Obligate anaerobes are microorganisms that are killed by normal atmospheric concentrations of oxygen (21% O2). Oxygen tolerance varies between species, some capable of surviving in up to 8% oxygen, others losing viability unless the oxygen concentration is less than 0.5%. An important distinction needs to be made here between the obligate anaerobes and the microaerophiles. Microaerophiles, like the obligate anaerobes, are damaged by normal atmospheric concentrations of oxygen. However, microaerophiles metabolise energy aerobically, and obligate anaerobes metabolise energy anaerobically. Microaerophiles therefore require oxygen (typically 2-10% O2) for growth. Obligate anaerobes do not.
Historically, it was widely accepted that obligate (strict) anaerobes die in presence of oxygen due to the absence of the enzymes superoxide dismutase and catalase, which would convert the lethal superoxide formed in their cells due to the presence of oxygen. While this is true in some cases, these enzyme activities have been identified in some obligate anaerobes, and genes for these enzymes and related proteins have been found in their genomes, such as Clostridium butyricum and Methanosarcina barkeri, among others. However, these organisms are still incapable of growing in the presence of oxygen. There are several hypotheses addressing why strict anaerobes are sensitive to oxygen:
- Dissolved oxygen increases the redox potential of a solution, and high redox potential inhibits the growth of some strict anaerobes. For example, methanogens grow at a redox potential lower than -0.3 V.
- Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form disulfide, thus inactivating certain enzymes. Organisms may not be able to grow with these essential enzymes deactivated.
- Growth may be inhibited due to a lack of reducing equivalents for biosynthesis, because electrons are exhausted in reducing oxygen.
It is most likely a combination of these mechanisms that accounts for oxygen sensitivity in obligate anaerobes.
Obligate anaerobes metabolise energy by fermentation or anaerobic respiration. In anaerobic respiration, an electron acceptor other than oxygen is used. Examples include sulfate, nitrate, iron, manganese, mercury, and carbon monoxide. The energy yield of these processes (ie. the number of ATP molecules generated) is less than in aerobic respiration.
Bacteroides and Clostridium species are examples of non-spore forming and spore-forming strict anaerobes, respectively. Other obligate anaerobes include Peptostreptococcus, Fusobacterium, Porphyromonas, Prevotella, Veillonella, Propionibacterium, and Actinomyces.
- Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 309–326, 378–384. ISBN 0-8385-8529-9.
- Kim, Byung Hong and Geoffrey Michael Gadd. Bacterial Physiology and Metabolism. Cambridge University Press, Cambridge, UK. 2008.
- "ANAEROBIC BACILLI". IndState.edu. Archived from the original on 29 January 2009.