Staphylococcus Aureus

Characteristics

• Catalase positive (all staphylococci)
• Coagulase positive
• Cause disease by actual infection AND/OR toxin
• Colonisation in 20-40% of healthy population - skin, nasal cavity, mucous membrane
• Spread by direct contact, contamination of objects, food

 

Pathogenesis

Cell wall virulence factors:

• Protein A - binds to the Fc moiety of IgG (thus antiopsonin and antiphagocytic)
• Fibronectin-binding protein (FnBP) - promotes attachment to cells

 

Cytolytic exotoxin (hemolysins)

(Type alpha, beta, gamma, delta)

Destorys mammalian cells and red blood cells.

 

Superantigen exotoxin

Stimulates up to 20% of the available T cells (thus causing toxin shock via IL-2, IFN-gamma, TNF-alpha)

• Enterotoxin (type A, B, C, D, E, G)
  - Stimulates the vomiting centre.
  - More heat stable than S. aureus.
• Toxic shock syndrome toxin (TSST-1) (Type F)
  - Doesn't cause food poisoning.
  - Causes classic toxic shock syndrom
• Exfoliatin (exfoliative toxin)
  - Causes scalded skin syndrome in children.
  - Attacks intercellular adhesive of the stratum granulosum
     -> causes epithelial desquamation/bullae

 

Clinical significance

1. Localised skin infection

• Sty (external hordeolum)
• Furuncle
• Carbuncles
• Impetigo (s. pyogenes more common cause)

2. Deep localised infections

• Abscess
• Osteomyelitis
• Arthritis

3. Acute endocarditis

Associated with IV drug use

4. Septicaemia

5. Pneumonia

Severe and necrotising

6. Nosocomial infection

7. Toxinoses

Diseases caused by toxins

7.1. Toxic shock syndrome

• High fever, vomiting, diarrhoea, hypotension
• Rash (resembling sunburn - diffuse erythema then desquamation)
• Multiorgan involvement (esp GI, renal, or hepatic)

Most cases are caused by the use of certain types of tampons in women with vaginal colonisation -> growth of S. aureus -> release of TSST.

7.2 Staphylococcal gastroenteritis

• Protein-rich food, improperly refrigerated
• Short incubation period (<6 hours)
• Nausea, vomiting, diarrhoea

7.3 Scalded skin syndrome

Superficial bullae.

May be infected locally or caused by toxin produced elsewhere.

 

Laboratory identification

Colony morphology - yellow and hemolytic

Coagulase positive (distinguishes from other staph)

Mannitol positive (distinguishes from most other staph)

 

Treatment

Almost all s. aureus are resistent to penicillin G.

Methicillin-resistant s. aureus (MRSA) - modified penicillin-binding protein -> low affinity to penicillin

First line: oxacillin, nafcillin

Second line: vancomycin

Prevention/Immunity

Barrier precaution, handwashing etc.

No vaccine,

No lasting immunity

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Things to revise/add later:

Bibliography: LWW microbiology

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