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

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Abstract
Presumptive identification of Salmonella or Shigella organisms directly upon the original isolation plate was accomplished by applying a drop of specifically sensitized protein A-containing Staphylococcus aureus over a suspected colony or several colonies of organisms grown, in this case, on MacConkey agar. The plate is tilted to allow mixing of the particles with specific antigen that is 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 2-min. The polyvalent Salmonella antiserum was slower in developing strong agglutination (1.5-2 min) compared to particles sensitized with group-specific antisera (15-45 s). A high-titer antiserum was important for a test reagent to have the required sensitivity.