, a metazoan that metabolises with hydrogen, lacking mitochondria and instead using hydrogenosomes.]] 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. Human muscle cells function as facultative anaerobes during vigorous exercise and this creates a build-up of the byproduct lactic acid in the muscles, resulting in the characteristic burning and aching sensations.
First observationIn 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.Gest, Howard. (2004) The discovery of microorganisms by Robert Hooke and Antoni van Leeuwenhoek, Fellows of the Royal Society, in: 'The Royal Society May 2004 Volume: 58 Issue: 2: pp. 12. In 1913 Martinus Beijerinck repeated Van Leeuwenhoek's experiment and identified Clostridium butyricum as a prominent anaerobic bacterium in the sealed pepper infusion tube liquid. Beijerinck commented: '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.'
ClassificationFor 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 floor.
- Aerotolerant organisms, which cannot use oxygen for growth, but tolerate its presence.
- Facultative anaerobes, which can grow without oxygen but use oxygen if it is present.