Butyrate is produced by several fermentation processes performed by obligate anaerobic bacteria. This fermentation pathway was discovered by Louis Pasteur in 1861. Examples of butyrate-producing species of bacteria:
*Clostridium butyricum
*Clostridium kluyveri
*Clostridium pasteurianum
*Faecalibacterium prausnitzii
*Fusobacterium nucleatum
*Butyrivibrio fibrisolvens
*Eubacterium limosum
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 () and two molecules of hydrogen () are formed as waste products. Subsequently,  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
Other pathways to butyrate include succinate reduction and crotonate disproportionation.
Several species form acetone and n-butanol in an alternative pathway, which starts as butyrate fermentation. Some of these species are:
*Clostridium acetobutylicum, the most prominent acetone and butanol producer, used also in industry
*Clostridium beijerinckii
*Clostridium tetanomorphum
*Clostridium aurantibutyricum
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
For commercial purposes Clostridium species are used preferably for butyric acid or butanol production.
The most common species used for probiotics is the Clostridium butyricum.