Identification of Salmonella bacteria by co-agglutination, using antibodies against synthetic disaccharide-protein antigens O2, O4 and O9, adsorbed to protein A-containing staphylococci.

Protein A-containing staphylococci sensitized with antisera against synthetic Salmonella O-antigens 2, 4 and 9, representative of serogroups A, B and D, respectively, were used for identification of Salmonella bacteria by co-agglutination. Out of 416 Salmonella bacteria tested the reagents correctly identified all 24 serogroup A strains, 119 serogroup B strains and 39 serogroup D strains. Unexpected agglutination was registered with two of 144 strains belonging to serogroup C 2 with reagent containing antiserum against synthetic O antigen 4. No agglutination occurred when 24 non-Salmonella bacterial strains were tested. Approximately 10(8)bacteria were required for positive co-agglutination. As compared to standard slide agglutination with conventional anti-Salmonella O factor sera, the co-agglutination metod was favourable in that the reactions were stronger, although the concentration of antiserum used was from 20 to 200 times lower. The co-agglutination method could also be used for detection of soluble antigens in the form of lipopolysaccharides from Salmonella bacteria in concentrations of 1 microgram/ml. When the sensitivity of the co-agglutination technique was compared with indirect immunofluorescence (IFL), the IFL method was shown to be at least 1000 times more sensitive.