What is the Gram stain result for Staphylococcus aureus?

• A. Gram-negative cocci
• B. Gram-positive cocci
• C. Gram-positive rods
• D. Gram-negative rods

Answer: (B)

Staphylococcus aureus is a Gram-positive cocci. In the Gram stain, thick peptidoglycan in the cell wall traps the crystal violet-iodine complex, so these bacteria appear purple under the microscope. They are characteristically seen in grape-like clusters, which helps distinguish them from other shapes and staining patterns.

This is not Gram-negative cocci, which would appear pink after counterstaining due to a thinner peptidoglycan layer and an outer membrane. It’s not Gram-positive rods, which are different in shape, nor Gram-negative rods.