Anaerobe Bacterial Culture

Anaerobic infections are usually caused by leakage of normal flora into a sterile body site, following disruption of a mucosal barrier. The most commonly isolated gram-negative anaerobic pathogens include Bacteroides, Fusobacterium, Prevotella, and Porphyromonas. Gram-positive anaerobic pathogens include Anaerococcus and a variety of Clostridium species. The most frequent sites of infection include skin and soft tissue, pleuropulmonary and abdominal spaces, and female genital tract. Anaerobic infections are characterized by suppuration or abscess formation and tissue necrosis. When anaerobes are suspected to be the causative agent of an infection, physicians must specifically request an anaerobic culture. 

Specimens for anaerobic culture should be collected only from an acceptable site and in such a way as to avoid contamination with normal flora. This is best accomplished by aspiration with a needle and syringe or with tissue or biopsy samples. Swabs are strongly discouraged since they provide a limited quantity of specimen, allow exposure to oxygen that is lethal to anaerobes, and are often contaminated with normal flora. For these reasons, swabs should only be used during surgery and only when aspiration or biopsy is not possible. In the instance that a swab must be collected, the eSwab transport system should be utilized. 

Appropriate specimens are inoculated onto blood agar, phenylethyl alcohol agar, lysed blood agar containing gentamicin and vancomycin, and into thioglycollate broth tubes which are incubated anaerobically for 48 hours. 

Obligate anaerobic organisms may be identified by Gram stain and the use of differential media, biochemical tests, MALDI-TOF mass spectrometry, RT PCR, or 16S ribosomal RNA gene sequencing.

Reference

Procop GW et al. eds: The anaerobic bacteria. In: Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Wolters Kluwer Lippincott Williams and Wilkins; 2017:983-1073.