Aerobic fermentation

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Aerobic fermentation or aerobic glycolysis is a metabolic process by which cells metabolize sugars via fermentation in the presence of oxygen and occurs through the repression of normal respiratory metabolism. Preference of aerobic fermentation over aerobic respiration is referred to as the Crabtree effect in yeast, [1] [2] and is part of the Warburg effect in tumor cells. While aerobic fermentation does not produce adenosine triphosphate (ATP) in high yield, it allows proliferating cells to convert nutrients such as glucose and glutamine more efficiently into biomass by avoiding unnecessary catabolic oxidation of such nutrients into carbon dioxide, preserving carbon-carbon bonds and promoting anabolism. [3]

Contents

Aerobic fermentation in yeast

Aerobic fermentation evolved independently in at least three yeast lineages ( Saccharomyces , Dekkera , Schizosaccharomyces ). [4] It has also been observed in plant pollen, [5] trypanosomatids, [6] mutated E. coli, [7] and tumor cells. [8] Crabtree-positive yeasts will respire when grown with very low concentrations of glucose or when grown on most other carbohydrate sources. [1] The Crabtree effect is a regulatory system whereby respiration is repressed by fermentation, except in low sugar conditions. [1] When Saccharomyces cerevisiae is grown below the sugar threshold and undergoes a respiration metabolism, the fermentation pathway is still fully expressed, [9] while the respiration pathway is only expressed relative to the sugar availability. [4] [10] This contrasts with the Pasteur effect, which is the inhibition of fermentation in the presence of oxygen and observed in most organisms. [9]

The evolution of aerobic fermentation likely involved multiple successive molecular steps, [9] which included the expansion of hexose transporter genes, [11] copy number variation (CNV) [12] [13] and differential expression in metabolic genes, and regulatory reprogramming. [14] Research is still needed to fully understand the genomic basis of this complex phenomenon. Many Crabtree-positive yeast species are used for their fermentation ability in industrial processes in the production of wine, beer, sake, bread, and bioethanol. [15] Through domestication, these yeast species have evolved, often through artificial selection, to better fit their environment. [15] Strains evolved through mechanisms that include interspecific hybridization, [15] horizontal gene transfer (HGT), gene duplication, pseudogenization, and gene loss. [16]

Origin of Crabtree effect in yeast

Approximately 100 million years ago (mya), within the yeast lineage there was a whole genome duplication (WGD). [17] A majority of Crabtree-positive yeasts are post-WGD yeasts. [4] It was believed that the WGD was a mechanism for the development of the Crabtree effect in these species due to the duplication of alcohol dehydrogenase (ADH) encoding genes and hexose transporters. [2] However, recent evidence has shown that aerobic fermentation originated before the WGD and evolved as a multi-step process, potentially aided by the WGD. [2] The origin of aerobic fermentation, or the first step, in Saccharomyces Crabtree-positive yeasts likely occurred in the interval between the ability to grow under anaerobic conditions, horizontal transfer of anaerobic DHODase (encoded by URA1 with bacteria), and the loss of respiratory chain Complex I. [9] A more pronounced Crabtree effect, the second step, likely occurred near the time of the WGD event. [9] Later evolutionary events that aided in the evolution of aerobic fermentation are better understood and outlined in the section discussing the genomic basis of the Crabtree effect.

Driving forces

It is believed that a major driving force in the origin of aerobic fermentation was its simultaneous origin with modern fruit (~125 mya). [2] These fruits provided an abundance of simple sugar food source for microbial communities, including both yeast and bacteria. [2] Bacteria, at that time, were able to produce biomass at a faster rate than the yeast. [2] Producing a toxic compound, like ethanol, can slow the growth of bacteria, allowing the yeast to be more competitive. [2] However, the yeast still had to use a portion of the sugar it consumes to produce ethanol. [2] Crabtree-positive yeasts also have increased glycolytic flow, or increased uptake of glucose and conversion to pyruvate, which compensates for using a portion of the glucose to produce ethanol rather than biomass. [9] Therefore, it is believed that the original driving force was to kill competitors. [4] This is supported by research that determined the kinetic behavior of the ancestral ADH protein, which was found to be optimized to make ethanol, rather than consume it. [13]

Further evolutionary events in the development of aerobic fermentation likely increased the efficiency of this lifestyle, including increased tolerance to ethanol and the repression of the respiratory pathway. [4] In high sugar environments, S. cerevisiae outcompetes and dominants all other yeast species, except its closest relative Saccharomyces paradoxus . [18] The ability of S. cerevisiae to dominate in high sugar environments evolved more recently than aerobic fermentation and is dependent on the type of high-sugar environment. [18] Other yeasts' growth is dependent on the pH and nutrients of the high-sugar environment. [18]

Genomic basis of the Crabtree effect

The genomic basis of the Crabtree effect is still being investigated, and its evolution likely involved multiple successive molecular steps that increased the efficiency of the lifestyle.

