Butyrate fermentation is a process that produces butyric acid via anaerobic bacteria. This process occurs commonly in clostridia which can be isolated from many anaerobic environments such as mud, fermented foods, and intestinal tracts or feces.[1] Clostridium can ferment carbohydrates into butyric acid, producing byproducts including hydrogen gas, carbon dioxide, and acetate. Butyrate fermentation is currently being utilized in the production of a variety of biochemicals and biofuels.
Butyrate in humans originates from the anaerobic microbes that ferment dietary fibers in the lower intestinal tract. Butyrate plays an important role in immune and inflammatory responses, as well as the formation of the intestinal barrier. The presence of short-chain fatty acids lowers the pH of the gut allowing optimal growth for butyrate-producing bacteria. The two major metabolic pathways used for butyrate fermentation are butyryl-CoAphosphorylation and acetate CoA transferase.
Microbial Biosynthesis
One pathway for butyrate biosynthesis. Relevant enzymes: acetoacetyl-CoA thiolase, NAD- and NADP-dependent 3-hydroxybutyryl-CoA dehydrogenase, 3-hydroxybutyryl-CoA dehydratase, and NAD-dependent butyryl-CoA dehydrogenase.
Butyrate is produced by several fermentation processes performed by obligateanaerobicbacteria.[2] This fermentation pathway was discovered by Louis Pasteur in 1861.[1] Examples of butyrate-producing species of bacteria include:
The pathway starts with the glycolytic cleavage of glucose to two molecules of pyruvate, as happens in most organisms. Pyruvate is oxidized into acetyl coenzyme A catalyzed by pyruvate:ferredoxin oxidoreductase. Two molecules of carbon dioxide (CO2) and two molecules of hydrogen (H2) are formed as waste products. Subsequently, ATP is produced in the last step of the fermentation. Three molecules of ATP are produced for each glucose molecule, a relatively high yield. The balanced equation for this fermentation is
C6H12O6 → C4H8O2 + 2CO2 + 2H2
Other pathways to butyrate include succinate reduction and crotonate disproportionation.
These bacteria begin with butyrate fermentation, as described above, but, when the pH drops below 5, they switch into butanol and acetone production to prevent further lowering of the pH. Two molecules of butanol are formed for each molecule of acetone.
The change in the pathway occurs after acetoacetyl CoA formation. This intermediate then takes two possible pathways:
acetoacetyl CoA → acetoacetate → acetone
acetoacetyl CoA → butyryl CoA → butyraldehyde → butanol
Butyrate can be produced by dietary fibers through two different metabolic pathways. The first metabolic pathway is, butyryl-CoA is phosphorylated to form butyryl-phosphorylated to form butyryl-phosphate and transformed to butyrate via butyrate kinase. The second pathway, the CoA part of butyryl-CoA is transferred to acetate via butyryl-CoA: acetate CoA-transferase, leading to the formation of butyrate and acetyl-CoA. These metabolic pathways are how the butyrate is produced.[3]
Applications for Commercial Use
For commercial purposes Clostridium species are used preferably for butyric acid or butanol production. Butyric acid that is produced via butyrate fermentation is a common food additive and found within products including butter, milk, cheese, and vegetable oils. Some species within the genus Clostridium are capable of producing biochemicals and biofuels. This fermentation process is able to produce acetone, butanol, and ethanol and is one of the first commercial fermentation processes used for bulk chemical production. This species has also been used in therapy, research, and even cosmetics (such as perfumes). It has also been applied to bioprocesses such as in the manufacturing of yogurt, with the most common species used for probiotics being Clostridium butyricum.[4]
Roles in Metabolism
Butyrate, one of the main products from gut microbial fermentation, plays many metabolic roles in the homeostasis of the human body. Butyrate is found to increase energy expenditure to counteract High Fat Diet (HFD) obesity. This is due to butyrate activating thermogenesis, which is a function in adipose tissue to dispel chemical energy by uncoupling protein to energy usage and body temperature. Butyrate also promotes fatty acid oxidation and decreases HFD-induced triglycerides elevation and reduces the respiratory exchange ratio. In metabolic disorders, such as obesity and diabetes, there is a dysfunction in glucose homeostasis due to the decrease in insulin sensitivity and pancreatic β cell dysfunction, which can lead to reduced insulin secretion. Butyrate is shown to help the regulation of glucose homeostasis by improving pancreatic β cell development and improving insulin sensitivity. It is also shown that children with β cell autoimmunity, there is a low abundance of butyrate-producing intestinal bacteria.[5]
Inflammation of The Gut
When butyrate is present in the intestine, IFN-γ, TNF-α, IL-6, and IL-8 are inhibited. These are proinflammatory cytokines which increase inflammation and can cause tissue destruction. Butyrate is also capable of inducing IL-10 and TGF-β which are anti-inflammatory cytokines. Short-chain fatty acids are capable of modifying neutrophil recruitment, which improves immune response. This shows clinical significance in inflammatory bowel disease due to its chronic inflammatory nature. In inflammatory bowel disease, it is seen that there is a reduction of butyrate-producing bacteria which greatly diminishes the defense mechanisms of the mucosal barrier of the gut.[6]
1 2 White, David; Drummond, James; Fuqua, Clay (2012). The physiology and biochemistry of prokaryotes (4thed.). New York: Oxford University Press. ISBN978-0-19-539304-0.
