An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It may react negatively or even die if oxygen is present. (In contrast, an aerobic organism (aerobe) is an organism that can survive and grow in an oxygenated environment.) An anaerobic organism may be unicellular (e.g. protozoans, bacteria) or multicellular. For practical purposes, there are three categories of anaerobe: obligate anaerobes, which are harmed by the presence of oxygen; aerotolerant organisms, which cannot use oxygen for growth but tolerate its presence; and facultative anaerobes, which can grow without oxygen but use oxygen if it is present.
1 First observation 2 Classification 3 Energy metabolism
4 Culturing anaerobes 5 Multicellularity 6 References
In his letter of 14 June 1680 to The Royal Society, Antonie van
Leeuwenhoek described an experiment he carried out by filling two
identical glass tubes about halfway with crushed pepper powder, to
which some clean rain water was added. Van Leeuwenhoek sealed one of
the glass tubes by using a flame and left the other glass tube open.
Several days later, he discovered in the open glass tube ‘a great
many very little animalcules, of divers sort having its own particular
motion.’ Not expecting to see any life in the sealed glass tube, Van
Leeuwenhoek saw to his surprise ‘a kind of living animalcules that
were round and bigger than the biggest sort that I have said were in
the other water.’ The conditions in the sealed tube had become quite
anaerobic owing to consumption of oxygen by aerobic microorganisms.
'We thus come to the remarkable conclusion that, beyond doubt, Van Leeuwenhoek in his experiment with the fully closed tube had cultivated and seen genuine anaerobic bacteria, which would happen again only after 200 years, namely about 1862 by Pasteur. That Leeuwenhoek, one hundred years before the discovery of oxygen and the composition of air, was not aware of the meaning of his observations is understandable. But the fact that in the closed tube he observed an increased gas pressure caused by fermentative bacteria and in addition saw the bacteria, prove in any case that he not only was a good observer, but also was able to design an experiment from which a conclusion could be drawn.' 
Aerobic and anaerobic bacteria can be identified by growing them in
test tubes of thioglycollate broth:
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.
2: Obligate anaerobes are poisoned by oxygen, so they gather at the
bottom of the tube where the oxygen concentration is lowest.
For practical purposes, there are three categories of anaerobe:
Obligate anaerobes, which are harmed by the presence of oxygen.
Two examples of obligate anaerobes are Clostridium botulinum and the
bacteria which live near hydrothermal vents on the deep-sea ocean
Some obligate anaerobes use fermentation, while others use anaerobic
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) in general use alcohol (ethanol) fermentation when oxygen becomes limiting:
C6H12O6 (glucose) + 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
Since normal microbial culturing occurs in atmospheric air, which is
an aerobic environment, the culturing of anaerobes poses a problem.
Therefore, a number of techniques are employed by microbiologists when
culturing anaerobic organisms, for example, handling the bacteria in a
glovebox filled with nitrogen or the use of other specially sealed
containers, or techniques such as injection of the bacteria into a
dicot plant, which is an environment with limited oxygen. The GasPak
System is an isolated container that achieves an anaerobic environment
by the reaction of water with sodium borohydride and sodium
bicarbonate tablets to produce hydrogen gas and carbon dioxide.
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^ a b Danovaro R; Dell'anno A; Pusceddu A; Gambi C; et al. (April
2010). "The first metazoa living in permanently anoxic conditions".
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^ a b Gest, Howard. (2004) The discovery of microorganisms by Robert
Hooke and Antoni van Leeuwenhoek, Fellows of the Royal Society, in: