Infectious Disease · Bacterial Infections
The facts most likely to be tested
MRSA is defined by the mecA gene which encodes PBP2a, resulting in altered penicillin-binding proteins that confer resistance to all beta-lactam antibiotics.
Vancomycin remains the first-line intravenous treatment for severe, systemic MRSA infections, requiring trough level monitoring to prevent nephrotoxicity.
Community-acquired MRSA (CA-MRSA) typically presents as purulent skin and soft tissue infections (SSTIs), such as furuncles or abscesses, often caused by the Panton-Valentine leukocidin (PVL) toxin.
Trimethoprim-sulfamethoxazole (TMP-SMX), clindamycin, or doxycycline are the preferred oral outpatient treatments for uncomplicated purulent SSTIs suspected to be MRSA.
Linezolid is the preferred agent for MRSA pneumonia due to its superior lung tissue penetration and inhibition of bacterial toxin production.
Daptomycin is indicated for MRSA bacteremia and right-sided endocarditis, but it is strictly contraindicated for MRSA pneumonia because it is inactivated by pulmonary surfactant.
Decolonization of persistent MRSA carriers involves the use of intranasal mupirocin and chlorhexidine body washes.
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A 24-year-old male presents to the urgent care clinic with a painful, erythematous, fluctuant nodule on his right buttock that has enlarged over the past 3 days. He denies fever or chills, and his vital signs are stable. Physical examination reveals a 3-cm tender abscess with surrounding erythema and induration. There is no evidence of systemic toxicity or cellulitis. The patient has no known drug allergies.
What is the most appropriate management for this patient?
Incision and drainage
The patient presents with a classic purulent SSTI; the most appropriate management for a localized abscess is incision and drainage, with antibiotics reserved for cases with systemic signs or severe infection.
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Etiology / Epidemiology
Common in healthcare settings and contact sports; colonization occurs in the nares.
Clinical Manifestations
Presents as furuncles or carbuncles; fluctuance is the hallmark of abscess formation.
Diagnosis
Culture and sensitivity is the gold standard; oxacillin/cefoxitin resistance confirms diagnosis.
Treatment
Outpatient: TMP-SMX or clindamycin; Inpatient: vancomycin.
Prognosis
Risk of sepsis and endocarditis; monitor for renal failure with vancomycin.
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Epidemiology & Etiology
MRSA is caused by *Staphylococcus aureus* with an altered penicillin-binding protein (PBP2a). Major risk factors include recent hospitalization, hemodialysis, and long-term care facility residence. Community-acquired strains often affect healthy individuals via skin-to-skin contact.
Pertinent Anatomy
Infection typically involves the dermis and subcutaneous tissue. Deep-seated infections may involve bone (osteomyelitis) or heart valves (endocarditis), requiring surgical evaluation.
Pathophysiology
The *mecA* gene encodes PBP2a, which has low affinity for beta-lactam antibiotics. This renders all penicillins and cephalosporins ineffective. The organism produces exotoxins that cause tissue necrosis and systemic inflammatory response.
Clinical Manifestations
Patients present with erythematous, tender, indurated skin lesions. Look for honey-colored crusts in impetigo or fluctuant masses in abscesses. Red flags include fever, hypotension, or rapid spread suggesting necrotizing fasciitis.
Diagnosis
Culture and sensitivity is the definitive test. PCR for the *mecA* gene provides rapid identification. Resistance is defined by an oxacillin MIC ≥ 4 mcg/mL.
Treatment
For uncomplicated skin infections, incision and drainage is primary. Oral TMP-SMX or doxycycline are first-line for outpatient MRSA. Inpatient severe infections require vancomycin; monitor trough levels to avoid nephrotoxicity.
Prognosis
Complications include bacteremia, septic emboli, and osteomyelitis. Patients on vancomycin require serial creatinine monitoring to prevent acute kidney injury.
Differential Diagnosis
Cellulitis: usually non-purulent and lacks fluctuance
Spider bite: often misdiagnosed; lacks central punctum
Necrotizing fasciitis: characterized by pain out of proportion to exam
Furunculosis: localized to hair follicle
Erysipelas: superficial, well-demarcated borders