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Microbiology · Staphylococci

Staphylococcus aureus !

  • Bacterium
  • Gram-positive
  • Contact
  • Exam priority: high

A common skin and nose bacterium that causes boils, cellulitis, bone infection, endocarditis and toxic shock; some strains are resistant (MRSA).

Updated 2026-11-01

Also called: golden staph, MRSA (methicillin-resistant), MSSA (methicillin-susceptible)

At a glance

Gram
Gram-positive
Shape
Cocci in clusters
Oxygen
Facultative anaerobe
Family
Staphylococcaceae
Stain
Gram positive; catalase and coagulase positive

Morphology

  • Gram-positive cocci in grape-like clusters
  • Golden, beta-haemolytic colonies on blood agar
  • Ferments mannitol on mannitol salt agar

Transmission

  • Direct contact with infected skin or with carriers (nostrils)
  • Contaminated objects and surfaces
  • Health-care workers' hands in hospital
  • Food carrying preformed enterotoxin (food poisoning)

Isolation precautions

What the care team adds to standard precautions.

Contact
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

Virulence factors

  • Protein A: binds the Fc portion of IgG and blocks opsonisation
  • Coagulase: builds a fibrin coat that shields the bacterium
  • TSST-1 and enterotoxins: superantigens that switch on T cells en masse
  • Heat-stable enterotoxins: vomiting a few hours after a meal
  • Exfoliative toxins: scalded skin syndrome in children
  • Panton-Valentine leukocidin: kills white cells; linked to community MRSA

Diseases it causes

Clinical picture

  • A fluctuant abscess full of pus
  • Fever, low blood pressure and a red rash that peels on palms and soles (toxic shock)
  • Sudden vomiting soon after eating (food poisoning)

Diagnosis

  • Gram stain: Gram-positive cocci in clusters
  • Catalase positive and coagulase positive
  • Culture on blood agar and mannitol salt agar
  • mecA or PBP2a detection to confirm MRSA
  • Blood cultures when bacteraemia or endocarditis is suspected

Treatment

MSSA: anti-staphylococcal penicillins (oxacillin) or first-generation cephalosporins (cefazolin). MRSA: vancomycin, linezolid or daptomycin depending on the site; clindamycin and other oral agents are also used for skin infection. MRSA resists nearly all beta-lactams through PBP2a; draining abscesses is part of treatment.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Hand hygiene
  • Covering draining wounds
  • Contact precautions for patients with MRSA
  • No sharing of towels or personal items

Test yourself

Think of the answer, then tap the question to see it.

Which gene explains MRSA's resistance to methicillin?

mecA, which encodes PBP2a, a penicillin-binding protein with low affinity for beta-lactams.

Which S. aureus toxin causes toxic shock syndrome?

TSST-1, a superantigen that massively activates T cells and releases cytokines.

Why does S. aureus food poisoning start so quickly?

Because the heat-stable enterotoxin is already formed in the food; the bacterium does not need to grow in the gut.

Which precautions does nursing apply to a patient with MRSA?

Standard precautions plus contact precautions: gown and gloves, dedicated equipment and hand hygiene.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Methicillin-resistant Staphylococcus aureus (MRSA) Basics (CDC, 2025); Staphylococcal Infections (MedlinePlus (NLM), 2025); Toxic shock syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024); Food poisoning: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.