Expansion of hexose transporter genes

Hexose transporters (HXT) are a group of proteins that are largely responsible for the uptake of glucose in yeast. In S. cerevisiae, 20 HXT genes have been identified and 17 encode for glucose transporters (HXT1-HXT17), GAL2 encodes for a galactose transporter, and SNF3 and RGT2 encode for glucose sensors. [19] The number of glucose sensor genes have remained mostly consistent through the budding yeast lineage, however glucose sensors are absent from Schizosaccharomyces pombe . Sch. pombe is a Crabtree-positive yeast, which developed aerobic fermentation independently from Saccharomyces lineage, and detects glucose via the cAMP-signaling pathway. [20] The number of transporter genes vary significantly between yeast species and has continually increased during the evolution of the S. cerevisiae lineage. Most of the transporter genes have been generated by tandem duplication, rather than from the WGD. Sch. pombe also has a high number of transporter genes compared to its close relatives. [11] Glucose uptake is believed to be a major rate-limiting step in glycolysis and replacing S. cerevisiae's HXT1-17 genes with a single chimera HXT gene results in decreased ethanol production or fully respiratory metabolism. [12] Thus, having an efficient glucose uptake system appears to be essential to ability of aerobic fermentation. [20] There is a significant positive correlation between the number of hexose transporter genes and the efficiency of ethanol production. [11]

CNV in glycolysis genes

A scheme of transformation of glucose to alcohol by alcoholic fermentation. Ethanol Fermentation english.png
A scheme of transformation of glucose to alcohol by alcoholic fermentation.

After a WGD, one of the duplicated gene pair is often lost through fractionation; less than 10% of WGD gene pairs have remained in S. cerevisiae genome. [12] A little over half of WGD gene pairs in the glycolysis reaction pathway were retained in post-WGD species, significantly higher than the overall retention rate. [12] This has been associated with an increased ability to metabolize glucose into pyruvate, or higher rate of glycolysis. [17] After glycolysis, pyruvate can either be further broken down by pyruvate decarboxylase (Pdc) or pyruvate dehydrogenase (Pdh). The kinetics of the enzymes are such that when pyruvate concentrations are high, due to a high rate of glycolysis, there is increased flux through Pdc and thus the fermentation pathway. [12] The WGD is believed to have played a beneficial role in the evolution of the Crabtree effect in post-WGD species partially due to this increase in copy number of glycolysis genes. [20]

CNV in fermentation genes

The fermentation reaction only involves two steps. Pyruvate is converted to acetaldehyde by Pdc and then acetaldehyde is converted to ethanol by alcohol dehydrogenase (Adh). There is no significant increase in the number of Pdc genes in Crabtree-positive compared to Crabtree-negative species and no correlation between number of Pdc genes and efficiency of fermentation. [20] There are five Adh genes in S. cerevisiae. [20] Adh1 is the major enzyme responsible for catalyzing the fermentation step from acetaldehyde to ethanol. [13] Adh2 catalyzes the reverse reaction, consuming ethanol and converting it to acetaldehyde. [13] The ancestral, or original, Adh had a similar function as Adh1 and after a duplication in this gene, Adh2 evolved a lower KM for ethanol. [13] Adh2 is believed to have increased yeast species' tolerance for ethanol and allowed Crabtree-positive species to consume the ethanol they produced after depleting sugars. [13] However, Adh2 and consumption of ethanol is not essential for aerobic fermentation. [13] Sch. pombe and other Crabtree positive species do not have the ADH2 gene and consumes ethanol very poorly. [13]

