Aerobic and anaerobic bacteria can be identified by growing them in liquid culture:
1. Obligate aerobe
2. Obligate anaerobe
3. Facultative anaerobic organism (continuum with "Facultative aerobic organism")
4. Microaerophile

Obligate anaerobes are anaerobic organisms that fail to grow in the presence of oxygen.^[1]

[edit] Metabolism

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:

1. 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.

2. 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 without these deactivated enzymes.

3. 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.^[2]

Instead of oxygen, obligate anaerobes use alternate electron acceptors for cellular respiration such as sulfate, nitrate, iron, manganese, mercury, and carbon monoxide. The energy yield of these respiratory processes is less than oxygen respiration, and not all of these electron acceptors are created equal.

• The most favorable (after oxygen) is sulfate. In marine sediments this leads to large amounts of sulfate reduction, which most of us are familiar with as the rotten egg smell and black material that can be found just a few centimeters below the sediment surface.
• Next in line is nitrate.
• Then the metal ions.
• Lastly a zone of methanogenesis is found. Very little energy is obtained from methanogenesis and vast amounts of substrate need to be turned over to make a living.

[edit] Examples

Bacteroides and Clostridium species are examples of non-spore forming and spore-forming strict anaerobes, respectively.

Other obligate anaerobes include Peptostreptococcus, Veillonella,^[3] and Actinomyces.

[edit] References