300px|thumb|Aerobic and anaerobic [[bacteria can be identified by growing them in test tubes of thioglycollate broth: <br /> 1: Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest. <br /> 2: Obligate anaerobes are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest. <br /> 3: Facultative anaerobes can grow with or without oxygen because they can metabolise energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more ATP than either fermentation or anaerobic respiration. <br /> 4: Microaerophiles need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top. <br /> 5: Aerotolerant organisms do not require oxygen and cannot utilise it even if present; they metabolise energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube.<br /> Both facultative anaerobes and aerotolerant organisms will undergo fermentation in the absence of oxygen, but the facultative anaerobes will switch to aerobic metabolism when oxygen is present (a phenomenon known as the Pasteur effect). The Pasteur effect is sometimes used to distinguish between facultative anaerobes and aerotolerant organisms, in the lab.]]
Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O<sub>2</sub>). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environments with an oxygen concentration greater than 0.5%.
Obligate anaerobes, which die when normal amounts of oxygen are present, are contrasted with obligate aerobes, which die without oxygen. Bacteria that fall in between these two extremes may be classified as either facultative anaerobes, which can use oxygen but also survive without it, or microaerophiles, which need lower levels of oxygen. Aerotolerant organisms are indifferent to the presence or absence of oxygen.
Oxygen sensitivity
The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors including oxidative stress and enzyme production. Oxygen can also damage obligate anaerobes in ways not involving oxidative stress.
Because molecular oxygen contains two unpaired electrons in the highest occupied molecular orbital, it is readily reduced to superoxide () and hydrogen peroxide () within cells. Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known as reactive oxygen species (ROS), highly reactant products that are damaging to microbes, including obligate anaerobes. The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O<sub>2</sub>) is thought to reflect the quantity of superoxide dismutase and catalase being produced. In the absence of oxygen, the mutated samples grew normally. For example, methanogens grow at a redox potential lower than -0.3 V.
Fermentation differs from anaerobic respiration in that the pyruvate generated from glycolysis is broken down without the involvement of an electron transport chain (i.e. there is no oxidative phosphorylation). Numerous fermentation pathways exist such as lactic acid fermentation, mixed acid fermentation, 2-3 butanediol fermentation where organic compounds are reduced to organic acids and alcohol. Obligate anaerobes are also found in the digestive tracts of humans and other animals as well as in the first stomach of ruminants.
Examples of obligately anaerobic fungal genera include the rumen fungi Neocallimastix, Piromonas, and Sphaeromonas.
See also
- Aerobic respiration
- Fermentation