MI5.{1,3-4} | Anaerobic & Skin/Soft-Tissue Infections — Summary & Reflection

REFLECT

A 60-year-old truck driver with poorly controlled Type 2 diabetes presents with a foot ulcer that has been present for three weeks. He now has a fever of 39°C, the wound smells offensive, and palpation reveals crepitus. His WBC is 22,000/µL. The orthopedic team is considering IV antibiotics alone. You believe surgical debridement is needed urgently. What microbiological arguments would you make? What specific specimens would you send, and what media would you request? How does the presence of crepitus change the likely pathogen spectrum from simple cellulitis?

KEY TAKEAWAYS

Key takeaways from this SDL:

1. Anaerobes require devitalised tissue: Low redox potential, absent O₂, ischaemic/necrotic milieu.
2. Gas gangrene (C. perfringens): Alpha toxin (lecithinase) → rapid myonecrosis; Gram stain shows rods WITHOUT PMNs; double-zone haemolysis + Nagler test; surgical debridement mandatory.
3. Tetanus (C. tetani): Drum-stick spore; tetanospasmin blocks inhibitory GABA/glycine → spastic paralysis; vaccine-preventable; TIG for post-exposure.
4. Bacterial SSTIs by depth: Impetigo (epidermis) → cellulitis/erysipelas (dermis/subcutis) → necrotising fasciitis (fascia) → myonecrosis (muscle).
5. NF type I vs. II: Polymicrobial (anaerobes + aerobes) vs. Group A Strep; both require emergency surgery.
6. Fungal SSTIs: Dermatophytes (KOH, hyphae, SDA) → sporotrichosis (cigar yeast, lymphangitis) → mycetoma (grains in sinuses).
7. Viral SSTIs: Tzanck smear for HSV/VZV (multinucleated giant cells); molluscum contagiosum bodies; measles Koplik spots.
8. Systemic infections with skin: Meningococcaemia (petechiae), secondary syphilis (palmoplantar rash), rickettsia (eschar), dengue (haemorrhages).