Which organism matches the following features: gram-positive cocci, catalase-positive, coagulase-positive, beta-hemolytic, ferments mannitol, protein A virulence factor, and nasal colonization?

• A. Staphylococcus aureus
• B. Staphylococcus epidermidis
• C. Streptococcus pyogenes
• D. Streptococcus pneumoniae

Answer: (A)

This combination of features points to Staphylococcus aureus. It is a gram-positive coccus that grows in grape-like clusters and is catalase-positive, which sets it apart from the catalase-negative streptococci. The fact that it is coagulase-positive distinguishes it from other coagulase-negative staphylococci such as S. epidermidis. Beta-hemolysis on blood agar is typical for S. aureus, and its ability to ferment mannitol is what makes it turn mannitol salt agar yellow, a classic differential test. Protein A is a distinctive virulence factor of S. aureus that binds the Fc region of IgG, helping the bacteria evade phagocytosis. Nasal colonization is common with S. aureus, providing a reservoir for infection. The other organisms either lack coagulase, do not ferment mannitol, or do not produce Protein A, making them a poorer fit for the described profile.