Brettanomyces is a non-spore forming genus of yeast in the family Saccharomycetaceae, and is often colloquially referred to as "Brett". The genus name Dekkera is used interchangeably with Brettanomyces, as it describes the teleomorph or spore forming form of the yeast. The cellular morphology of the yeast can vary from ovoid to long "sausage" shaped cells. The yeast is acidogenic and when grown on glucose rich media produce large amounts of acetic acid. Brettanomyces is important to both the brewing and wine industries due to the sensory compounds it produces.
In most beer styles, Brettanomyces is viewed as a contaminant and the characteristics it imparts are considered unwelcome "off-flavours". However, in some styles -- particularly certain traditional Belgian ales -- it is appreciated and encouraged. Lambic and gueuze owe their unique flavour profiles to Brettanomyces. (Full article...)