Cellulitis and Skin Abscess
System: Skin and Soft Tissue Infections · Parent MOC: MOC - Diseases by System
One-Sentence Definition
Cellulitis is spreading dermal/subcutaneous infection; abscess is a localized pus collection — often overlapping clinically and dominated by streptococci (diffuse) vs S. aureus (purulent).
Clinical Presentation
- Cellulitis: warmth, erythema, edema, tenderness, ± fever; borders may be diffuse
- Abscess: fluctuance, central pustule; “spider bite” lore often MRSA abscess
- Red flags for Necrotizing Soft Tissue Infection: pain out of proportion, crepitus, rapid spread, systemic toxicity
Pathogens
| Syndrome | Organism | Notes |
|---|---|---|
| Nonpurulent cellulitis | Streptococcus pyogenes ± other β-hemolytic strep | Often no culture |
| Purulent / abscess | Staphylococcus aureus (MSSA/MRSA) | I&D is key |
| Water / trauma special | Vibrio, Aeromonas, Pseudomonas (selected) | Exposure history |
Pathogenesis
- Breach of skin barrier → bacterial replication in soft tissue
- Abscess = walling-off + neutrophils; antibiotics penetrate poorly without drainage
Diagnosis
| Step | Test | Note |
|---|---|---|
| Clinical | Exam | Mainstay for typical cellulitis |
| Purulent | I&D + culture/AST | Always culture drained pus |
| Blood cultures | If severe / immunocompromised | Bacteremia risk |
| Imaging | US for drainable collection; CT/MRI if deep/nec fasc concern |
Treatment Principles
- Abscess: incision & drainage ± antibiotics (MRSA coverage when indicated)
- Nonpurulent cellulitis: cover β-hemolytic strep (± MRSA if risk)
- Elevate limb; treat tinea/toe web fissures as portals
- Escalate early if necrotizing signs → Necrotizing Soft Tissue Infection
Prevention
- Skin care, wound hygiene; decolonize selected recurrent MRSA cases
- Glycemic control in diabetics
Differential / Pitfalls
- DVT, stasis dermatitis, gout, necrotizing infection
- Antibiotics alone for undrained abscess → failure
Learning Aids
Clinical Example
Example
Case: Fluctuant thigh boil; afebrile otherwise.
Question: First intervention and likely organism?
Answer: I&D ± culture; Staphylococcus aureus. Antibiotics adjunctive per size/risk, not a substitute for drainage.
Related
- Necrotizing Soft Tissue Infection · Skin and Soft Tissue Infections
- Staphylococcus aureus · Streptococcus pyogenes
Active Recall
- Purulent vs nonpurulent empiric organism focus?
- Why drain abscesses?
- Three red flags suggesting necrotizing infection?