The effect of various stress treatments on the coagulase test in Staphylococcus aureus.

Coagulase production of 77 coagulase-positive Staphylococcus aureus strains stressed before plating on Baird-Parker agar were studied directly from Baird-Parker agar and from BHI broth on strains subcultured first from baird-Parker agar. Reduced pH, heat treatment and repeated freezing and lyophilization caused false-negative coagulase reactions carried out directly from Baird-Parker agar. No false-negative reactions were observed after simple freezing. The different stress treatments had no effect on coagulase production carried out from BHI broth. Coagulase tubes inoculated directly from Baird-Parker agar have to be incubated for longer time for a positive reaction than tubes inoculated from BHI broth. In the case of foods not heavily processed, it is possible to reduce the time for identification of S. aureus by performing the coagulase test directly from Baird-Parker agar.