Differential expression

In Crabtree-negative species, respiration related genes are highly expressed in the presence of oxygen. However, when S. cerevisiae is grown on glucose in aerobic conditions, respiration-related gene expression is repressed. Mitochondrial ribosomal proteins expression is only induced under environmental stress conditions, specifically low glucose availability. [20] Genes involving mitochondrial energy generation and phosphorylation oxidation, which are involved in respiration, have the largest expression difference between aerobic fermentative yeast species and respiratory species. [20] In a comparative analysis between Sch. pombe and S. cerevisiae, both of which evolved aerobic fermentation independently, the expression pattern of these two fermentative yeasts were more similar to each other than a respiratory yeast, C. albicans. However, S. cerevisiae is evolutionarily closer to C. albicans. [14] Regulatory rewiring was likely important in the evolution of aerobic fermentation in both lineages. [20]

Domestication and aerobic fermentation

A close up picture of ripening wine grapes. The light white "dusting" is a film that also contains wild yeasts. Wine Grapes with dusting that contains yeast.jpg
A close up picture of ripening wine grapes. The light white "dusting" is a film that also contains wild yeasts.

Aerobic fermentation is essential for multiple industries, resulting in human domestication of several yeast strains. Beer and other alcoholic beverages, throughout human history, have played a significant role in society through drinking rituals, providing nutrition, medicine, and uncontaminated water. [15] [21] During the domestication process, organisms shift from natural environments that are more variable and complex to simple and stable environments with a constant substrate. This often favors specialization adaptations in domesticated microbes, associated with relaxed selection for non-useful genes in alternative metabolic strategies or pathogenicity. [16] Domestication might be partially responsible for the traits that promote aerobic fermentation in industrial species. Introgression and HGT is common in Saccharomyces domesticated strains. [16] Many commercial wine strains have significant portions of their DNA derived from HGT of non-Saccharomyces species. HGT and introgression are less common in nature than is seen during domestication pressures. [16] For example, the important industrial yeast strain Saccharomyces pastorianus is an interspecies hybrid of S. cerevisiae and the cold tolerant S. eubayanus. [15] This hybrid is commonly used in lager-brewing, which requires slow, low temperature fermentation. [15]

Aerobic fermentation in acetic acid bacteria

Acetic acid bacteria (AAB) incompletely oxidize sugars and alcohols, usually glucose and ethanol, to acetic acid, in a process called AAB oxidative fermentation (AOF). After glycolysis, the produced pyruvate is broken down to acetaldehyde by pyruvate decarboxylase, which in turn is oxidized to acetic acid by acetaldehyde dehydrogenase. Ethanol is first oxidized to acetaldehyde by alcohol dehydrogenase, which is then converted to acetic acid. Both of these processes either generate NAD(P)H, or shuttle electrons into the electron transport chain via ubiquinol. [22] This process is exploited in the use of acetic acid bacteria to produce vinegar.

Tumor cells

One of the hallmarks of cancer is altered metabolism or deregulating cellular energetics. [23] Cancers cells often have reprogrammed their glucose metabolism to perform lactic acid fermentation, in the presence of oxygen, rather than send the pyruvate made through glycolysis to the mitochondria. This is referred to as the Warburg effect and is associated with high consumption of glucose and a high rate of glycolysis. [24] ATP production in these cancer cells is often only through the process of glycolysis and pyruvate is broken down by the fermentation process in the cell's cytoplasm.

This phenomenon is often seen as counterintuitive, since cancer cells have higher energy demands due to the continued proliferation and respiration produces significantly more ATP than glycolysis alone (fermentation produces no additional ATP). Typically, there is an up-regulation in glucose transporters and enzymes in the glycolysis pathway (also seen in yeast). [25] There are many parallel aspects of aerobic fermentation in tumor cells that are also seen in Crabtree-positive yeasts. Further research into the evolution of aerobic fermentation in yeast such as S. cerevisiae can be a useful model for understanding aerobic fermentation in tumor cells. This has a potential for better understanding cancer and cancer treatments. [8]

Aerobic fermentation in other non-yeast species

Plants

Alcoholic fermentation is often used by plants in anaerobic conditions to produce ATP and regenerate NAD+ to allow for glycolysis to continue. For most plant tissues, fermentation only occurs in anaerobic conditions, but there are a few exceptions. In the pollen of maize (Zea mays) [26] and tobacco (Nicotiana tabacum & Nicotiana plumbaginifolia), the fermentation enzyme ADH is abundant, regardless of the oxygen level. In tobacco pollen, PDC is also highly expressed in this tissue and transcript levels are not influenced by oxygen concentration. Tobacco pollen, similar to Crabtree-positive yeast, perform high levels of fermentation dependent on the sugar supply, and not oxygen availability. In these tissues, respiration and alcoholic fermentation occur simultaneously with high sugar availability. [5] Fermentation produces the toxic acetaldehyde and ethanol, that can build up in large quantities during pollen development. It has been hypothesized that acetaldehyde is a pollen factor that causes cytoplasmic male sterility. Cytoplasmic male sterility is a trait observed in maize, tobacco and other plants in which there is an inability to produce viable pollen. It is believed that this trait might be due to the expression of the fermentation genes, ADH and PDC, a lot earlier on in pollen development than normal and the accumulation of toxic aldehyde. [5]

