Anaerobic organism

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenated environment. Anaerobes may be unicellular (e.g. protozoans,[1] bacteria[2]) or multicellular.[3]

Spinoloricus nov. sp., a metazoan that metabolises with hydrogen, lacking mitochondria and instead using hydrogenosomes.

First observation

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.[4]

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.' [4]

Classification
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.
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 adenosine triphosphate (ATP) than either fermentation or anaerobic respiration.
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.
5: Aerotolerant organisms do not require oxygen as they metabolise energy anaerobically. Unlike obligate anaerobes however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube.

For practical purposes, there are three categories of anaerobe:

  • Obligate anaerobes, which are harmed by the presence of oxygen.[5][6] 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.[7]
  • Facultative anaerobes, which can grow without oxygen but use oxygen if it is present.[7]

However, this classification has been questioned by the fact that recent research showed that human "obligate anaerobes" (such as Fineglodia magna or the methanogenic archaea Methanobrevibacter smithii) can be grown in aerobic atmosphere if the culture medium is supplemented with antioxidants such as ascorbic acid, glutathione and uric acid.[8][9][10][11]

Energy metabolism

Some obligate anaerobes use fermentation, while others use anaerobic respiration.[12] Aerotolerant organisms are strictly fermentative.[13] In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen, some of them ferment; some use anaerobic respiration.[7]

Fermentation

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) 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 methanogenesis.

Culturing anaerobes

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. 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.[14]

Recently, a French team evidenced a link between redox and gut anaerobes [15] based on clinical studies on severe acute malnutrition.[16] These findings led to the development of aerobic culture of "anaerobes" by the addition of antioxidants in the culture medium.[17]

Multicellularity

Except for three species of anaerobic loricifera, all known complex multicellular life is aerobic, i.e. needs oxygen to survive. In February 2020, the journal PNAS published the discovery of a new 10-celled anaerobic organism: Henneguya salminicola Henneguya zschokkei.[18]

In 2010 three species of anaerobic loricifera were discovered in the hypersaline anoxic L'Atalante basin at the bottom of the Mediterranean Sea. They lack mitochondria which contain the oxidative phosphorylation pathway, which in all other animals combines oxygen with glucose to produce metabolic energy, and thus they consume no oxygen. Instead these loricifera derive their energy from hydrogen using hydrogenosomes.[19][3]

Some organisms metabolise primarily using glycogen, for example the Nereid (worm)s and some polychaetes,[20] or the juvenile form of the pork parasite Trichinella spiralis which causes trichinosis.[21]

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References
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  3. Danovaro R; Dell'anno A; Pusceddu A; Gambi C; et al. (April 2010). "The first metazoa living in permanently anoxic conditions". BMC Biology. 8 (1): 30. doi:10.1186/1741-7007-8-30. PMC 2907586. PMID 20370908.
  4. 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.
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  8. La Scola, B.; Khelaifia, S.; Lagier, J.-C.; Raoult, D. (2014). "Aerobic culture of anaerobic bacteria using antioxidants: a preliminary report". European Journal of Clinical Microbiology & Infectious Diseases. 33 (10): 1781–1783. doi:10.1007/s10096-014-2137-4. ISSN 0934-9723. PMID 24820294.
  9. Dione, N.; Khelaifia, S.; La Scola, B.; Lagier, J.C.; Raoult, D. (2016). "A quasi-universal medium to break the aerobic/anaerobic bacterial culture dichotomy in clinical microbiology". Clinical Microbiology and Infection. 22 (1): 53–58. doi:10.1016/j.cmi.2015.10.032. PMID 26577141.
  10. Khelaifia, S.; Lagier, J.-C.; Nkamga, V. D.; Guilhot, E.; Drancourt, M.; Raoult, D. (2016). "Aerobic culture of methanogenic archaea without an external source of hydrogen". European Journal of Clinical Microbiology & Infectious Diseases. 35 (6): 985–991. doi:10.1007/s10096-016-2627-7. ISSN 0934-9723.
  11. Traore, S.I.; Khelaifia, S.; Armstrong, N.; Lagier, J.C.; Raoult, D. (2019). "Isolation and culture of Methanobrevibacter smithii by co-culture with hydrogen-producing bacteria on agar plates". Clinical Microbiology and Infection. 25 (12): 1561.e1–1561.e5. doi:10.1016/j.cmi.2019.04.008. PMID 30986553.
  12. Pommerville, Jeffrey (2010). Alcamo's Fundamentals of Microbiology. Jones and Bartlett Publishers. p. 177. ISBN 9781449655822.
  13. Slonim, Anthony; Pollack, Murray (2006). Pediatric Critical Care Medicine. Lippincott Williams & Wilkins. p. 130. ISBN 9780781794695.
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  18. Scientists discovered the first animal that doesn't need oxygen to live
  19. Oxygen-Free Animals Discovered-A First, National Geographic news
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