Staphylococcus aureus

Staphylococcus aureus

Staphylococcus aureus

Staphylococcus aureus is catalase positive, coagulase positive, facultative anaerobe, non-motile, non-sporing and occasionally capsulated.


Distinguishing Features

  • Small, yellow colonies on blood agar
  • β-hemolytic
  • Coagulase positive (all other Staphylococcus species are negative)
  • Ferments mannitol on mannitol salt agar

Reservoir

  • Normal flora
    • Nasal mucosa (25% of population are carriers)
    • Skin

Transmission

  • Hands
  • Sneezing
  • Surgical wounds
  • Contaminated food
    • Custard pastries
    • Potato salad
    • Canned meats

Staphylococcus aureus


Source: Microbiology In Pictures

Predisposing Factors for Infection

  • Surgery/wounds
  • Foreign body (tampons, surgical packing, sutures)
  • Severe neutropenia (<500/μL)
  • Intravenous drug abuse
  • Chronic granulomatous disease
  • Cystic fibrosis

Pathogenesis

  • Protein A binds Fc component of IgG, inhibits phagocytosis
  • Enterotoxins: fast acting, heat stable
  • Toxic shock syndrome toxin-1 (TSST-1): superantigen
  • Coagulase: converts fibrinogen to fibrin clot
  • Cytolytic toxin (α toxin): pore-forming toxin, Panton-Valentine leukocidin (PVL), forms pores in infected cells and is acquired by bacteriophage; associated with increased virulence, MRSA strains
  • Exfoliatins: skin-exfoliating toxins (involved in scalded skin syndrome [SSS]) and bullous impetigo.

Clinical Manifestation

1. Diseases: Gastroenteritis (food poisoning)—toxin ingested performed in food

Clinical Symptoms: 2–6 hours after ingesting toxin: nausea, abdominal pain, vomiting, followed by diarrhea.

Pathogenicity Factors: Enterotoxins A–E preformed in food.

2. Diseases: Infective endocarditis (acute) (most common cause)

Clinical Symptoms: Fever, malaise, leukocytosis, heart murmur (may be absent initially).

Pathogenicity Factors: Fibrin-platelet mesh, cytolytic toxins.

3. Diseases: Abscesses and mastitis

Clinical Symptoms: Subcutaneous tenderness, redness and swelling; hot.

Pathogenicity Factors: Coagulase, cytolysins

4. Diseases: Toxic shock syndrome

Clinical Symptoms: Fever, hypotension, scarlatiniform rash that desquamates (particularly on palms and soles), multiorgan failure.

Pathogenicity Factors: TSST-1.


5. Diseases: Impetigo

Clinical Symptoms: Erythematous papules to bullae.

Pathogenicity Factors: Coagulase, exfoliatins.

6. Diseases: Scalded skin syndrome.

Clinical Symptoms: Diffuse epidermal peeling.

Pathogenicity Factors: Coagulase, exfoliatins.

7. Diseases: Pneumonia.

Clinical Symptoms: Productive pneumonia with rapid onset, high rate of necrosis, and high fatality; nosocomial, ventilator, postinfluenza, IV drug abuse, cystic fibrosis, chronic granulomatous disease, etc. Salmon-colored sputum.

Pathogenicity Factors: Coagulase, cytolysins.

8. Diseases: Surgical infections

Clinical Symptoms: Fever with cellulitis and/or abscesses

Pathogenicity Factors: Coagulase, exfoliatins, ± TSSTs

9. Diseases: Osteomyelitis (most common cause)

Clinical Symptoms: Bone pain, fever, ± tissue swelling, redness; lytic bone lesions on imaging.

Pathogenicity Factors: Cytolysins, coagulase.

10. Diseases: Septic arthritis.

Clinical Symptoms: Monoarticular joint pain; inflammation.

Pathogenicity Factors: Cytolysins, coagulase

Laboratory Diagnosis

  • Direct smear microscopy: Pus cells with gram positive cocci in cluster
  • Culture:
    • Nutrient agar: Golden yellow pigmented colonies (pigments made up of beta carotene)
    • Blood agar: Colonies with narrow zone of β-hemolysis
    • Selective media:
      • Mannitol salt agar (yellow colonies due to mannitol fermentation)
      • Salt milk agar
      • Ludlam’s medium
  • Culture smear microscopy: Gram-positive cocci in clusters
  • Biochemical identification
    • Catalase test-positive
    • Tests differentiating staphylococci from micrococci- Of test (shows fermentative pattern)
    • Tests differentiating S. aureus (positive) from CoNs (negative)
      • Coagulase test (slide and tube)-positive
      • Heat stable thermo nuclease test-positive
      • DNase test-positive
      • Phosphatase test-positive
      • Mannitol sugar is fermented
      • Black colored colonies on potassium tellurite agar
      • Gelatin liquefaction-positive
      • Protein A detection

Typing of S. aureus

  • MC method for typing of S. aureus – Phage typing (pattern method)
  • National reference centre for phage typing—in Maulana Azad Medical College, Delhi.
  • Epidemic strain of S. aureus is Phage type 80/81. It causes outbreaks in hospitals

Treatment

  • Gastroenteritis is self-limiting.
  • Nafcillin/oxacillin are drugs of choice because of widespread penicillinase producing stains.
  • Mupirocin for topical treatment.
  • For methicillin-resistant Staphylococcus aureus (MRSA): vancomycin
  • For vancomycin-resistant Staphylococcus aureus (VRSA) or vancomycin intermediate S. aureus (VISA): quinupristin/dalfopristin.

Staphylococcus aureus


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