Brettanomyces or Dekkera is any one of nine different species of naturally-occurring yeast (including B. intermidious, B. lambicus, B. bruxellensis, D. bruxulensis, etc.) that can impart distinct sensory character to wine. Europeans refer to this fairly common phenomenon as Dekkera.
Some wine drinkers consider its presence a flaw, while some consider it an enhancement, depending upon the severity or concentration, as well as their level of sensory experience and taste preferences. It is quite a controversial phenomenon and the mere mention of "Brett" can bring lively and impassioned discussion to gatherings of wine drinkers.
Contributing to the controversy is Brett's potential for complexity. In very low concentrations it may be difficult to identify and can add aromatic notes that can be savory, smoky, or spicy. An increased level can be very unpleasant, even noxious and overpower any sense of freshness or fruitiness. This unpleasantness may be variously described as smelling like antiseptic, Band-Aid, rodent cage, sweaty horse, wet dog, rancid, or such. Four separate byproducts (esterases, tetrahydropyradines, volatile fatty acids, and volatile phenols) contribute to these aromas. Character and intensity may depend upon the relative concentration and mix of these compounds.
Brettanomyces can populate many winery sources, especially hard-to-clean areas, such as cooperage, must lines, and hoses. A favorite environment is oak barrels, especially if filled with wine that has high pH (3.6+) but low free sulfur levels and that contains trace amounts of residual sugar and where the barrels are stored in relatively warm conditions (above 68° F). High levels of ripeness and sugar content, combined with "minimalist" winemaking techniques may be factors contributing to a seeming rise in Brett levels in California wines.
Although old barrels are almost mythologically blamed, there are indications that new barrels are much more susceptible to Brett infection. Barrel age and hygiene, however, seem to be less essential to preventing Brett than more interventionist winemaking procedures, such as sterile filtering. Targeted use of sulfur (large additions at well-timed intervals) and diligent temperature control, along with conscientious equipment cleaning can prevent Brett from becoming established.
Because it favors an environment low in carbon dioxide and high in oxygen, wine stored in barrels following fermentation is the most susceptible, especially if the environment is warmed to promote malolactic fermentation. Brett seems also to have a "stealth mode" where it is present in wine but undetected and lying in wait until conditions are right for it to reveal its smelly presence.
Although it is slow-growing and easily dominated by other yeast species, once it takes hold, Brettanomyces can be very difficult to control. Brett can survive on ethanol, malic acid, some amino acids, as well as much higher and much lower levels of residual sugar, most conditions ineffective to supporting normal wine yeasts. This tenaciousness often infects wineries insidiously, living in empty used barrels and infecting other barrels during racking or topping operations, gradually colonizing the entire winery and growing in flavor influence from imperceptible to dominant.
Brett character is almost exclusively found in red wines, since acids necessary to form some of the indicative aromas are exclusively extracted from grape skins. Certain grape varieties (Mourvédre) have naturally higher concentrations of these precursor acids. Some famous and expensive wines have such predominant Brett aroma that this has become almost a trademark of house style, a trait of brand recognition. "If you can't beat 'em, join 'em" may be the unspoken justification for defenders of Brett character in these wines. However, it is misleading to attribute these aromas to be indicative of either vineyard or terroir.
The Aroma Dictionary has additional information and more technical explanations on Brettanomyces Character in Wine, by Richard Gawel.
On the Wine Anorak, Jaime Goode has discussions with several winemakers on the causes, effects, and impact of Brettanomyces.