Staphylococcus aureus

One-Sentence Definition

Staphylococcus aureus is a Gram-positive coccus in clusters; a versatile pathogen causing skin infection, bacteremia, pneumonia, toxinoses, and device infections, with MRSA as a major AMR phenotype.

Taxonomy & Morphology

• Gram-positive cocci, catalase-positive, coagulase-positive
• β-hemolytic colonies on blood agar; golden pigment common
• Gram Stain: grape-like clusters

Virulence Highlights

• Adhesins, coagulase, proteases
• Toxins: cytotoxins, PVL (some strains), TSST-1, enterotoxins
• Biofilm on devices
• Protein A (immune evasion)

Clinical Syndromes

• SSTI (abscess, cellulitis), osteomyelitis, endocarditis
• Bacteremia / sepsis, hospital pneumonia
• Food poisoning (preformed enterotoxin), scalded skin, TSS

Diagnosis

• Culture of relevant site + blood cultures when invasive disease suspected
• Rapid MRSA PCR on swabs/blood culture bottles (lab-dependent)
• Antimicrobial Susceptibility Testing essential

Treatment Notes (conceptual)

• MSSA: anti-staph β-lactams preferred when susceptible
• MRSA: vancomycin/daptomycin/linezolid etc. per syndrome and AST
• Source control (drain abscess, remove infected device)

AMR

• MRSA: mecA/mecC → PBP2a
• Also: vancomycin-intermediate phenotypes; inducible clindamycin resistance (D-test)

Related Notes

• Capsule (some strains) · Biofilm · Normal Microbiota (nasal carriage)
• Diseases: Cellulitis and Skin Abscess · Infective Endocarditis · Sepsis · CLABSI · Acute Osteomyelitis · Septic Arthritis · Hospital-Acquired Pneumonia
• MOCs: MOC - Bacteriology · MOC - Diseases by System · MOC - Antimicrobial Resistance (AMR)

Learning Aids

Diagrams

• Morphology cue: Figure - Gram Envelope Comparison (Gram+ cocci)
• Relapsing device infection: Figure - Bacterial Growth Curve + Biofilm

Clinical Example

Example

Case: Injection-drug user with fever; blood cultures grow Gram-positive cocci in clusters.
Question: Likely ID, must-not-miss complication workup?
Answer: Staphylococcus aureus bacteremia until proven otherwise. Examine for endocarditis (echo per guidelines), metastatic foci, remove foci/devices; tailor MSSA vs MRSA therapy with AST.

Videos

Video	Why watch
Khan Academy — Gram staining	Read the smear: clusters = staph clue

Learning Media Hub

Active Recall

1. Coagulase-positive vs CoNS — clinical meaning?
2. What gene defines most MRSA?
3. Why is source control critical in S. aureus bacteremia?