What Gram stain result characterizes Staphylococcus aureus?

• A. Gram-positive cocci in clusters; catalase-positive and coagulase-positive.
• B. Gram-negative cocci in clusters; catalase-positive.
• C. Gram-positive bacilli in chains; catalase-negative.
• D. Gram-negative rods in grape-like clusters; oxidase-positive.

Answer: (A)

Staphylococcus aureus is a Gram-positive coccus that forms grape-like clusters on Gram stain, and it is catalase-positive and coagulase-positive. This combination—Gram-positive cocci in clusters with both catalase and coagulase activity—best fits the organism. The Gram stain shows purple spherical cells arranged in clusters, not in chains or as Gram-negative forms. The catalase test distinguishes staphylococci (catalase-positive) from streptococci (catalase-negative), while the coagulase test specifically identifies S. aureus among the Staphylococcus species. Other options describe different morphologies or Gram stain results (e.g., Gram-negative or bacilli, clusters vs. chains) or different biochemical profiles, which don’t match S. aureus.