Anaerobic organism

Aerobic and anaerobic bacteria can be identified by growing them in liquid culture:
1: Obligate aerobic (oxygen-needing) bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen.
2: Obligate anaerobic bacteria gather at the bottom to avoid oxygen.
3: Facultative bacteria gather mostly at the top, since aerobic respiration is the most beneficial one; but, as lack of oxygen does not hurt them, they can be found all along the test tube.
4: Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen but at a low concentration.
5: Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present. Most such species are unicellular microbes, though some are near-microscopic metazoa and some deep-sea worms. Some largely unicellular anaerobic microbes are protists, but most of the anaerobic microbes are bacteria or Archaea. For practical purposes there are three categories:

• obligate anaerobes, which cannot use oxygen for growth and are even harmed by it
• aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it
• facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present

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

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.

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 + 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.

Antibiotic Potency

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

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. Although cephamycins are typically effective against anaerobes, resistance can arise due to decreased cell permeability or production of different penicillin binding proteins (PBP).

Metronidazole

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. Metronidazole metabolites become incorporated into cellular DNA and form unstable molecules which inhibit protein synthesis ultimately killing the cell. Because metronidazole requires a reduced environment unique to anaerobes, it is not effective against aerobic bacteria.

Aminoglycosides

A major class of antibiotics, the aminoglycosides (streptomycin, kanamycin…) 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.

See also

• Aerobic organism
• Anaerobic infection
• Anaerobic digestion
• Biogas
• Digester
• Facultative anaerobic organism
• Hypoxia (environmental)
• Fermentation (biochemistry)
• Waste management
• Oxygen catastrophe
• Spinoloricus Cinzia