Skin and Soft Tissue Infections

SSTI Basics & Layers - Skin Deep Dive

• SSTIs: Microbial invasion of skin & underlying soft tissues, ranging from superficial to deep.

• Affected Layers & Examples:

  • Epidermis: Impetigo (crusted lesions)
  • Dermis: Erysipelas (raised, sharp border), Cellulitis (spreading erythema)
  • Subcutaneous Tissue: Abscess (pus collection), deeper Cellulitis
  • Fascia: Necrotizing Fasciitis (tissue necrosis)
  • Muscle: Myositis, Pyomyositis

• Classification: Based on anatomical depth, severity, and presence of purulence.

⭐ Necrotizing fasciitis often presents with pain out of proportion to visible skin changes, a critical diagnostic clue (Type II is commonly Group A Streptococcus).

Superficial Bacterial Blitz - Surface Skirmishes

• Impetigo: Superficial (S. aureus, Strep. pyogenes).
  • Non-bullous: Honey-colored crusts (commonest).
  • Bullous: S. aureus (toxin); flaccid bullae.
  • Tx: Topical mupirocin. Oral Abx for widespread.
• Folliculitis: Hair follicle infection.
  • S. aureus (common), P. aeruginosa (hot tub).
  • Perifollicular papules/pustules.
  • Tx: Antiseptics; topical/oral Abx if extensive.
• Furuncle (Boil): Deep folliculitis (S. aureus).
  • Painful, fluctuant nodule.
  • Tx: Warm compress, I&D. Abx if systemic/cellulitis.
• Carbuncle: Coalesced furuncles, deeper (S. aureus).
  • Multiple sinuses; systemic symptoms.
  • Tx: I&D + systemic Abx.

⭐ Bullous impetigo is caused by exfoliative toxins A and B produced by S. aureus.

Deep & Deadly Infections - Flesh Eaters' Feast

• Necrotizing Fasciitis (NF): Rapidly spreading infection of fascia & subcutaneous tissue.
  • Types: I (Polymicrobial), II (GAS - S. pyogenes), III (Clostridium spp.).
  • Signs: Pain out of proportion, crepitus, bullae, skin necrosis. 📌 "Flesh-eating bacteria".
  • LRINEC score >6 suggests NF.
  • Rx: Urgent surgical debridement + Broad-spectrum Abx (e.g., Piperacillin-tazobactam + Clindamycin +/- Vancomycin/Linezolid).

    ⭐ Group A Strep (Type II NF) is a common cause of Streptococcal Toxic Shock Syndrome (StrepTSS).

• Gas Gangrene (Clostridial Myonecrosis): Life-threatening muscle necrosis.
  • Agent: C. perfringens (alpha-toxin).
  • Signs: Sudden severe pain, bronze/purple skin, crepitus, foul discharge, systemic toxicity.
  • Rx: Surgical debridement + Penicillin G + Clindamycin. Hyperbaric oxygen (HBO) as adjunctive therapy.

Key Non-Bacterial & Specific SSTIs - Viral, Fungal & Oddballs

• Viral:
  • HSV: Painful grouped vesicles. Tzanck: multinucleated giant cells.
  • VZV: Chickenpox (vesicular crops), Shingles (dermatomal).
  • Molluscum Contagiosum: Poxvirus. Umbilicated papules. Henderson-Patterson bodies.
• Fungal:
  • Dermatophytes (Tinea): Ringworm. KOH: hyphae.
  • Candida: Intertrigo, satellite pustules. Pseudohyphae.
  • Sporotrichosis: "Rose gardener's disease". Lymphocutaneous nodules. Cigar-shaped yeasts.
• Oddballs/Specific:
  • Cutaneous TB: Lupus vulgaris (apple-jelly nodules).
  • Leprosy: Nerve thickening, anesthetic patches.
  • Ecthyma Gangrenosum: P. aeruginosa (neutropenia). Necrotic ulcers.

⭐ > Sporotrichosis, caused by Sporothrix schenckii, often presents as a fixed cutaneous or lymphocutaneous infection after traumatic inoculation of soil, plants, or organic matter contaminated with the fungus.

SSTI Dx & Rx Strategies - Plan of Attack

• Dx: Clinical exam (local/systemic signs). Gram stain, C&S from pus/aspirate. Blood cultures if systemic. Imaging for deep infections.
• Rx:
  • Incision & Drainage (I&D) for abscesses.
  • Empiric Abx: Cover Staph aureus, Strep pyogenes. Consider MRSA coverage (e.g., vancomycin, linezolid).
  • Definitive Abx: Tailor based on C&S.

⭐ For purulent SSTIs (abscess, furuncle, carbuncle), incision and drainage (I&D) is the cornerstone of management; antibiotics are often adjunctive.

High‑Yield Points - ⚡ Biggest Takeaways

• S. aureus is the most common SSTI cause; MRSA is a key concern.
• S. pyogenes causes erysipelas (superficial, sharp border) and cellulitis (deeper, diffuse).
• Necrotizing fasciitis: surgical emergency, often polymicrobial or Group A Strep.
• Impetigo: honey-colored crusts, by S. aureus or S. pyogenes.
• Diabetic foot infections: often polymicrobial, need broad-spectrum coverage.
• Folliculitis, furuncles, carbuncles: hair follicle infections, usually S. aureus.
• Gas gangrene (clostridial myonecrosis): crepitus, muscle necrosis, caused by Clostridium perfringens.