Obligate anaerobe

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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 and cannot utilise it even if present; they metabolise energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube. Both facultative anaerobes and aerotolerant organisms will undergo fermentation in the absence of oxygen, but the facultative anaerobes will switch to aerobic metabolism when oxygen is present (a phenomenon known as the Pasteur effect). The Pasteur effect is sometimes used to distinguish between facultative anaerobes and aerotolerant organisms, in the lab. Anaerobic.png
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 and cannot utilise it even if present; they metabolise energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube.
Both facultative anaerobes and aerotolerant organisms will undergo fermentation in the absence of oxygen, but the facultative anaerobes will switch to aerobic metabolism when oxygen is present (a phenomenon known as the Pasteur effect). The Pasteur effect is sometimes used to distinguish between facultative anaerobes and aerotolerant organisms, in the lab.

Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). [1] [2] Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environments with an oxygen concentration greater than 0.5%. [3]

Contents

Oxygen sensitivity

The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors including oxidative stress and enzyme production. Oxygen can also damage obligate anaerobes in ways not involving oxidative stress.[ citation needed ]

Because molecular oxygen contains two unpaired electrons in the highest occupied molecular orbital, it is readily reduced to superoxide (O−
2) and hydrogen peroxide (H
2O
2) within cells. [1] A reaction between these two products results in the formation of a free hydroxyl radical (OH.). [4] Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known as reactive oxygen species (ROS), highly reactant products that are damaging to microbes, including obligate anaerobes. [4] Aerobic organisms produce superoxide dismutase and catalase to detoxify these products, but obligate anaerobes produce these enzymes in very small quantities, or not at all. [1] [2] [3] [5] (The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O2) is thought to reflect the quantity of superoxide dismutase and catalase being produced. [2] [3] ).

Carlioz and Touati (1986) performed experiments which support the idea that reactive oxygen species may be toxic to anaerobes. E. coli, a facultative anaerobe, was mutated by a deletion of superoxide dismutase genes. In the presence of oxygen, this mutation resulted in the inability to properly synthesize certain amino acids or use common carbon sources as substrates during metabolism. [6] In the absence of oxygen, the mutated samples grew normally. [6] In 2018, Lu et al. found that in Bacteroides thetaiotaomicron, an obligate anaerobe found in the mammalian digestive tract, exposure to oxygen results in increased levels of superoxide which inactivated important metabolic enzymes. [6]

Dissolved oxygen increases the redox potential of a solution, and high redox potential inhibits the growth of some obligate anaerobes. [3] [5] [7] For example, methanogens grow at a redox potential lower than -0.3 V. [7] Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form disulfide, thus inactivating certain enzymes (e.g. nitrogenase). Organisms may not be able to grow with these essential enzymes deactivated. [1] [5] [7] Growth may also be inhibited due to a lack of reducing equivalents for biosynthesis because electrons are exhausted in reducing oxygen. [7]

Energy metabolism

Obligate anaerobes convert nutrients into energy through anaerobic respiration or fermentation. In aerobic respiration, the pyruvate generated from glycolysis is converted to acetyl-CoA. This is then broken down via the TCA cycle and electron transport chain. Anaerobic respiration differs from aerobic respiration in that it uses an electron acceptor other than oxygen in the electron transport chain. Examples of alternative electron acceptors include sulfate, nitrate, iron, manganese, mercury, and carbon monoxide. [8]

Fermentation differs from anaerobic respiration in that the pyruvate generated from glycolysis is broken down without the involvement of an electron transport chain (i.e. there is no oxidative phosphorylation). Numerous fermentation pathways exist such as lactic acid fermentation, mixed acid fermentation, 2-3 butanediol fermentation where organic compounds are reduced to organic acids and alcohol. [8] [4]

The energy yield of anaerobic respiration and fermentation (i.e. the number of ATP molecules generated) is less than in aerobic respiration. [8] This is why facultative anaerobes, which can metabolise energy both aerobically and anaerobically, preferentially metabolise energy aerobically. This is observable when facultative anaerobes are cultured in thioglycolate broth. [1]

Ecology and Examples

Obligate anaerobes are found in oxygen-free environments such as the intestinal tracts of animals, the deep ocean, still waters, landfills, in deep sediments of soil. [9] Examples of obligately anaerobic bacterial genera include Actinomyces , Bacteroides , Clostridium , Fusobacterium , Peptostreptococcus , Porphyromonas , Prevotella , Propionibacterium , and Veillonella . Clostridium species are endospore-forming bacteria, and can survive in atmospheric concentrations of oxygen in this dormant form. The remaining bacteria listed do not form endospores. [5]

