Anaerobic and Aerobic Culture

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 that are pathologic synergists. Anaerobes frequently recovered from closed postoperative wound infections include Bacteroides fragilis, 50 percent; Bacteroides melaninogenicus, 25 percent; Peptostreptococcus prevotii, 15 percent; and Fusobacterium sp, 25 percent. Anaerobes are seldom recovered in pure culture (10 to 15 percent 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 B melaninogenicus 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)

87070, 87075

Important: Sterile preparation of the aspiration site is imperative.

Gram-staining is recommended with all anaerobic cultures (additional charge).

The requisition form must state:

• The specific site of the specimen;
• The age of patient;
• The current antibiotic therapy;
• The patient’s clinical diagnosis; and
• The time of collection.

Container/Tube: Anaerobic transport or aerobic/anaerobic bacterial swab transport containing gel preservative

Sample Volume: Swab 0.5 mL pus or other fluid or tissue from an aspirated site. Ship specimens to the Lab in an anaerobic transporter.

Storage: Store specimens at room temperature.

• Under these conditions, aerobes and anaerobes will survive 24 to 72 hours when properly collected in the anaerobic transport tube.

Stability (collection to time of analysis/testing):

• Ambient: Unstable
• Refrigerated: Unstable
• Frozen: 14 days

Rejection Criteria:

• Unlabeled specimen or name discrepancy between specimen and request label
• Specimen not received in appropriate anaerobic transport tube
• Swab not stored in oxygen-free atmosphere (any swab is suboptimal)
• Refrigerated specimens
• Specimen received after prolonged delay in transport (usually more than 72 hours)
  • Note: Refrigeration inhibits viability of certain anaerobic organisms.
• Specimens from sites that have anaerobic bacteria as indigenous flora will not be cultured anaerobically
  (e.g., throat, feces, colostomy stoma, rectal swabs, bronchial washes, cervical-vaginal mucosal swabs, sputa, skin and superficial wounds, voided or catheterized urine, ulcer surfaces, drainages onto contaminated surfaces).

This test is used to isolate and identify anaerobic pathogenic organisms, as well as to determine the susceptibility of isolates. 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.

This test includes:

• The isolation and identification (additional CPT coding of 87076, 87077, 87143 or other code, depending on the methods required) of potential anaerobic and aerobic pathogens
• Susceptibility testing if culture results warrant at an additional charge. CPT coding for microbiology and virology procedures often cannot be determined before the culture is performed.

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 less than five percent 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.

Bronchoscopically obtained specimens are not ideal as the instrument becomes contaminated by organisms normally contaminating the oropharynx during insertion. Culture of specimens from sites harboring endogenous anaerobic organisms or contaminated by endogenous organisms may be misleading with regard to etiology and selection of appropriate therapy.