Trypanosomatids

When grown in glucose-rich media, trypanosomatid parasites degrade glucose via aerobic fermentation. [6] In this group, this phenomenon is not a pre-adaptation to/or remnant of anaerobic life, shown through their inability to survive in anaerobic conditions. [27] It is believed that this phenomenon developed due to the capacity for a high glycolytic flux and the high glucose concentrations of their natural environment. The mechanism for repression of respiration in these conditions is not yet known. [27]

E. coli mutants

A couple of Escherichia coli mutant strains have been bioengineered to ferment glucose under aerobic conditions. [7] One group developed the ECOM3 (E. coli cytochrome oxidase mutant) strain by removing three terminal cytochrome oxidases (cydAB, cyoABCD, and cbdAB) to reduce oxygen uptake. [7] After 60 days of adaptive evolution on glucose media, the strain displayed a mixed phenotype. [7] In aerobic conditions, some populations' fermentation solely produced lactate, while others performed mixed-acid fermentation. [7]

Myc and HIF-1 regulate glucose metabolism and stimulate the Warburg effect. Warburgh Effect.gif
Myc and HIF-1 regulate glucose metabolism and stimulate the Warburg effect.

Related Research Articles

<span class="mw-page-title-main">Glycolysis</span> Catabolic pathway

Glycolysis is the metabolic pathway that converts glucose into pyruvate, and in most organisms, occurs in the liquid part of cells, the cytosol. 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.

<span class="mw-page-title-main">Yeast</span> Informal group of fungi

Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitute 1% of all described fungal species.

<span class="mw-page-title-main">Phosphorylation</span> Chemical process of introducing a phosphate

In biochemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology. Protein phosphorylation often activates many enzymes.

Pyruvic acid (IUPAC name: 2-oxopropanoic acid, also called acetoic acid) (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.

<span class="mw-page-title-main">Alcohol dehydrogenase</span> Group of dehydrogenase enzymes

Alcohol dehydrogenases (ADH) (EC 1.1.1.1) are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to NADH. In humans and many other animals, they serve to break down alcohols that are otherwise toxic, and they also participate in the generation of useful aldehyde, ketone, or alcohol groups during the biosynthesis of various metabolites. In yeast, plants, and many bacteria, some alcohol dehydrogenases catalyze the opposite reaction as part of fermentation to ensure a constant supply of NAD+.

<span class="mw-page-title-main">Cellular respiration</span> Process to convert glucose to ATP in cells

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.

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available. Anaerobic glycolysis is an effective means of energy production only during short, intense exercise, providing energy for a period ranging from 10 seconds to 2 minutes. This is much faster than aerobic metabolism. The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules). The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.

Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

<span class="mw-page-title-main">Ethanol fermentation</span> Biological process that produces ethanol and carbon dioxide as by-products

Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish where it provides energy when oxygen is scarce.

In oncology, the Warburg effect is the observation that most cancer cells release energy predominantly not through the 'usual' citric acid cycle and oxidative phosphorylation in the mitochondria as observed in normal cells, but through a less efficient process of 'aerobic glycolysis' consisting of a high level of glucose uptake and glycolysis followed by lactic acid fermentation taking place in the cytosol, not the mitochondria, even in the presence of abundant oxygen. This observation was first published by Otto Heinrich Warburg, who was awarded the 1931 Nobel Prize in Physiology for his "discovery of the nature and mode of action of the respiratory enzyme". The precise mechanism and therapeutic implications of the Warburg effect, however, remain unclear.

<span class="mw-page-title-main">Mixed acid fermentation</span> Biochemical conversion of six-carbon sugars into acids in bacteria

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.

