infectious diseases
Mixed Anaerobic Infections
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. Only specimens from normally sterile sites should be cultured anaerobically because commensal anaerobic contaminants may easily be mistaken for pathogens. Gram stains and aerobic cultures should be obtained for all specimens. Cultures for anaerobes, particularly if mishandled, may be falsely negative. Antibiotic susceptibility testing of anaerobes is exacting, and data may not be available for≥1 week after initial culture. However, if the species is known, susceptibility patterns can usually be predicted. Therefore, many laboratories do not routinely test anaerobic organisms for susceptibility. 