Classical Diagnosis
Depending upon the type of infection present, an appropriate specimen is obtained accordingly and sent to the laboratory for definitive identification by using biochemical or enzyme-based tests. A Gram stain is first performed to guide the way, which should show typical Gram-positive bacteria, cocci, in clusters. Second, the isolate is cultured on mannitol salt agar, which is a selective medium with 7–9% NaCl that allows S. aureus to grow, producing yellow-colored colonies as a result of mannitol fermentation and subsequent drop in the medium's pH.
Furthermore, for differentiation on the species level, catalase (positive for all Staphylococcus species), coagulase (fibrin clot formation, positive for S. aureus), DNAse (zone of clearance on DNase agar), lipase (a yellow color and rancid odor smell), and phosphatase (a pink color) tests are all done. For staphylococcal food poisoning, phage typing can be performed to determine whether the staphylococci recovered from the food were the source of infection.
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