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We have previously described a surface oligosaccharide antigen (epitope C) present in fresh isolates of Neisseria gonorrhoeae and in variants grown in subcutaneous chambers, but poorly formed by variants repeatedly subcultured in vitro. We have now investigated the presence of antibodies to epitope C in sera from normal individuals and from patients with gonorrhoea. Sera were analysed by Western blotting and ELISA, and compared with a pool of sera from normal individuals with no known history of gonorrhoea. Antigenic extracts and monoclonal antibody to the C epitope were used for competition and inhibition studies. Only the sera from patients contained antibodies to epitope C. Antibodies to several other gonococcal antigens were found in sera from patients, and also in normal sera. Collectively, the results indicate that epitope C is expressed in humans, that patients with gonorrhoea develop antibodies to it, and that such antibodies are absent in sera of normal individuals.

Serratia marcescens New CDC O14:H12 contains major outer membrane proteins of 43.5 kDal, 42 kDal (the porins) and 38 kDal (the OmpA protein) which can be separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Immunoblotting of whole cell or outer membrane preparations using antiserum raised against the whole cells revealed similar complex patterns of antigens. The OmpA protein was the major immunogen, although six other outer membrane proteins were also detected; the porins reacted only weakly with antibodies in this system. Immunoabsorption of antisera with whole cells showed that only the O antigenic chains of lipopolysaccharide and the H (flagella) antigens were accessible to antibody on the cell surface. Failure to detect the OmpA protein and other envelope antigens in this way suggests that their antigenic sites are not able to react with antibodies and are possibly masked by the O antigen.

Two O14:H12 strains of Serratia marcescens with different sensitivities to killing by normal pooled human serum were investigated. Complement binding, studied by measuring hydrophobicity and using rocket immunoelectrophoresis with anti-human C3, showed the sensitive cells (S1220) rapidly bound and fixed complement whereas the resistant cells (4444-60) bound less C3b. The strains had identical membrane protein composition. Crossed immunoelectrophoresis suggested that in S1220 cells the polysaccharide material including LPS was less antigenic and present in smaller amounts than in 4444-60 cells. This was confirmed by examining extracted polysaccharide material chemically and by SDS-PAGE. The resistant strain had 33% more phenol-extractable polysaccharide material than the sensitive strain, possibly comprising LPS with longer O antigen chain lengths, or a microcapsule of O antigen polysaccharide. Extra polysaccharide material on the surface of the resistant strain prevents complement components binding and reaching the hydrophobic membrane where lytic lesions occur.