Which laboratory test is commonly used to differentiate Staphylococcus aureus from other Staphylococcus species?

• A. Coagulase test positive
• B. Catalase test positive
• C. Oxidase test positive
• D. Gram stain Gram-negative rods

Answer: (A)

Coagulase production distinguishes Staphylococcus aureus from other Staphylococcus species. S. aureus makes coagulase, an enzyme that converts fibrinogen to fibrin and causes plasma to clot, so a positive coagulase test points to S. aureus. Other Staphylococcus species are coagulase-negative, so they won’t clot plasma. The catalase test being positive is true for all Staphylococcus species and hence doesn’t separate S. aureus from its relatives. The oxidase test helps identify certain Gram-negative organisms and is not useful for differentiating Staphylococcus species. A Gram stain showing Gram-positive cocci in clusters confirms staphylococci but doesn’t specify the species.