Anaerobic organism

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 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.

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. An anaerobic organism may be unicellular (e.g. protozoans,[1] bacteria[2]) or multicellular (e.g. Nereid (worm) polychaetes,[3] juvenile Trichinella spiralis (pork worm) parasites[4]). For practical purposes, there are three categories of anaerobe:

Energy metabolism

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.[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.[8]

References

  1. ↑ Upcroft P, Upcroft JA. "Drug Targets and Mechanisms of Resistance in". pp. 150–164. doi:10.1128/CMR.14.1.150-164.2001. PMC 88967. PMID 11148007.
  2. ↑ Levinson, W. (2010). Review of Medical Microbiology and Immunology (11th ed.). McGraw-Hill. pp. 91–93. ISBN 978-0-07-174268-9.
  3. ↑ Schöttler, U. (November 30, 1979). "On the Anaerobic Metabolism of Three Species of Nereis (Annelida)" (PDF). Marine Ecology Progress Series 1: 249–54. doi:10.3354/meps001249. ISSN 1616-1599. Retrieved February 14, 2010.
  4. ↑ Roberts, Larry S., John Janovay (2005). Foundations of Parasitology (7th ed.). New York: McGraw-Hill. pp. 405–407.
  5. ↑ Prescott LM, Harley JP, Klein DA (1996). Microbiology (3rd ed.). Wm. C. Brown Publishers. pp. 130–131. ISBN 0-697-29390-4.
  6. ↑ Brooks GF, Carroll KC, Butel JS, Morse SA (2007). Jawetz, Melnick & Adelberg's Medical Microbiology (24th ed.). McGraw Hill. pp. 307–312. ISBN 0-07-128735-3.
  7. ↑ 7.0 7.1 7.2 Hogg, S. (2005). Essential Microbiology (1st ed.). Wiley. pp. 99–100. ISBN 0-471-49754-1.
  8. ↑ "GasPak System". Accessed May 3, 2008.