What is the Gram stain reaction of Staphylococcus aureus?

The Gram stain reaction of Staphylococcus aureus is Gram-positive. Its thick, peptidoglycan-rich cell wall retains the crystal violet–iodine complex during Gram staining, so the cells appear purple under a microscope.

S. aureus is a spherical, or coccal, bacterium that commonly forms grape-like clusters. It is a facultative anaerobe, allowing it to grow with or without oxygen, and it is usually found on human skin and in the upper respiratory tract. Many people carry it without symptoms, but it can cause skin infections, pneumonia, bloodstream infections, and other diseases when it enters vulnerable tissue.

Laboratories use the Gram stain as an early identification step, then combine it with tests such as catalase and coagulase reactions. Staphylococci are catalase-positive, while S. aureus is classically coagulase-positive. It also grows on high-salt media and can ferment mannitol, producing yellow colonies on mannitol salt agar.

Gram-positive does not mean harmless, and it does not indicate antibiotic susceptibility. Methicillin-resistant S. aureus, or MRSA, remains Gram-positive even though it is resistant to several important antibiotics. The bacterium is also nonmotile and does not form endospores.