Method for identifying Salmonella and Shigella directly from the primary isolation plate by coagglutination of protein A-containing staphylococci sensitized with specific antibody.

A technique is described that allows presumptive identification of either Salmonella or Shigella organisms directly upon the original isolation plate, in this case, MacConkey agar. This was accomplished by applying a drop of specifically sensitized protein A-containing Staphylococcus aureus over a "suspected" colony or several colonies of organisms grown on MacConkey agar. The plate is tilted to and fro to allow mixing of the particles with specific antigen that ir readily solubilized from the colony and observing for agglutination of the sensitized particles by use of a dissecting microscope. The agglutination can frequently be seen within 15 s, increasing in intensity over a 2-min period. The polyvalent Salmonella antiserum was slower in developing strong agglutination (1.5 to 2 min) compared to particles sensitized with group-specific antisera (15 to 45 s). A high-titer antiserum was important for a test reagent to have the required sensitivity.