Staphylococci are part of the normal flora and important human pathogens. There are more than 26 species but only a few are pathogenic. Staphylococcus aureus is the most invasive species, which can be differentiated from other species by its possession of the enzyme coagulase.
Staphylococcus aureus
Asymptomatic carriage of S. aureus is found in up to 40% of healthy people, in the nose, skin, axilla or perineum. This is important in healthcare workers especially if they carry an invasive or resistant strain (e.g. MRSA).
Pathogenesis
Staphylococcus aureus has many potential pathogenicity determinants such as coagulase, which catalyses the conversion of fibrinogen to fibrin thereby providing protection.
| Determinant | Activity | Effect |
| Coagulase | Converts fibrinogen to fibrin | May form protective barrier |
| Adhesion molecules | Bind fibronectin and collagen | Assist adherence |
| Lytic enzymes | Lipase | Breaks down host tissue |
| Protein toxins | Panton–Valentine leucocidin (PVL) toxin | Tissue damage |
| Toxic shock syndrome toxin (TSST) | Shock and toxicity | |
| Enterotoxins | Diarrhoea | |
| Biofilm formation | Slower growth in an extracellular matrix | Difficult to treat with antibiotics, adhere to plastics |
Clinical Importance
Staphylococcus aureus causes a wide range of infectious syndromes.
- Primary skin infection – impetigo, which is transmitted from person to person.
- Secondary skin infections – associated with eczema, surgical wounds, intravenous devices, burns.
- Pneumonia – rare, but may follow influenza and progress rapidly with cavity formation (see Chapter 34).
- Endocarditis – can be rapid and destructive; associated with intravenous drug misuse or colonization of intravenous devices (see Chapter 48).
- Osteomyelitis (see Chapter 52).
- Septic arthritis (see Chapter 52).
Laboratory Diagnosis
Staphylococcus aureus grows readily on most laboratory media. Selective medium contains high salt, to which S. aureus is relatively tolerant. Phenotypic identification depends on demonstrating coagulase, catalase enzymes and typical ‘cluster of grapes’ morphology on Gram stain. Typing by molecular means can support interventions to control outbreaks.
Antibiotic Susceptibility
The history of the susceptibility of S. aureus is a lesson in the history of antimicrobial chemotherapy.
1 It was initially susceptible to penicillin, but strains that produced β-lactamase soon predominated, so methicillin and related agents (e.g. flucloxacillin) were introduced and replaced penicillin.
2 Methicillin-resistant S. aureus (MRSA) emerged. Resistance is caused by possession of the mecA gene, which codes for a penicillin-binding protein that binds the drug less well. Glycopeptides, such as vancomycin or teicoplanin, started to be required for these strains.
3 Intermediate or heteroresistance to glycopeptides emerged as an increasing issue and fully glycopeptide-resistant strains (GRSA) have now been described, resistance being mediated by the vanA vanB genes acquired from enterococci.
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