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Subject: Anaerobic Culture
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Anaerobic cultures are indicated for evaluation of patient specimens taken from sites with signs and symptoms of bacterial infection (e.g., swelling, redness, heat, pus, or exudate). Infections associated with anaerobic pathogens include surgical and traumatic wounds, sinusitis and pararespiratory infections, pelvic and intra-abdominal infections, osteomyelitis, myositis, gangrene, and necrotic wounds, abscesses, and actinomycosis and infections associated with fistula formation.
Anaerobic cultures are used for the detection of common anaerobic bacterial pathogens from patient specimens. Site-specific aerobic bacterial cultures (e.g., tissue culture, abscess culture, wound culture) are recommended, if available. Specimens are inoculated on several types of anaerobic culture media. (see “Aerobic Culture” for a general discussion about media). Media should be fresh and prereduced. Typical media for aerobic cultures include
Supportive agar media, like Schaedler agar or CDC anaerobic blood agar
Selective/differential agar media:
Phenylethyl alcohol or CNA agar for anaerobic gram-positive pathogens
Kanamycin–vancomycin–laked blood agar, for anaerobic gram-negative bacilli
Bacteroide bile–esculin agar, for Bacteroides fragilis group
Egg yolk agar, for characterization of Clostridium species
Cycloserine–cefoxitin–egg yolk–fructose agar (CCFA), for Clostridium difficile
Broth, like enriched thioglycolate medium or chopped meat broth
Turnaround time: Incubation for 5–7 days
Additional time is required for positive cultures for additional testing required for isolation, confirmation as anaerobic (aerotolerance testing), identification, susceptibility testing, and further characterization, as needed. Anaerobic infections are frequently polymicrobial; final results may require several weeks for full laboratory evaluation, if needed.
See “Aerobic Culture” for a general discussion of collection and transport instructions
Because of the anaerobic endogenous flora, specimens from the following sites should not be submitted for anaerobic culture: sputum or bronchoscopically collected lower respiratory specimens, swabs from skin or mucosal surfaces, specimens from the GI tract (including fistulae, stoma surfaces, and so on), superficial ulcers or eschars, including decubitus ulcers, vaginal or cervical swabs or urine (except suprapubic aspirate urine).
When submitting samples, ensure that sufficient specimen is collected for all of the diagnostic testing required (e.g., aerobic, fungal, and/or mycobacterial cultures and stains). Minimize exposure to atmospheric oxygen and transport in an anaerobic transport system. Do not refrigerate or freeze specimens for anaerobic culture. Note the following: Specimens collected and transported for anaerobic culture are also acceptable for aerobic bacterial, fungal, or mycobacterial culture, provided a sufficient volume of specimen is provided.
Expected results: No anaerobic pathogen isolated.
Anaerobic infections are frequently polymicrobial. Initial isolation and aerotolerance testing may require repeated subculture of the primary culture media. Many anaerobic pathogens are slow growing and biochemically indolent, making identification, susceptibility testing, and further characterization of isolates in the laboratory much slower than most aerobic bacterial pathogens. Therefore, patient care decisions must often be made before results of testing are available, limiting the clinical utility of extensive workup of mixed anaerobic cultures.
Anaerobic culture may be significantly compromised by collection and transport conditions that are not strictly anaerobic or because of refrigeration during transport. Inadequate site preparation may result in false-positive cultures due to specimen contamination with endogenous flora. Contaminated cultures may also mask the recognition of slow-growing or fastidious anaerobic pathogens in the culture.