By Dr. M. M. Chengappa and Amy Burklund
Anaerobic bacteria are defined as bacteria that do not grow in the presence of oxygen. Anaerobes vary greatly in their sensitivity to oxygen. Aerotolerant anaerobes can grow on the surface of agar plates with low levels of oxygen, whereas obligate anaerobes die, or their growth is inhibited almost immediately when exposed to oxygen.
Obligate anaerobes are unable to initiate growth in a culture medium unless the medium is anaerobic, i.e., the oxidation-reduction (redox) potential (Eh) of the medium is low (negative (Eh) and the incubation is under an oxygen-free incubator. The redox potential provides a useful scale for measuring the degree of anaerobiosis. Generally, anaerobic bacteria do not grow if the Eh is above -100 mV. Normal body tissue has a positive Eh (+120 to +240 mV); however, decrease in blood flow would result in a reduction in the Eh to that tissue. Thus, abscesses have an Eh of -20 to +40 mV, whereas necrotic abscesses have an Eh of -250 to -150 mV. Therefore, anaerobes are more often associated with necrotic abscesses. Aerotolerant anaerobes can grow on agar medium, if incubation is under anaerobic condition. Anaerobic bacteria are often overlooked or missed unless the specimen is properly collected and transported to the laboratory where appropriate anaerobic culture methods are employed for isolation.
Collection and transport of samples
Aspirates, swabs and tissue samples are the preferred specimens for anaerobic culture. Some anaerobes, particularly some of the gram negative, nonspore-formers, are very sensitive to oxygen. Special commercial sample collection and transport systems are available for the submission of materials suspected of containing diagnostically significant anaerobes. Swabs should be protected from drying and exposure to oxygen or oxidizing conditions. Small tissue samples should be processed immediately or placed in commercial anaerobic transport containers. For large tissue specimens, maintenance of anaerobic conditions is not critical.
Clostridial Infections (Blackleg, Malignant Edema): Affected tissue or aspirate from the affected tissue are the best samples.
Necrotic enteritis or Enterotoxemia (Clostridial infection): Place several ounces of fresh small intestinal contents in a jar or plastic bag or tie off a section of the affected intestine. Submit ASAP on ice.
Clinical conditions suggestive of anaerobic infections are:
- Deep tissue wounds
- Draining tracts
- Pleural, pericardial, and peritoneal effusions
Specific infections where anaerobes are involved are:
- Clostridium spp. – A variety of species and disease in all animals
- Dichelobacter nodosus – Foot rot in sheep
- Fusobacterium necrophorum – Calf diphtheria, liver abscesses and foot rot in cattle
- Actinomyces suis – Urinary tract infections in sows.
Nonspecific infections where anaerobes are involved.
Generally caused by nonspore forming, gram negative rods and cocci and are often polymicrobial (mixed) infections.
Specimens or conditions where anaerobic culture is not advised.
Sites that contain a population of anaerobic bacteria as part of the normal flora are often unrewarding (e.g., feces, vagina, distal urethra, and oral cavity) unless the diagnostician is looking for a specific species or type of anaerobic organism. Anaerobic culture of the urinary tract is not routinely performed because the recovery of these microorganisms from this site is extremely rare.
Dr. Chengappa is the KSVDL bacteriology section head and Amy Burkland is the bacteriology section administrator.
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