Staphylococcus aureus: Clinical Features, Pathogenesis, and Treatment

Staphylococcus aureus

Genus Features and Identification

Staphylococcus aureus is a Gram-positive coccal bacterium, commonly found in clusters. It is catalase-positive and forms small, yellow colonies on blood agar. It is notable for its ability to ferment mannitol on mannitol salt agar and for its beta-hemolytic activity, which is visible as clear zones around colonies on blood agar.

• Gram-positive: Thick peptidoglycan cell wall, stains violet in Gram stain.

• Catalase-positive: Produces catalase enzyme, distinguishing it from Streptococcus species.

• Beta-hemolysis: Complete lysis of red blood cells on blood agar.

• Mannitol fermentation: Produces acid, turning mannitol salt agar yellow.

Reservoirs and Transmission

S. aureus is part of the normal flora of the nasal mucosa and skin. About 25% of the population are carriers. Transmission occurs via direct contact (hands), respiratory droplets (sneezing), surgical wounds, and contaminated foods.

• Reservoir: Nasal mucosa, skin.

• Transmission: Hands, sneezing, surgical wounds, contaminated foods (e.g., custard pastries, potato salad, canned meats).

Predisposing Factors for Infection

Individuals with surgical wounds, severe neutropenia, IV drug abuse, chronic granulomatous disease, or cystic fibrosis are at increased risk for S. aureus infection.

• Surgery and wound packing

• Severe neutropenia

• IV drug abuse

• Chronic granulomatous disease

• Cystic fibrosis

Disease and Pathogenesis

S. aureus produces several virulence factors that contribute to its pathogenicity:

• Protein A: Binds Fc region of IgG, inhibiting phagocytosis.

• Coagulases: Convert fibrinogen to fibrin, promoting abscess formation.

• Exfoliative toxins: Cause skin peeling in scalded skin syndrome.

• TSST-1: Superantigen toxin causing toxic shock syndrome.

• Panton-Valentine leukocidin (PVL): Forms pores in leukocytes, leading to cell lysis.

Clinical Diseases and Symptoms

S. aureus is associated with a wide range of diseases, each with distinct clinical presentations and pathogenic mechanisms.

Diseases	Clinical Symptoms	Pathogenicity
Gastroenteritis (food poisoning)	2-6 hours after ingesting toxin: nausea, abdominal pain, vomiting, followed by diarrhea	Enterotoxins A-E preformed in food, fast acting and heat stable
Acute Infective Endocarditis (IV drug abuse)	Fever, malaise, leukocytosis, heart murmur (may be absent initially)	Fibrin platelet mesh, cytolytic (alpha) toxins: PVL, forms pores in infected cells
Toxic Shock Syndrome	Fever, hypotension, scarlatiniform rash (esp. on palms and soles), multi-organ failure	TSST-1: superantigen toxin, nonspecific binding of MHC class II and T-cell receptors, cytokine storm
Osteomyelitis (most common cause)	Bone pain, fever, tissue swelling, redness; lytic bone lesions on imaging	Cytolysin toxin and coagulases; lytic bone lesions
Pneumonia (post-viral, nosocomial, etc.)	Productive pneumonia with rapid onset, high fatality; salmon-colored sputum	Cytolysin toxin and coagulase; converts fibrinogen to fibrin clot
Abscesses and mastitis	Subcutaneous tenderness, redness, swelling, hot	Cytolysin toxin and coagulases; localized boils
Impetigo, Scalded Skin Syndrome	Erythematous papules to bullae; diffuse epidermal peeling	Coagulase and exfoliative toxins
Surgical infections from wound packing	Fever with cellulitis and/or abscesses	Coagulase, exfoliative, TSSTs

Treatment and Antibiotic Resistance

Gastroenteritis caused by S. aureus is self-limiting. For other infections, penicillin was historically used, but resistance is widespread. Nafcillin and oxacillin are now preferred. Methicillin-resistant S. aureus (MRSA) is treated with vancomycin. MRSA acquires resistance by altering its penicillin-binding protein. For vancomycin-resistant S. aureus (VRSA) or vancomycin-intermediate S. aureus (VISA), quinupristin/dalfopristin are used.

• Penicillin: Historically used, now limited due to resistance.

• Nafcillin/Oxacillin: Drugs of choice for most infections.

• Vancomycin: Used for MRSA.

• Quinupristin/Dalfopristin: Used for VRSA/VISA.

Example: MRSA resistance is due to altered penicillin-binding protein, making beta-lactam antibiotics ineffective.

Additional info: S. aureus is a major cause of nosocomial infections and is notable for its ability to form biofilms on medical devices, contributing to persistent infections.