Diagnosis of Mixed Anaerobic Infections Clinical suspicion Gram stain and culture Clinical clues to the presence of anaerobic organisms include Infection adjacent to mucosal surfaces that bear anaerobic flora Ischemia, tumor, penetrating trauma, foreign body, or perforated viscus Spreading gangrene involving skin, subcutaneous tissue, fascia, and muscle Feculent odor in pus or infected tissues Abscess formation Gas in tissues Septic thrombophlebitis Failure to respond to antibiotics that do not have significant activity against anaerobic bacteria Anaerobic infection should be suspected when any wound smells foul or when a Gram stain of pus from an infected site shows mixed pleomorphic bacteria but aerobic cultures show no growth.