Serum Bactericidal Assay (SBA) with colony forming units (CFU) counting (C-SBA)

The Serum Bactericidal Assay (SBA) is a functional immunological assay used to measure the complement‑mediated bactericidal activity of antibodies present in human serum. The assay evaluates the ability of antibodies to bind antigens on the bacterial surface, activate the classical complement pathway, trigger the MAC formation, and consequently kill the target organism. In general, the assay involves live bacteria, usually grown until the mid-exponential phase, incubated with serially diluted sera, and an exogenous source of complement.

In the classical or conventional version of the SBA (C-SBA), after the incubation time, the reaction mix is plated in solid agar plates and incubated overnight.

The CFUs are counted after overnight incubation. The readout is expressed as the reciprocal titer of the serum which kills ≥50% of the bacteria when compared to the negative control wells (bacteria with complement at the selected concentration).

The SBA is widely applied in vaccine development and immunogenicity assessment, and in some cases is referenced in regulatory guidance as a correlate or surrogate of protection for some bacterial pathogens. It is the established correlate of protection for meningococcal vaccines.

When appropriately validated and qualified, serum bactericidal assay (SBA) data can serve as valuable evidence to support the clinical development and regulatory approval of bacterial vaccines. In VisMederi, this assay is mainly used to evaluate immunogenicity of vaccines against Klebsiella spp.