Anaerobic bacteria

Anaerobic bacteria are widely distributed in nature. Many anaerobes are common soil bacteria while many others make up part of the normal flora. The sensitivity of anaerobes to oxygen may be due to several factors, including the genetic inability to make enzymes such as superoxide dismutase (SOD), catalase or various peroxidases. In the absence of these enzymes, oxygen products that include superoxide, hydroxy radical and singlet oxygen can cause damage to cellular constituents. These highly reactive oxygen products are formed as outlined below:

O2 Arrow O2-  (superoxide)
O2-  +  H2O2 Arrow OH-  +  O2  +  OH  (free hydroxy radical)
O2-  +  OH Arrow OH-  +  O2*  (singlet oxygen)

Notably, it is the superoxide moiety that is responsible for generation of the hydroxy radical and the singlet oxygen species. Therefore, aerobic bacterial species have evolved enzymes designed to eliminate superoxide (superoxide dismutase) thereby reducing the formation of the more potent species. Unfortunately, superoxide dismutase itself produces a harmful product (hydrogen peroxide) which must then be eliminated by other enzymes (catalase and peroxidase). The pathways for these enzymes are outlined below:

2  O2-  +  2  H+ Arrow O2  +  H2O2  (superoxide dismutase) 
2  H2O2 Arrow 2  H2O  +  O2  (catalase)
H2O2  +  H2R Arrow 2  H2O  +  R  (peroxidase)

Medical Relevance of Anaerobic Bacteria

The fact that most of the human normal flora is composed of anaerobic bacteria suggests that anaerobic infections might be of medical concern. Indeed, anaerobic infections can occur in a variety of body sites and involve many different genera. Most of the normal anaerobic flora are not overtly pathogenic; rather, they are considered to be opportunistic. That is, if given the opportunity, they can inflict serious and occasionally life-threatening disease. These types of infections most often occur due to trauma, injury or surgery. In general, a loss of natural barriers that introduce these bacteria into normally sterile body sites may result in infection. The sites commonly involved in anaerobic infection include the following:

  1. intraabdominal infections
  2. pulmonary infections
  3. pelvic infections
  4. brain abscesses
  5. skin and soft tissue
  6. oral and dental infections
  7. bacteremia and endocarditis


The diagnosis of an anaerobic infection is based primarily on symptoms, the patient's medical history, and location of the infection. A foul-smelling infection or drainage from an abscess is helpful in the diagnosis of an anaerobic infection. This foul smell is produced by anaerobic bacteria and occurs in one third to one half of patients late in the infection. Other clues to anaerobic infection include tissue necrosis and gas production at the infection site. A sample from the infected site may be obtained, using a swab or a needle and syringe, to determine which bacteria are causing the infection. Because these bacteria can be easily killed by oxygen, they rarely grow in the laboratory cultures of tissue or pus samples. The recent medical history of the patient is helpful in diagnosing an anaerobic infection. A patient who has or recently had surgery, dental work, tumors, blood vessel disease, or injury are susceptible to these types of infection. Diagnostic tests may include blood tests to see if bacteria are in the bloodstream, and X-rays to look at internal infections.


Treatment of these infections can sometimes be difficult but, generally, moderate to broad spectrum antibiotics are usually effective.

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