Skin & Soft-Tissue Infections
A boards-focused walkthrough of skin & soft-tissue infections: the purulent-vs-nonpurulent and severity framework, exposure-specific organisms, classic buzzwords, and the next-best-step decisions — especially recognizing necrotizing fasciitis and choosing I&D versus antibiotics.
The framework that drives every answer
Skin & soft-tissue infections (SSTIs) are stratified two ways on the exam, and both dictate the answer.
First, purulent vs. nonpurulent — this fixes the organism and therefore empiric coverage:
- Nonpurulent (cellulitis, erysipelas): β-hemolytic streptococci (mostly Group A strep), ± MSSA. Cover strep; MRSA is not routine.
- Purulent (abscess, furuncle, carbuncle): *S. aureus*, including CA-MRSA. Primary therapy is incision & drainage (I&D).
Second, severity (mild → moderate → severe) decides oral vs. IV and whether antibiotics are added at all. Systemic toxicity, immunocompromise, or failed oral therapy escalates care.
The high-yield traps are the exposure-specific organisms (bites, water, gardening) and spotting the one surgical emergency — necrotizing fasciitis — early. Cellulitis itself is a clinical diagnosis; imaging and blood cultures are usually unnecessary.
- Cellulitis — deeper dermis/subcutis; poorly demarcated, warm, tender erythema; usually unilateral. Bilateral lower-leg redness is almost always stasis dermatitis, not cellulitis.
- Erysipelas — upper dermis + superficial lymphatics; sharply demarcated, raised, fiery-red plaque with abrupt fever; face/shins. Group A strep → penicillin.
- Abscess / furuncle / carbuncle — fluctuant, purulent; S. aureus / CA-MRSA → I&D.
- Impetigo — honey-colored crusts (nonbullous: strep/staph) or flaccid bullae (bullous = S. aureus exfoliative toxin). May trigger post-streptococcal glomerulonephritis (antibiotics do not reliably prevent it); rheumatic fever does NOT follow skin strep.
- Erythrasma — intertriginous plaques, coral-red Wood-lamp fluorescence; Corynebacterium minutissimum.
- Necrotizing fasciitis — pain out of proportion, rapid spread, systemic toxicity = surgical emergency.
Exposure clues (organism · exposure · treatment)
| Organism | Exposure / buzzword | Treatment |
|---|---|---|
| Pasteurella multocida | Cat/dog bite, cellulitis < 24 h | Amoxicillin-clavulanate |
| Capnocytophaga canimorsus | Dog bite in asplenic/cirrhotic → fulminant sepsis, DIC | Amoxicillin-clavulanate / IV β-lactam |
| Eikenella corrodens | Human clenched-fist ("fight") bite | Amoxicillin-clavulanate |
| Vibrio vulnificus | Salt water / raw oysters + cirrhosis; hemorrhagic bullae | Doxycycline + ceftriaxone |
| Aeromonas hydrophila | Fresh-water laceration; leeches | Fluoroquinolone or doxycycline |
| Sporothrix schenckii | Rose thorn / gardener; ascending nodules | Itraconazole |
| Mycobacterium marinum | Fish tank / aquarium | Clarithromycin (± ethambutol/rifampin) |
| Pseudomonas aeruginosa | Hot-tub folliculitis; nail-puncture wound | Self-limited / antipseudomonal |
Vignette: A 58-year-old man with diabetes develops severe left-leg pain 1 day after a minor scratch. The pain is out of proportion to a modest patch of erythema; edema extends beyond the redness with violaceous skin, tense hemorrhagic bullae, and crepitus. T 39.2 °C, HR 122, BP 86/52. Labs: WBC 22k, Na 128, ↑CK, ↑CRP.
Diagnosis: Necrotizing fasciitis (here Type I / polymicrobial, favored by diabetes; Type II = Group A strep ± S. aureus). The deranged labs feed the LRINEC score — supportive, but never a reason to wait.
Next best step: Emergent surgical exploration & debridement — the definitive diagnostic and therapeutic move. Do NOT delay the OR for CT/MRI. Start broad-spectrum IV antibiotics: vancomycin + piperacillin-tazobactam + clindamycin (clindamycin suppresses exotoxin production). Resuscitate; consider IVIG in streptococcal toxic shock.
Vignette: A 24-year-old wrestler has a painful, fluctuant 3-cm nodule on the thigh with central pus and a thin rim of surrounding erythema. He is afebrile and well-appearing, with no comorbidities.
Diagnosis: Cutaneous abscess, most likely CA-MRSA (classic in contact-sport athletes). If it is unclear whether a drainable collection is present, point-of-care ultrasound confirms it.
Next best step: Incision & drainage — the primary therapy for any drainable abscess; antibiotics alone are inadequate. Send a wound culture.
Add an oral MRSA-active antibiotic (TMP-SMX or doxycycline) for moderate-risk features: surrounding cellulitis, size > 2 cm, multiple lesions, systemic signs, immunocompromise, or failure of I&D alone. This patient qualifies (3 cm and surrounding cellulitis), so I&D plus TMP-SMX is appropriate. Reserve I&D alone for a truly small (< 2 cm), simple, fully drained abscess without surrounding cellulitis in a healthy host.
Memorize these one-line associations (and the images behind them):
- Honey-colored crusts → impetigo
- Sharply demarcated, raised, fiery-red facial plaque ("St. Anthony's fire") → erysipelas (Group A strep)
- Pain out of proportion + crepitus + hemorrhagic bullae → necrotizing fasciitis
- Raw oysters / sea water + cirrhosis + hemorrhagic bullae → *Vibrio vulnificus*
- Rose gardener + ascending (nodular) lymphangitis → sporotrichosis
- Coral-red Wood-lamp fluorescence → erythrasma (Corynebacterium)
- Itchy pustules after a hot tub → Pseudomonas folliculitis
- Fish-tank cleaner + skin nodule → *Mycobacterium marinum*
- Grouped painful vesicles on a red base → HSV (viral — do not I&D)
- Nonpurulent, mild cellulitis → oral strep coverage: cephalexin, dicloxacillin, or penicillin. MRSA coverage is not routine.
- Purulent / abscess → I&D first; add an MRSA-active oral drug (TMP-SMX, doxycycline, clindamycin) by severity.
- Moderate–severe / systemic toxicity / immunocompromised → IV therapy covering MRSA: vancomycin (or linezolid, daptomycin).
- Necrotizing fasciitis → surgery + broad IV antibiotics + clindamycin — do not wait for imaging.
- Animal / human bite → amoxicillin-clavulanate; update tetanus; assess rabies; leave most bite wounds open (no primary closure).
- Adjuncts: elevate the limb, mark the border, and treat the entry portal (tinea pedis, ulcers). Uncomplicated cellulitis course ≈ 5 days, extend if not improving.
- Not improving? Reconsider undrained abscess, DVT, stasis dermatitis, or a resistant organism.
Vignette: A 60-year-old man with cirrhosis has fever and a rapidly worsening, exquisitely painful left leg 2 days after eating raw oysters on the coast. Exam shows spreading erythema with hemorrhagic bullae; BP 84/50. He recalls wading in warm seawater.
Diagnosis: *Vibrio vulnificus* — classic in liver disease / hemochromatosis (iron overload). It can arise from ingestion or wound exposure and progress to necrotizing infection and septic shock.
Next best step: Blood and wound cultures, aggressive resuscitation, and empiric doxycycline + a third-generation cephalosporin (ceftriaxone or cefotaxime), plus early surgical debridement if the infection is necrotizing. Mortality is high — treat on suspicion; do not wait for culture confirmation.
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