Diagnostic microbiology – coagulase production Which staphylococcal species is characteristically coagulase-positive and therefore a key pathogen in human disease?

Introduction / Context:
The coagulase test differentiates pathogenic Staphylococcus aureus from coagulase-negative staphylococci (CoNS). Coagulase converts fibrinogen to fibrin, aiding immune evasion and abscess formation, and remains a cornerstone of bench-top identification.

Given Data / Assumptions:

• Tube or slide coagulase assays detect enzyme activity.
• S. aureus is typically coagulase-positive.
• Common CoNS include S. epidermidis, S. saprophyticus, and others.

Concept / Approach:
Coagulase positivity correlates with virulence traits of S. aureus, including Protein A expression, cytotoxins, and surface adhesins. In contrast, CoNS are frequent skin commensals and opportunists (notably in device-associated infections) but lack coagulase.

Step-by-Step Solution:
Recall the canonical rule: coagulase-positive = S. aureus.Eliminate CoNS and micrococci.Select “Staphylococcus aureus.”

Verification / Alternative check:
Clinical labs routinely confirm S. aureus by coagulase and agglutination tests (e.g., detecting clumping factor, Protein A).

Why Other Options Are Wrong:

• S. epidermidis, S. saprophyticus, S. hominis: coagulase-negative species.
• Micrococcus spp.: distinct genus, coagulase-negative.

Common Pitfalls:
Confusing slide coagulase (clumping factor) with tube coagulase (free coagulase); both are associated with S. aureus identification.

Final Answer:
Staphylococcus aureus.