Determination of bactericidal activity of serum against Vibrio cholerae outer membrane vesicles in BALB/c mice

Serum bactericidal assay is the gold standard index of protection against Vibrio cholerae. The outer membrane vesicles (OMVs) which are released during the bacterial growth have intrinsic immune stimulatory properties, based on their nature and composition. In this study, the induction of serum bactericidal activities in immunized BALB/c mice was determined using different vaccine regimens, using V. cholerae O1 (El Tor biotype) OMVs.

A single clone of V. cholerae O1 (El Tor biotype) isolated during the 2005 outbreak in Iran, was used. A detergent-centrifugation procedure was used for OMVs production. Various vaccination regimens were inoculated into female mice via an oral route. The vaccine formulas included V. cholerae OMVs, killed whole cells of V. cholerae (WC), combination of WC-OMV and licensed cholera vaccine (Dukoral).The serum vibriocidal activity of mice sera was determined by measuring the complement-mediated lysis.

Electron microscopy of the purified OMVs from the isolated V. cholerae revealed the spherical-shaped vesicles of the size range 20-300 nm. In vitro reactivity of mice sera bactericidal capability against regimens of vaccination showed a significant immune response of antibody titers in comparison with negative control groups. Also, there was a significant increase in serum bactericidal titer of WC-OMV obtained from wild-type V. cholerae which had a satisfactory reactivity as Dukoral cholera vaccine.

The results indicated that the combination of WC-OMV from the local strain is able to induce a high level of bactericidal antibody responses and it can be useful in optimization of the vaccine formula.