Evaluation of Coagulase Activity and Protein A Production for the Identification of Staphylococcus aureus

The activities of coagulase and thermostable nuclease (TNase) and the production of protein A were studied in 338 bacterial strains. These included 213 isolates of Staphylococcus aureus to determine which characteristic was most specific for the identification of S. aureus . The evaluation of different protocols for interpretation of coagulase results was also undertaken. Protein A was analyzed by a sandwich enzyme-linked immunosorbent assay using microtiter plates coated with antiprotein A antibodies. Coagulase activities were determined according to the criteria recommended by Association of Official Analytical Chemists (AOAC; any degree of clot formation is a positive reaction), American Public Health Association (APHA; coagulase activities ≥ 3+ are positive reactions), and the Bacteriological Analytical Manual (BAM; only 4+ reaction is positive). It was found that the AOAC protocol, which had a test sensitivity of 97.7% and a specificity of 95.1% and could be completed within six hours, was more practical than the methods used by APHA and BAM. Compared with coagu1ase and TNase, protein A was a better marker of S. aureus ; a high sensitivity (100%) and specificity (96.8%) were obtained by using protein A for the identification of S. aureus .