Brettanomyces is any one of nine different species of naturally-occurring yeast, and is often colloquially referred to as 'Brett'. In the wild, Brettanomyces lives on the skins of fruit. The term Brettanomyces comes from the Greek for "British fungus" - due to investigating it as a cause of spoilage in English ales.
When Brettanomyces grows in wine it produces several compounds that can alter the palate and bouquet. At low levels some winemakers argue that the presence has a positive effect on wine, contributing to complexity, and giving an aged character to young red wines. Many wines even rely on Brett to give their distinctive character, however when the levels of the sensory compounds exceed the sensory threshold, their perception is almost always negative.
As Brett can potentially spoil a wine it is generally seen in wine as a wine fault. Wines that have been contaminated with Brettanomyces taints are often referred to as "Bretty", "mousy", "metallic", or as having "Brett character".
Brett is typically isolated from barrel aged red wines - this is because red wines are far higher in polyphenol content, and generally have a higher pH, both factors which encourages development, but Brett has also been be found in some white wines. In some cases the yeast has caused contamination in sparkling wines produced by the Methode champenoise.
It is thought Brett can be introduced to a winery by insects such as fruit flies, or by purchasing Brett contaminated wine barrels. Once the yeast is in a winery it is hard to remove and is spread readily by unsanitised equipment. Brett is best controlled by the addition of sulphur dioxide to which the yeast is particularly sensitive. Alternatively the wine can be bottled after sterile filtration, which physically removes the yeast. Wines that are vinified to low residual sugar levels, such as <1.0g/L, are also less likely to be spoiled as the main growth substrate has been limited.