Several species of the Mycobacterium, Streptomyces, and Rhodococcus genera are examples of obligate anaerobe found in soil. [10] Obligate anaerobes are also found in the digestive tracts of humans and other animals as well as in the first stomach of ruminants. [11]

Examples of obligately anaerobic fungal genera include the rumen fungi Neocallimastix , Piromonas , and Sphaeromonas . [12]

See also

Related Research Articles

<span class="mw-page-title-main">Obligate aerobe</span>

An obligate aerobe is an organism that requires oxygen to grow. Through cellular respiration, these organisms use oxygen to metabolise substances, like sugars or fats, to obtain energy. In this type of respiration, oxygen serves as the terminal electron acceptor for the electron transport chain. Aerobic respiration has the advantage of yielding more energy than fermentation or anaerobic respiration, but obligate aerobes are subject to high levels of oxidative stress.

<span class="mw-page-title-main">Electron transport chain</span> Cellular electron transfer

An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. The electrons that transferred from NADH and FADH2 to the ETC involves 4 multi-subunit large enzymes complexes and 2 mobile electron carriers. Many of the enzymes in the electron transport chain are membrane-bound.

<span class="mw-page-title-main">Aerobic organism</span> Organism that thrives in an oxygenated environment

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. In contrast, an anaerobic organism (anaerobe) is any organism that does not require oxygen for growth. Some anaerobes react negatively or even die if oxygen is present. The ability to exhibit aerobic respiration may yield benefits to the aerobic organism, as aerobic respiration yields more energy than anaerobic respiration. In July 2020, marine biologists reported that aerobic microorganisms (mainly), in "quasi-suspended animation", were found in organically-poor sediments, up to 101.5 million years old, 250 feet below the seafloor in the South Pacific Gyre (SPG), and could be the longest-living life forms ever found.

An anaerobic organism or anaerobe is any organism that does not require molecular 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 or multicellular. Most fungi are obligate aerobes, requiring oxygen to survive. However, some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen will disrupt their metabolism or kill them. Deep waters of the ocean are a common anoxic environment.

<i>Clostridium</i> Genus of Gram-positive bacterium, which includes several significant human pathogens

Clostridium is a genus of anaerobic, Gram-positive bacterium. Species of Clostridium inhabit soils and the intestinal tract of animals, including humans. This genus includes several significant human pathogens, including the causative agents of botulism and tetanus. It also formerly included an important cause of diarrhea, Clostridioides difficile, which was reclassified into the Clostridioides genus in 2016.

Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.

<span class="mw-page-title-main">Facultative anaerobic organism</span> Beings that can respire with and without oxygen

A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent.

<span class="mw-page-title-main">Microaerophile</span>

A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O2; typically 2–10% O2) for optimal growth. A more restrictive interpretation requires the microorganism to be obligate in this requirement. Many microaerophiles are also capnophiles, requiring an elevated concentration of carbon dioxide (e.g. 10% CO2 in the case of Campylobacter species).

In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can be applied to any desymmetrizing reaction of the following type, regardless of whether it is a redox or some other type of process:

<span class="mw-page-title-main">Capnophile</span> Microorganism that flourishes in a carbon dioxide rich environment

Capnophiles are microorganisms that thrive in the presence of high concentrations of carbon dioxide.

<span class="mw-page-title-main">Aerotolerant anaerobe</span>

Aerotolerant anaerobes use fermentation to produce ATP. They do not use oxygen, but they can protect themselves from reactive oxygen molecules. In contrast, obligate anaerobes can be harmed by reactive oxygen molecules.

<i>Bacteroides</i> Genus of bacteria

Bacteroides is a genus of Gram-negative, obligate anaerobic bacteria. Bacteroides species are non endospore-forming bacilli, and may be either motile or nonmotile, depending on the species. The DNA base composition is 40–48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids. They also contain meso-diaminopimelic acid in their peptidoglycan layer.

<i>Bacteroides fragilis</i> Species of bacterium

Bacteroides fragilis is an anaerobic, Gram-negative, pleomorphic to rod-shaped bacterium. It is part of the normal microbiota of the human colon and is generally commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, or trauma.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

The Pasteur effect describes how available oxygen inhibits ethanol fermentation, driving yeast to switch toward aerobic respiration for increased generation of the energy carrier adenosine triphosphate (ATP).

