typically oxidize ethanol to form acetic acid (vinegar) and are used for commercial vinegar production. This family of Gram-negative bacteria (Acetobacteriaceae) encompasses some 10 genera of which the most common are Acetobacter, Gluconobacter, and Gluconacetobacter. See gram stain. These bacteria generate energy by the incomplete oxidation of sugars, alcohols, and other carbon compounds to produce organic acids as end products. With ethanol as a carbon source, they accumulate acetic acid. Upon prolonged incubation, most can “overoxidize” the acetate to produce water and carbon dioxide (Gluconobacter strains are unable to further metabolize the acetate). Acetic acid bacteria must have a generous supply of oxygen and cannot grow in its absence. Colonies of acetic acid bacteria can be detected on culture media containing insoluble calcium carbonate and ethanol as carbon source. The acid produced by the bacteria dissolves the chalk, leaving a clear zone around the colony.

Acetic acid bacteria are widely distributed, particularly on plants and fruits and in the air, and will cause severe spoilage of beer if allowed access in the presence of oxygen. Because they are acid- and ethanol-tolerant and not inhibited by hop compounds, they grow rapidly in beer, producing acid off-flavors and turbidity. However, as long as the beer is properly stored and oxygen levels are low, acetic acid bacteria are not a problem. It is for this reason that they most commonly cause problems in cask-conditioned beer during dispense.

See also acetic acidandbacteria.