Clinical microbiology screening: Which Staphylococcus species is known to ferment mannitol under anaerobic conditions (a property exploited on mannitol salt agar to differentiate pathogenic staphylococci)?

Introduction / Context:
Differentiating staphylococci in the laboratory often relies on carbohydrate fermentation patterns and enzyme tests. Mannitol fermentation, especially under high salt conditions, is a classic screening step. This question targets recognition of the species most consistently able to ferment mannitol anaerobically.

Given Data / Assumptions:

• Comparison is among common staphylococcal species encountered in clinical labs.
• Fermentation refers to anaerobic metabolism of mannitol with acid production.
• Mannitol salt agar (with 7.5% NaCl) is a standard differential medium.

Concept / Approach:
Staphylococcus aureus typically ferments mannitol, producing acid that turns indicator media yellow. This trait is useful for preliminary separation of S. aureus from coagulase-negative staphylococci such as S. epidermidis, which usually does not ferment mannitol. S. saprophyticus can be variable, but is not the canonical mannitol-fermenting staphylococcus used for routine screening.

Step-by-Step Solution:
Recall the differential principle of mannitol salt agar: salt selects for staphylococci; mannitol fermentation differentiates species. Identify the species most consistently positive for mannitol fermentation: S. aureus. Recognize that the test can be observed aerobically and anaerobically; fermentation indicates anaerobic acid production. Select “Staphylococcus aureus.”

Verification / Alternative check:
Clinical algorithms combine mannitol fermentation with catalase and coagulase testing. Coagulase-positive, mannitol-fermenting isolates are highly suggestive of S. aureus.

Why Other Options Are Wrong:

• S epidermidis: Classically mannitol-negative; colonies keep the medium pink/red.
• S saprophyticus: Variable; not the prototypical mannitol fermenter used for differentiation.
• None of these: Incorrect because S. aureus does ferment mannitol.

Common Pitfalls:
Overreliance on a single plate reading; always confirm with coagulase and additional biochemical tests. Some atypical strains exist.

Final Answer:
Staphylococcus aureus