Susceptibility of Vibrio cholerae O139 to antibody-dependent, complement-mediated bacteriolysis

Volunteer studies with Vibrio cholerae O1 have shown that the best correlat e of a vaccine's protective efficacy is its propensity to elicit serum bact ericidal responses in its recipients. Attempts to detect such responses fol lowing infection with ii cholerae O139, however, have met with varying succ ess. Using a tube-based assay which involves viable counting, we now report that strains of serogroup O139 can appear to be sensitive or resistant to a fixed concentration of complement in the presence of antibody, depending on assay conditions. Susceptibility to lysis is critically dependent on the availability of complement, but with O139 indicator strains this is not si mply determined by the concentration of serum added to the reaction mix. Th e nature of the assay diluent and the concentration of indicator bacteria c an also dramatically affect bactericidal end points, whereas such variables have minimal significance with O1 indicator bacteria. Although some labora tories use unencapsulated mutant strains to seek evidence of seroconversion following exposure to V. cholerae O139, this is not necessary, and our fin dings question the significance of capsule expression as a determinant of c omplement sensitivity when antibody is present. The medium used for growth of the indicator strain and the particular strain used appeared to be unimp ortant. Each of seven O139 isolates tested was found to be lysed by antibod y and complement in our standard assay system, which allowed the detection of significant serum bactericidal responses in 9 of 11 cases of O139 diseas e.