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Cellulitis and skin and soft tissue infections
Skin infection splits into two types: non-purulent cellulitis (mainly streptococci — cephalexin or penicillin) and purulent abscess (Staphylococcus aureus — drainage, with antibiotics as needed); pain out of proportion, bullae and crepitus call for thinking of necrotising infection and summoning a surgeon.
Why it matters in the exam
The question presents redness, local warmth, swelling and tenderness of a leg, sometimes with fever, and asks about treatment. Non-purulent cellulitis is treated against streptococci (oral cephalexin, or intravenous cefazolin when there are systemic signs) for 5 days, extended if there is no improvement; the limb is elevated and fungal infection between the toes, a common portal of entry, is treated. Erysipelas is a superficial infection with a raised, sharp border. Blood tests, cultures and imaging are not needed in an ordinary case.
In an abscess the main treatment is drainage; antibiotics (TMP-SMX or doxycycline to cover MRSA) are added when there are systemic signs, a large abscess, immunosuppression or a site that is hard to drain. Well-known distractors: "bilateral cellulitis" of both legs is usually stasis dermatitis, not infection; and an animal bite is treated with amoxicillin-clavulanate (Pasteurella). Necrotising fasciitis — pain greater than the findings, bullae, necrosis, crepitus and a toxic state — is a surgical emergency (see the surgery chapter). In a diabetic with a foot ulcer, an immunosuppressed patient or after water exposure, the organisms differ and treatment is broader; and cellulitis that keeps returning in the same leg requires treating the chronic oedema and the portal of entry to prevent another attack.
Key points
- Non-purulent: streptococci — cephalexin or cefazolin, 5 days.
- Purulent (abscess): drainage; anti-MRSA antibiotics when indicated.
- Bite: amoxicillin-clavulanate.
- Bilateral: usually stasis dermatitis.
- Pain out of proportion, bullae, crepitus — necrotising infection: surgeon.
Summary table
| Type | Clue | Treatment | Exam note |
|---|---|---|---|
| Non-purulent cellulitis | Diffuse redness, no pus | Oral cephalexin or intravenous cefazolin; 5 days | Streptococci |
| Erysipelas | Raised, sharp border, superficial | Penicillin or cephalexin | Sometimes on the face |
| Abscess | Purulent lump, fluctuance | Drainage; TMP-SMX or doxycycline when indicated | S. aureus including MRSA |
| Animal bite | Dog or cat | Amoxicillin-clavulanate | Pasteurella |
| "Bilateral cellulitis" | Both legs, itch, chronic swelling | Treat venous congestion | Usually not an infection |
| Necrotising infection | Pain out of proportion, bullae, crepitus, toxic state | Surgeon at once + broad antibiotics | An emergency |
A free exam question
אילו מהמחוללים הבאים אינו זיהום אופייני לאחר נשיכת חיה? (the question as examined, in Hebrew)
- א. Capnocytophaga spp.
- ב. Erysipelothrix rhusiopathiae
- ג. Streptobacillus spp.
- ד. Leptospira species
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Sources
- IDSA Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections, 2014 — Purulent versus non-purulent infection, cellulitis, abscess, bites
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Infections of the Skin, Muscles, and Soft Tissues
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.