User:Simrankmann/Facultative anaerobic organism

Lead
"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."

Changes made to Lead:

Facultative anaerobes are versatile organisms that can adapt to many environments, generating ATP in oxygen (aerobic conditions) and absence of oxygen (anaerobic conditions). In aerobic conditions, they can use oxygen as a final electron acceptor; however they can reduce other substances such as nitrate to substitute for oxygen in anaerobic conditions. Therefore, using various electron acceptors makes these organisms metabolically flexible. Since these organisms are able to adapt to multiple environments, they also play a crucial role in antibiotic susceptibility.

Habitat
Facultative anaerobes can be found in various different habitats. Enteric bacteria can be found in the human gut, most commonly as food-borne pathogens such as gram-negative E. coli and Salmonella enterica. Not only that but many of these organisms can also be found in the soil and in wetlands, in which they can perform nitrate respiration when oxygen levels are lowered.

Aerobic Conditions
Aerobic respiration relies the presence of oxygen and the electron transfer chain (ETC). The ETC includes various protein complexes that are present in the bacteria's inner membrane, which include vital components such as dehydrogenases and electron carriers. In this mechanism, electrons pass through various complexes to release energy using a proton gradient. An essential part of this mechanism is that complex IV can oxidize cytochrome c and transfer electrons to oxygen, which is the final electron carrier. The protons generated by this step go to the ATP synthase, creating a proton gradient and generating ATP.

Anaerobic Conditions
When bacteria grows in anoxic conditions (without oxygen), it relies on the expression of terminal electron acceptors complexes to produce energy. These terminal electron acceptors are nitrate reductase, nitrite reductase or fumarate reductase.

Nitrate Reducation
When oxygen levels are depleted, many organisms can reduce nitrate into ammonium using an enzyme called nitrate reductase. Facultative anaerobic bacteria, such as Bacillus, Enterobacter, Erwinia and Pseudomonas, use nitrate as their final electron to generate ammonium, a process known as dissimilatory nitrate reduction to ammonium (DNRA). In this mechanism, there are two key steps. In the first step, nitrate is converted to nitrite. Whereas the second step is the conversion of nitrite to ammonium using pentaheme cytochrome c or nrf genes. In the end, the conversion to ammonium generates energy via electron transport phosphorylation.

Fumarate Reduction
Fumarate reductase plays a crucial role in anaerobic metabolism and is structurally similar to complex II (succinate dehydrogenase) of the ETC. The function of complex II is to convert succinate into fumarate; however, this anaerobic mechanism does the complete opposite. This reduction reaction occurs in the mitochondria, where fumarate is the terminal electron acceptor. The enzyme fumarate reductase converts fumarate to succinate to generate ATP in the anoxic conditions.

Fermentation
Facultative anaerobic organisms play a key role in the fermentation industry. An example of this is the eukaryotic, single-celled microorganism called yeast. Under aerobic conditions, it produces carbon dioxide and water. However under anaerobic conditions, it converts sugar to ethanol and carbon dioxide. This is important in the fermenting alcohols, where the yeast is kept in anaerobic conditions.

Infections & Antimicrobial Susceptibility
Since facultative anaerobes have the unique ability to adapt to both aerobic and anaerobic conditions, they are the most life-threatening pathogens. When considering infections, being able to grow in anaerobic conditions greatly benefits these organisms as they are able to thrive in organs such as the GI tract, brain, teeth, vagina, and uterus. In fact, a particular study looked at enteric gram negative facultative anaerobes response to antimicrobial treatments. They found that these bacteria have different susceptibility to antimicrobials depending on whether they are exposed to aerobic or anaerobic conditions.