Glycolysis is the metabolic pathway that converts glucose into pyruvate and, in most organisms, occurs in the liquid part of cells. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.
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. The sea floor is possibly one of the largest accumulation of anaerobic organisms on Earth, where microbes are primarily concentrated around hydrothermal vents. These microbes produce energy in absence of sunlight or oxygen through a process called chemosynthesis, whereby inorganic compounds such as hydrogen gas, hydrogen sulfide or ferrous ions are converted into organic matter.
Cellular respiration is the process by which biological fuels are oxidized in the presence of an inorganic electron acceptor, such as oxygen, to drive the bulk production of adenosine triphosphate (ATP), which contains energy. Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP, and then release waste products.
Acetyl-CoA is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production.
Butyric acid, also known under the systematic name butanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula CH3CH2CH2CO2H. It is an oily, colorless liquid with an unpleasant odor. Isobutyric acid is an isomer. Salts and esters of butyric acid are known as butyrates or butanoates. The acid does not occur widely in nature, but its esters are widespread. It is a common industrial chemical and an important component in the mammalian gut.
Gluconeogenesis (GNG) is a metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia). In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
Clostridium is a genus of anaerobic, Gram-positive bacteria. Species of Clostridium inhabit soils and the intestinal tracts 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.
Acidogenesis is the second stage in the four stages of anaerobic digestion:
Acetogenesis is a process through which acetyl-CoA or acetic acid is produced by prokaryote microorganisms either by the reduction of CO2 or by the reduction of organic acids, rather than by the oxidative breakdown of carbohydrates or ethanol, as with acetic acid bacteria. Acetyl-CoA can be incorporated into biomass or converted to acetic acid.
In biochemistry, mixed acid fermentation is the metabolic process by which a six-carbon sugar is converted into a complex and variable mixture of acids. It is an anaerobic (non-oxygen-requiring) fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.
Butanol may be used as a fuel in an internal combustion engine. It is more similar to gasoline than it is to ethanol. A C4-hydrocarbon, butanol is a drop-in fuel and thus works in vehicles designed for use with gasoline without modification. Both n-butanol and isobutanol have been studied as possible fuels. Both can be produced from biomass (as "biobutanol" ) as well as from fossil fuels (as "petrobutanol"). The chemical properties depend on the isomer (n-butanol or isobutanol), not on the production method.
Fermentation is a type of redox metabolism carried out in the absence of oxygen. During fermentation, organic molecules are catabolized and donate electrons to other organic molecules. In the process, ATP and organic end products are formed.
Acetone–butanol–ethanol (ABE) fermentation, also known as the Weizmann process, is a process that uses bacterial fermentation to produce acetone, n-butanol, and ethanol from carbohydrates such as starch and glucose. It was developed by chemist Chaim Weizmann and was the primary process used to produce acetone, which was needed to make cordite, a substance essential for the British war industry during World War I.
Butyryl-CoA is an organic coenzyme A-containing derivative of butyric acid. It is a natural product found in many biological pathways, such as fatty acid metabolism, fermentation, and 4-aminobutanoate (GABA) degradation. It mostly participates as an intermediate, a precursor to and converted from crotonyl-CoA. This interconversion is mediated by butyryl-CoA dehydrogenase.
In enzymology, a butyrate kinase is an enzyme that catalyzes the chemical reaction
Faecalibacterium is a genus of bacteria. The genus contains several species including Faecalibacterium prausnitzii, Faecalibacterium butyricigenerans, Faecalibacterium longum, Faecalibacterium duncaniae, Faecalibacterium hattorii, and Faecalibacterium gallinarum. Its first known species, Faecalibacterium prausnitzii is gram-positive, mesophilic, rod-shaped, and anaerobic, and is one of the most abundant and important commensal bacteria of the human gut microbiota. It is non-spore forming and non-motile. These bacteria produce butyrate and other short-chain fatty acids through the fermentation of dietary fiber. The production of butyrate makes them an important member of the gut microbiota, fighting against inflammation.
Clostridium butyricum is a strictly anaerobic endospore-forming Gram-positive butyric acid–producing bacillus subsisting by means of fermentation using an intracellularly accumulated amylopectin-like α-polyglucan (granulose) as a substrate. It is uncommonly reported as a human pathogen and is widely used as a probiotic in Japan, Korea, and China. C. butyricum is a soil inhabitant in various parts of the world, has been cultured from the stool of healthy children and adults, and is common in soured milk and cheeses. The connection with dairy products is shown by the name, the butyr- in butyricum reflects the relevance of butyric acid in the bacteria's metabolism and the connection with Latin butyrum and Greek βούτυρον, with word roots pertaining to butter and cheese.
Elizabeth McCoy was an American microbiologist and a professor at the University of Wisconsin–Madison.
Solventogenesis is the biochemical production of solvents by Clostridium species. It is the second phase of ABE fermentation.
Caproate fermentation is a metabolic process used by different bacteria to utilize different organic substrates for the production of caproic acid as well as other valuable byproducts. Caproic acid is a valuable compound in food industries as a flavor additive, feedstock for chemical industries, antimicrobial agents in the pharmaceutical industry, and more. Though this process is used by varying bacterial species, the most common species utilizing caproate fermentation in its metabolic process is Clostridium kluyveri. This species, as well as others, utilize caproate fermentation through the breakdown of varying substrates for energy production, waste management and increased ability for survival in different environments.
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