Hydrogen-oxidizing bacteria are a group of facultative autotrophs that can use hydrogen as an electron donor. They can be divided into aerobes and anaerobes. The former use hydrogen as an electron donor and oxygen as an acceptor while the latter use sulphate or nitrogen dioxide as electron acceptors. Species of both types have been isolated from a variety of environments, including fresh waters, sediments, soils, activated sludge, hot springs, hydrothermal vents and percolating water.

Geobacillus thermoglucosidasius is a thermophilic gram-positive bacterium, and a member of the Bacillota phylum. It was first isolated from soil in Japan in 1983.

<span class="mw-page-title-main">Fumarate reductase (quinol)</span>

Fumarate reductase (quinol) is an enzyme with systematic name succinate:quinone oxidoreductase. This enzyme catalyzes the following chemical reaction:

<span class="mw-page-title-main">Nickel superoxide dismutase</span>

Nickel superoxide dismutase (Ni-SOD) is a metalloenzyme that, like the other superoxide dismutases, protects cells from oxidative damage by catalyzing the disproportionation of the cytotoxic superoxide radical to hydrogen peroxide and molecular oxygen. Superoxide is a reactive oxygen species that is produced in large amounts during photosynthesis and aerobic cellular respiration. The equation for the disproportionation of superoxide is shown below:

Dissimilatory metal-reducing microorganisms are a group of microorganisms (both bacteria and archaea) that can perform anaerobic respiration utilizing a metal as terminal electron acceptor rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration. The most common metals used for this end are iron [Fe(III)] and manganese [Mn(IV)], which are reduced to Fe(II) and Mn(II) respectively, and most microorganisms that reduce Fe(III) can reduce Mn(IV) as well. But other metals and metalloids are also used as terminal electron acceptors, such as vanadium [V(V)], chromium [Cr(VI)], molybdenum [Mo(VI)], cobalt [Co(III)], palladium [Pd(II)], gold [Au(III)], and mercury [Hg(II)].

References

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  3. 1 2 3 4 Ryan KJ; Ray CG, eds. (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 309–326, 378–384. ISBN   0-8385-8529-9.
  4. 1 2 3 Hentges, David J. (1996), Baron, Samuel (ed.), "Anaerobes: General Characteristics", Medical Microbiology (4th ed.), Galveston (TX): University of Texas Medical Branch at Galveston, ISBN   978-0-9631172-1-2, PMID   21413255 , retrieved 2021-04-26
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  6. 1 2 3 Lu, Zheng; Sethu, Ramakrishnan; Imlay, James A. (2018-04-03). "Endogenous superoxide is a key effector of the oxygen sensitivity of a model obligate anaerobe". Proceedings of the National Academy of Sciences of the United States of America. 115 (14): E3266–E3275. doi: 10.1073/pnas.1800120115 . ISSN   0027-8424. PMC   5889672 . PMID   29559534.
  7. 1 2 3 4 Kim BH, Gadd GM (2008). Bacterial Physiology and Metabolism.
  8. 1 2 3 Hogg, S. (2005). Essential Microbiology (1st ed.). Wiley. pp. 99–100, 118–148. ISBN   0-471-49754-1.
  9. ↑ "Oxygen Requirements for Microbial Growth | Microbiology". courses.lumenlearning.com. Retrieved 2021-05-08.
  10. ↑ Berney, Michael et al. “An obligately aerobic soil bacterium activates fermentative hydrogen production to survive reductive stress during hypoxia.” Proceedings of the National Academy of Sciences of the United States of America vol. 111,31 (2014): 11479-84. doi:10.1073/pnas.1407034111
  11. ↑ Ueki, Atsuko; Kaku, Nobuo; Ueki, Katsuji (2018-08-01). "Role of anaerobic bacteria in biological soil disinfestation for elimination of soil-borne plant pathogens in agriculture". Applied Microbiology and Biotechnology. 102 (15): 6309–6318. doi:10.1007/s00253-018-9119-x. ISSN   1432-0614. PMID   29858952. S2CID   44123873.
  12. ↑ Carlile MJ, Watkinson SC (1994). The Fungi. Academic Press. pp. 33–34. ISBN   0-12-159960-4.
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