A clinical microbiology laboratory receives a Gram-positive coccus from a wound isolate. The organism is catalase-positive. Which additional test most definitively distinguishes Staphylococcus aureus from coagulase-negative staphylococci?
- A Hemolysis pattern on blood agar
- B Coagulase test (bound and free coagulase) ✓
- C Novobiocin sensitivity
- D DNase production
Correct answer: B. Coagulase test (bound and free coagulase)
Explanation
Coagulase production is the definitive test that distinguishes S. aureus (coagulase-positive) from coagulase-negative staphylococci such as S. epidermidis and S. saprophyticus. Bound coagulase (clumping factor) and free coagulase are both produced by S. aureus. Novobiocin sensitivity distinguishes S. saprophyticus from S. epidermidis among CoNS. Hemolysis and DNase are supportive but not definitive.
Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.
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