Quantitation of serum antibodies to surface antigens of Neisseria gonorrhoeae with radiolabeled protein A of Staphylococcus aureus.

Antibodies in human sera against surface antigens of Neisseria gonorrhoeae were detected with use of 125I-labeled protein A from Staphylococcus aureus. Serum was allowed to react with a suspension of whole gonococci, and the antibodies attached to the bacteria were quantitated with protein A, which reacts with the Fc fragment of IgG. Tests with five human sera with a gonococcoal complement-fixation titer of greater than or equal to 1:30 revealed no difference between use of freshly isolated gonococci and use of strains subcultured daily on artificial medium for 10 years. Antibodies cross-reacting with N. gonorrhoeae and Neisseria meningitidis were found in human sera. The results of the test with labeled protein A varied with the serum titer of complement-fixing antibody to N. gonorrhoeae. Acute and convalescent sera from six of seven patients, including one with disseminated gonococcal infection, showed significantly larger differences in antibodies to N. gonorrhoeae than did sera of women without evidence of genital infection. Complement-fixation titers changed significantly in only three of the six patients.