Makindo Medical Notes

Growth of Staph. aureus in blood cultures should never be dismissed as a ‘contaminant’ unless all possible underlying causes have been excluded, and repeat cultures are negative. Consider the possibility of endocarditis.

About

• A common skin commensal with significant pathogenic potential, often responsible for serious infections.
• Produces various toxins, including superantigens, which can trigger severe immune responses.

Characteristics

• Aerobic or facultative anaerobe and salt-tolerant.
• Gram-positive cocci appearing in grape-like clusters.
• Coagulase-positive (forms a fibrin clot).
• Catalase positive and beta-haemolytic on blood agar.
• Protein A on the surface binds to the Fc region of IgG, reducing opsonization and phagocytosis.

Virulence Factors

• Coagulase: Clots plasma to help evade immune defenses.
• Staphylokinase: Breaks down blood clots, aiding in spread.
• Hyaluronidase: Degrades connective tissue, facilitating tissue invasion.
• Haemolysin: Lyses red blood cells.
• Panton-Valentine leucocidin (PVL): Destroys white blood cells, linked to aggressive skin infections.
• Toxic Shock Syndrome Toxin (TSST-1): Superantigen that activates T cells and can cause toxic shock syndrome.
• Exfoliative toxins: Cause skin peeling, leading to scalded skin syndrome.
• Capsule: Polysaccharide capsule blocks phagocytosis.

Source

• Commonly found in the nasal mucosa, skin, and other moist body areas.
• Higher prevalence in healthcare workers, IV drug users, diabetics, and immunocompromised individuals.

Pathogenicity

• Food Poisoning: Heat-stable enterotoxins cause rapid-onset nausea, abdominal pain, and vomiting after ingesting contaminated foods.
• Skin Infections: Causes bullous impetigo, eczema, folliculitis, boils, mastitis, and cellulitis. Toxins can cause scalded skin syndrome and toxic epidermal necrolysis (Ritter’s disease).
• Toxic Shock Syndrome (TSS): Associated with tampon use and surgical packing. TSST-1 triggers a rapid, life-threatening cytokine storm.
• Endocarditis: Adheres to the heart endocardium, leading to infective endocarditis, especially in IV drug users or patients with indwelling IV catheters.
• Osteomyelitis and Pneumonia: Causes bone infections and pneumonia, particularly in immunocompromised patients or post-influenza infections.

Investigations

• Coagulase Test: Positive, distinguishing it from coagulase-negative staphylococci.
• DNAse production: Positive in S. aureus.
• Mannitol Salt Agar: Grows well, tolerates 5-10% NaCl, and forms golden colonies.
• Phage typing and PCR: For epidemiological studies and strain identification.

Management

• MSSA (Methicillin-sensitive S. aureus):
  • Flucloxacillin, cephalosporins, fusidic acid, and vancomycin are effective.
  • Other options include linezolid, erythromycin, and other macrolides.
• MRSA (Methicillin-resistant S. aureus):
  • Resistance is due to a modified penicillin-binding protein (PBP2a) encoded by the mecA gene.
  • Vancomycin or teicoplanin is typically used; consider linezolid or daptomycin for severe cases.

Sensitivity

• MSSA is generally sensitive to beta-lactams (flucloxacillin).
• MRSA requires treatment with glycopeptides like vancomycin or newer agents, depending on local susceptibility profiles.