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		O2-  (superoxide)
O2-  +  H2O2		OH-  +  O2  +  OH•  (free hydroxy radical)
O2-  +  OH•		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+		O2  +  H2O2  (superoxide dismutase)
2  H2O2		2  H2O  +  O2  (catalase)
H2O2  +  H2R		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

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