<span class="mw-page-title-main">Fermentation</span> Metabolic process

Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

<span class="mw-page-title-main">Warburg hypothesis</span> Hypothesis explaining cancer

The Warburg hypothesis, sometimes known as the Warburg theory of cancer, postulates that the driver of tumorigenesis is an insufficient cellular respiration caused by insult to mitochondria. The term Warburg effect in oncology describes the observation that cancer cells, and many cells grown in vitro, exhibit glucose fermentation even when enough oxygen is present to properly respire. In other words, instead of fully respiring in the presence of adequate oxygen, cancer cells ferment. The Warburg hypothesis was that the Warburg effect was the root cause of cancer. The current popular opinion is that cancer cells ferment glucose while keeping up the same level of respiration that was present before the process of carcinogenesis, and thus the Warburg effect would be defined as the observation that cancer cells exhibit glycolysis with lactate production and mitochondrial respiration even in the presence of oxygen.

<i>Kluyveromyces marxianus</i> Species of fungus

Kluyveromyces marxianus in ascomycetous yeast and member of the genus, Kluyveromyces. It is the sexual stage of Atelosaccharomyces pseudotropicalis also known as Candida kefyr. This species has a homothallic mating system and is often isolated from dairy products.

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). More generally, in the medical literature, the Pasteur effect refers to how the cellular presence of oxygen causes in cells a decrease in the rate of glycolysis and also a suppression of lactate accumulation. The effect occurs in animal tissues, as well as in microorganisms belonging to the fungal kingdom.

The Crabtree effect, named after the English biochemist Herbert Grace Crabtree, describes the phenomenon whereby the yeast, Saccharomyces cerevisiae, produces ethanol (alcohol) in aerobic conditions at high external glucose concentrations rather than producing biomass via the tricarboxylic acid (TCA) cycle, the usual process occurring aerobically in most yeasts e.g. Kluyveromyces spp. This phenomenon is observed in most species of the Saccharomyces, Schizosaccharomyces, Debaryomyces, Brettanomyces, Torulopsis, Nematospora, and Nadsonia genera. Increasing concentrations of glucose accelerates glycolysis which results in the production of appreciable amounts of ATP through substrate-level phosphorylation. This reduces the need of oxidative phosphorylation done by the TCA cycle via the electron transport chain and therefore decreases oxygen consumption. The phenomenon is believed to have evolved as a competition mechanism around the time when the first fruits on Earth fell from the trees. The Crabtree effect works by repressing respiration by the fermentation pathway, dependent on the substrate.

Cellular waste products are formed as a by-product of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP. One example of cellular respiration creating cellular waste products are aerobic respiration and anaerobic respiration.

Scheffersomyces stipitis is a species of yeast, belonging to the "CUG Clade" of ascomycetous yeasts. This is a group of fungi that substitute serine for leucine when the CUG codon is encountered. S. stipitis is distantly related to brewer's yeast, Saccharomyces cerevisiae, which uses the conventional codon system. Found, among other places, in the guts of passalid beetles, S. stipitis is capable of both aerobic and oxygen limited fermentation, and has the highest known natural ability of any yeast to directly ferment xylose, converting it to ethanol, a potentially economically valuable trait. Xylose is a hemicellulosic sugar found in all angiosperm plants. As such xylose constitutes the second most abundant carbohydrate moiety in nature. Xylose can be produced from wood or agricultural residues through auto- or acid hydrolysis. Ethanol production from such lignocellulosic residues does not compete with food production through the consumption of grain.

<span class="mw-page-title-main">Yeast in winemaking</span> Yeasts used for alcoholic fermentation of wine

The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy or neutral spirits to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients and the wine has not yet reached dryness, this is considered a stuck fermentation.

<span class="mw-page-title-main">Auto-brewery syndrome</span> Medical condition

Auto-brewery syndrome(ABS) (also known as gut fermentation syndrome, endogenous ethanol fermentation or drunkenness disease) is a condition characterized by the fermentation of ingested carbohydrates in the gastrointestinal tract of the body caused by bacteria or fungi. ABS is a rare medical condition in which intoxicating quantities of ethanol are produced through endogenous fermentation within the digestive system. The organisms responsible for ABS include various yeasts and bacteria, including Saccharomyces cerevisiae, S. boulardii, Candida albicans, C. tropicalis, C. krusei, C. glabrata, C. kefyr, C. parapsilosis, Klebsiella pneumoniae, and Enterococcus faecium. These organisms use lactic acid fermentation or mixed acid fermentation pathways to produce an ethanol end product. The ethanol generated from these pathways is absorbed in the small intestine, causing an increase in blood alcohol concentrations that produce the effects of intoxication without the consumption of alcohol.

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