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Isolate and identify anaerobic pathogenic organisms; determine susceptibility of isolates (extra charge). When actinomycetes are suspected a specific request must be made. Anaerobic cultures are indicated particularly when suspected infections are related to gastrointestinal tract, pelvic organs, associated with malignancy, related to use of aminoglycosides; or occur in a setting in which the diagnosis of gas gangrene or actinomycosis is considered. Anaerobic culture is especially indicated when an exudate has a foul odor or if the exudate has a grayish discoloration and is hemorrhagic. Frequently, more than one organism is recovered from an anaerobic infection.
The only sources for specimens with established validity for meaningful anaerobic culture in patients with pleuropulmonary infections are blood, pleural fluid, transtracheal aspirates, transthoracic pulmonary aspirates, and specimens obtained at thoracotomy. Pleural fluid is preferred for patients with empyema.1 Blood cultures yield positive results in <5% of cases of anaerobic pulmonary infection. Specimens received in anaerobic transport containers are not optimal for aerobic fungus cultures. Mycobacterium sp or Nocardia sp, which may cause abscesses, will not be recovered even if present, since extended incubation periods, aerobic incubation, and special media are necessary for their isolation. Cultures for these organisms should be specifically requested, and a separate specimen submitted.
IUDs will be cultured for Actinomyces sp only.
In open wounds, anaerobic organisms may play an etiologic role, whereas aerobes may represent superficial contamination. Serious anaerobic infections are often due to mixed flora, which are pathologic synergists. Anaerobes frequently recovered from closed postoperative wound infections include Bacteroides fragilis, approximately 50%; Prevotella melaninogenica, approximately 25%; Peptostreptococcus prevotii, approximately 15%; and Fusobacterium sp, approximately 25%. Anaerobes are seldom recovered in pure culture (10% to 15% of cultures). Aerobes and facultative bacteria when present are frequently found in lesser numbers than the anaerobes. Anaerobic infection is most commonly associated with operations involving opening or manipulating the bowel or a hollow viscus (eg, appendectomy, cholecystectomy, colectomy, gastrectomy, bile duct exploration, etc). The ratio of anaerobes to facultative species is normally about 10:1 in the mouth, vagina, and sebaceous glands and at least 1000:1 in the colon. Biopsy culture is particularly useful in establishing the diagnosis of anaerobic osteomyelitis,2 clostridial myonecrosis, intracranial actinomycosis, and pleuropulmonary infections. Anaerobic infections of soft tissue include anaerobic cellulitis, necrotizing fasciitis, clostridial myonecrosis (gas gangrene), anaerobic streptococcal myositis or myonecrosis, synergistic nonclostridial anaerobic myonecrosis, and infected vascular gangrene. These infections, particularly clostridial myonecrosis, necrotizing fasciitis, and nonclostridial anaerobic myonecrosis, may be fulminant and are frequently characterized by the presence of gas and foul-smelling necrotic tissue.3 Empiric therapy based on likely pathogens should be instituted as soon as appropriate cultures are collected.
Clinical symptoms suggestive of anaerobic infection include:
• Foul-smelling discharge
• Location of infection in proximity to a mucosal surface
• Necrotic tissue, gangrene, pseudomembrane formation
• Gas in tissues or discharges
• Endocarditis with negative routine blood cultures
• Infection associated with malignancy or other process producing tissue destruction
• Septic thrombophlebitis
• Bacteremic picture with jaundice
• Infection resulting from human or other bites
• Black discoloration of blood-containing exudates (may fluoresce red under ultraviolet light in P melaninogenica infections)
• Presence of “sulfur granules” in discharges (actinomycosis)
• Classical clinical features of gas gangrene
• Clinical setting suggestive for anaerobic infection (septic abortion, infection after gastrointestinal surgery, genitourinary surgery, etc)
1. Bartlett JG. Anaerobic bacterial infections of the lung. Chest. 1987 Jun; 91(6):901-909. PubMed 3556058
2. Hall BB, Fitzgerald RH Jr, Rosenblatt JE. Anaerobic osteomyelitis. J Bone Joint Surg Am. 1983 Jan; 65(1):30-35. PubMed 6848532
3. Finegold SM, George LW, Mulligan ME. Anaerobic infections. Part II. Dis Mon. 1985 Nov; 31(11):1-97. PubMed 3914407
