What is the Gram stain appearance and typical arrangement of Staphylococcus aureus?

• A. Gram-negative rods in chains
• B. Gram-positive cocci in grape-like clusters
• C. Gram-positive rods in singles
• D. Gram-variable cocci in pairs

Answer: (B)

Staphylococcus aureus is a Gram-positive cocci that tends to group in grape-like clusters when viewed under a light microscope after Gram staining. The Gram-positive part comes from a thick peptidoglycan layer in its cell wall, which retains the crystal violet stain and makes the cells appear purple. The grape-like clustering happens because these cocci divide in multiple planes and remain attached to one another, forming irregular, clustered arrangements rather than neat chains. This combination—Gram-positive staining and cocci arranged in clusters—is the classic appearance used to identify Staphylococcus aureus on Gram stain.