study:

• To evaluate the bactericidal and opsonizing capacity of pooled adult serum against various Hib strains.
• To assess the correlation between serum activity and bacterial source (cerebrospinal fluid vs. nasopharynx) and growth phase (exponential vs. stationary).

Main Methods:

• Bacterial strains from cerebrospinal fluid (CSF) and nasopharynx (NP) were tested against pooled adult serum.
• Assays included bactericidal activity (serum alone) and opsonization (serum with phagocytes).
• Bacterial density decrease of ≥1 log10 unit was considered significant.

Main Results:

• Pooled adult serum demonstrated significant bactericidal activity against most stationary-phase CSF strains and some exponential-phase CSF and stationary-phase NP strains.
• Exponential-phase NP strains were largely resistant to both bactericidal and opsonizing activity.
• Serum from an immunized adult showed activity comparable to or greater than pooled serum.

Conclusions:

• Bactericidal and opsonizing activity of adult serum against Hib varies depending on the strain's origin and growth phase.
• Immunity against invasive Hib disease appears to be more complex than solely relying on serum bactericidal and opsonizing functions.
• Stationary-phase CSF isolates may serve as a relevant experimental model for assessing immunity to invasive Hib infections.