Acetic acid bacteria
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|Acetic acid bacteria|
Acetic acid bacteria (AAB) derive their energy from the oxidation of ethanol to acetic acid during fermentation. They are Gram-negative, aerobic, rod-shaped bacteria. They are not to be confused with the genus Acetobacterium, which are anaerobic homoacetogenic facultative autotrophs and can reduce carbon dioxide to produce acetic acid, for example, Acetobacterium woodii.
The acetic acid bacteria are usually airborne and are ubiquitous in nature. They are actively present in environments where ethanol is being formed as a result of fermentation of sugars. They can be isolated from the nectar of flowers and from damaged fruit. Other good sources are fresh apple cider and unpasteurized beer that has not been filter sterilized. In these liquids, they grow as a surface film due to their aerobic nature and active motility. Vinegar is produced when acetic acid bacteria act on alcoholic beverages such as wine. Fruit flies or vinegar eels are considered as a common vector in propagating acetic acid bacteria in nature.
Some genera, such as Acetobacter, can eventually oxidize acetic acid to carbon dioxide and water using Krebs cycle enzymes. Other genera, such as Gluconobacter, do not further oxidize acetic acid, as they do not have a full set of Krebs cycle enzymes.
As these bacteria produce acid, they are usually acid-tolerant, growing well below pH 5.0, although the pH optimum for growth is 5.4-6.3.
Acetobacter is a genus of acetic acid bacteria characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. Several species are in this genus, and other bacteria are capable of forming acetic acid under various conditions, but all of the Acetobacter species are known by this characteristic ability.
Acetobacter is of particular importance commercially, because some species are used in the production of vinegar (intentionally converting the ethanol in wine to acetic acid), and they can destroy wine which they infect by producing excessive amounts of acetic acid or ethyl acetate, both of which can render the wine unpalatable. Acetobacter species are also used to intentionally acidify beer during long maturation periods in the production of traditional Flemish sour ales.
The growth of Acetobacter in wine can be suppressed through effective sanitation, by complete exclusion of air from wine in storage, and by the use of moderate amounts of sulfur dioxide in the wine as a preservative. Acetobacter species can be easily distinguished in the laboratory by the growth of colonies on a medium containing about 7% ethanol, and enough calcium carbonate to render it partially opaque. When Acetobacter colonies form enough acetic acid from the ethanol, the calcium carbonate around the colonies dissolves, forming a very distinct clear zone.
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