Which organism is classically described as catalase-positive, coagulase-positive, and capable of fermenting mannitol?

• A. Streptococcus pneumoniae
• B. Escherichia coli
• C. Staphylococcus aureus
• D. Enterococcus faecalis

Answer: (C)

The organism identified by this combination is Staphylococcus aureus. The first clue is catalase positivity, which distinguishes staphylococci (catalase-positive) from streptococci and enterococci (catalase-negative). The second clue is coagulase positivity, which sets apart S. aureus from other staphylococci that are coagulase-negative. The third clue is the ability to ferment mannitol, which on mannitol salt agar yields acid and turns the medium yellow, a trait characteristic of S. aureus. Taken together, catalase-positive, coagulase-positive, and mannitol fermentation points specifically to Staphylococcus aureus. The other organisms listed do not exhibit this exact combination: for example, streptococci are catalase-negative, enterococci are typically catalase-negative, and while some E. coli can ferment various sugars, the key differentiator here is coagulase positivity along with mannitol fermentation, which S. aureus uniquely shows among the options.