Background:
The global rise of antimicrobial resistance in Neisseria gonorrhoeae highlights the urgent need for alternative prevention strategies. Epidemiological evidence suggests that Bexsero, developed against Neisseria meningitidis, may provide partial protection against gonorrhea due to antigenic similarities. However, the mechanisms underlying this cross-protection remain poorly understood. The serum bactericidal assay (SBA) enables functional assessment of complement-mediated killing and cross-reactive immunity. The objective is to standardize an SBA for N. gonorrhoeae using clinical isolates from high-risk individuals in Nairobi and establish a platform to study cross-reactive immune responses induced by meningococcal vaccination.
Methods:
Samples from 30 syndromic individuals yielded three N. gonorrhoeae isolates; one was selected for assay development. Bacteria were cultured on chocolate agar and grown to log phase (OD₆₀₀ ≈ 0.075) in GC broth. Non-heat-inactivated serum from 12 participants was used to preserve complement activity. Equal volumes (20 µL) of bacteria and serially diluted serum were incubated at 37°C for 60 minutes. SBA buffer was added, and samples were plated on GCB agar. Colony-forming units (CFUs) were enumerated after overnight incubation (37°C, 5% CO₂), and percent killing was calculated relative to controls.
Results:
The optimized SBA showed clear serum-dependent bactericidal activity. Undiluted serum achieved >90% killing, with strong activity at 1:2 dilution. Killing decreased with dilution (~70% at 1:4, ~35% at 1:8, and <30% at 1:16), with ≥50% killing maintained up to ~1:4. Trends were consistent despite inter-individual variability.
Conclusion:
We established a reproducible SBA for N. gonorrhoeae as a platform to evaluate cross-reactive immunity. This assay provides a critical tool for investigating functional antibody responses and will be applied to evaluate cross-reactive immunity induced by the Bexsero against N. gonorrhoeae. Future work will assess how prior infection influences vaccine responses, define cross-reactive epitopes within NhbA, and evaluate outer membrane vesicle immunogenicity in a tonsil organoid model to inform next-generation gonorrhea vaccine design.