Rationale: The medical history and findings on physical examination should arouse suspicion of anaerobic infection. Predisposing factors (e.g., solid tumor), foul odor of lesion or drainage, gas or discoloration in tissue, and tissue necrosis, gangrene, or abscess point toward a narrowed differential diagnosis. Gas gangrene is commonly caused by clostridial species (e.g., C. perfringens). However, Gram-negative bacteria, which are much more common with surgical infections, may, rarely, cause gas in mixed infections of soft tissue. Gram-positive organisms (e.g., Strep. pyogenes, Staph. aureus) also commonly cause postsurgical infections, but usually not gas gangrene.