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Aerobic and anaerobic bacteria can be identified by growing them in liquid culture:
1: Obligate aerobic (oxygen-needing) bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen.
2: Obligate anaerobic bacteria gather at the bottom to avoid oxygen.
3: Facultative bacteria gather mostly at the top, since aerobic respiration is the most beneficial one; but as lack of oxygen does not hurt them, they can be found all along the test tube.
4: Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen but at a low concentration.
5: Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth, could possibly react negatively and may even die in its presence. There are three types: obligate anaerobes, which cannot use oxygen for growth and are even harmed by it; aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it; and facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present.



Obligate anaerobes may use fermentation or anaerobic respiration.
Aerotolerant organisms are strictly fermentative.
In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen some of them ferment, some use anaerobic respiration.



There are many anaerobic fermentative reactions.

Fermentative anaerobic organisms mostly use the lactic acid fermentation pathway:

C6H12O6 + 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP

The energy released in this equation is approximately 150 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose. This is only 5% of the energy per sugar molecule that the typical aerobic reaction generates.

Plants and fungi (e.g., yeasts) generally use alcohol (ethanol) fermentation when oxygen becomes limiting:

C6H12O6 + 2 ADP + 2 phosphate → 2 C2H5OH + 2 CO2 + 2 ATP

The energy released is about 180 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose.

Anaerobic bacteria and archaea use these and many other fermentative pathways, e.g., propionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis or methanogenesis.

Some anaerobic bacteria produce toxins (e.g., tetanus or botulinum toxins) that are highly dangerous to higher organisms, including humans.

Culturing anaerobes

Given that normal microbial culturing is undertaken in an aerobic environment, the culturing of anaerobes poses a problem. To overcome this, a number of techniques are employed by microbiologists. One way required the injection of the bacteria into a Dicot. The Dicot would then provide an environment without oxygen thus ensuring the survival of the anaerobes. The GasPak System is an isolated container which achieves an anaerobic environment by the reaction of water with sodium borohydride and sodium bicarbonate tablets to produce hydrogen gas and carbon dioxide. Hydrogen then reacts with oxygen gas on a palladium catalyst to produce more water, thereby removing oxygen gas. The issue with the Gaspak method is that an adverse reaction can take place where the bacteria may die which is why a thioglycollate medium should be used. The Thioglycollate supplies a medium mimicking that of a Dicot thus providing not only an anaerobic environment but all the nutrients needed for the bacteria to thrive.[1]


  1. ^ "GasPak System". Accessed May 3, 2008.

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