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
Things to revise/add later:
Bibliography: LWW microbiology