Anaerobic organism

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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:

Distribution[edit]

In human beings, anaerobic organisms are usually found in the gastrointestinal tract.[8] Some anaerobic bacteria produce clinically important toxins (e.g. tetanus).

Energy metabolism[edit]

Obligate anaerobes may use fermentation or anaerobic respiration.[citation needed] Aerotolerant organisms are strictly fermentative.[citation needed] In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen, some of them ferment; some use anaerobic respiration.[7]

Fermentation[edit]

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 + 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[edit]

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.[9]

Antibiotic potency[edit]

Certain antibiotics may or may not be effective against anaerobes or aerobes depending on the intracellular environment, cellular permeability, or enzymes produced by the organism. A few well-described mechanisms have been proposed as to why an anaerobe may or may not be susceptible to a given antibiotic; however, a few remain unclear.

Cephamycins[edit]

A specific class of β-lactam antibiotics called cephamycins (cefoxitin, cefotetan…) are particularly effective against anaerobes because of their ability to maintain structural integrity in the presence of plasmid and chromosomally-mediated β-lactamases.[10] Although cephamycins are typically effective against anaerobes, resistance can arise due to decreased cell permeability or production of different penicillin binding proteins (PBP).[11]

Metronidazole[edit]

Metronidazole, another antibiotic which has proven to be effective specifically against anaerobes, is a prodrug where its antimicrobial properties are only effective in anaerobic environments. Once inside the cell, metronidazole is metabolized and partially reduced by ferredoxin, a major component involved in the anaerobic electron transport chain.[10] Metronidazole metabolites become incorporated into cellular DNA and form unstable molecules which inhibit protein synthesis ultimately killing the cell.[12] Because metronidazole requires a reduced environment unique to anaerobes, it is not effective against aerobic bacteria.

Aminoglycosides[edit]

A major class of antibiotics, the aminoglycosides (streptomycin, kanamycin, etc.) are not effective against anaerobic bacteria because of their inability to reach the ribosome. In order to enter a cell, aminoglycosides require an energy-dependent phase using oxygen or nitrate in electron transport functions. Because anaerobes do not utilize oxygen or nitrate for electron transport mediated functions, aminoglycosides cannot enter anaerobic cells to inhibit ribosomal activity.[12]

See also[edit]

References[edit]

  1. ^ Upcroft P, Upcroft JA (2001). "Drug Targets and Mechanisms of Resistance in the Anaerobic Protozoa". Clinical Microbiology Reviews 14 (1): 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)". 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. ^ a b c Hogg, S. (2005). Essential Microbiology (1st ed.). Wiley. pp. 99–100. ISBN 0-471-49754-1. 
  8. ^ "Anaerobic bacteria – Overview". 
  9. ^ "GasPak System". Accessed May 3, 2008.
  10. ^ a b Bryan, L.E.; S.K. Kowand (1979). "Mechanism of Aminoglycoside Antibiotic Resistance in Anaerobic Bacteria: Clostridium Perfringens and Bacteroides Fragilis". Antimicrobial Agents and Chemotherapy. 1 15: 7–13. doi:10.1128/aac.15.1.7. 
  11. ^ Rasmussen, B.A; Bush, Tally (1994). Clinical Infectious Diseases 24: 110–20. 
  12. ^ a b Bosso, John A.; Randall A. Prince (1990). "Anti-Anaerobic Antimicrobial Agents: Cefoxitin, Cefotetan, Clindamycin, and Metronidazole". Texas Heart Institute Journal. 2 17: 77–85.