Who am I?
Most winemakers know the feared five-letter word “Brett” all too well. Brettanomycesbruxellensis is a famous red wine spoilage yeast, responsible for financial losses within the wine industry yearly. Contaminated wine can usually not be readily commercialised and ridding contaminated wineries of this yeast is a difficult task. B. bruxellensis produces various spoilage compounds that not only affect the aroma profile of the wine, but also the appearance of the wine, often resulting in a colour loss and haze. These compounds include acetic acid (Scheffers, 1961 & Freer, 2002) and fatty acids (Rozès et al., 1992; Malfeito-Ferreira et al., 1997 & Licker et al., 1998). However, B. bruxellensis is best known for the production of volatile phenolic compounds that are generally considered to have a negative impact on the organoleptic properties of the wine. Although the metabolic pathway leading to the production of these volatile phenols has been elucidated more than 20 years ago, the enzymes catalysing the 2-step reaction have only been identified recently, following the sequencing of B. bruxellensis’ genome (Curtin et al., 2012a; Piškur et al., 2012 & Crauwels et al., 2014). Concurrently, research has allowed significant advances in our global understanding of B. bruxellensis, especially concerning this yeast’s peculiar ability to survive and develop in a matrix as harsh as wine. This article provides an overview of these recent research findings.
My genetic make-up
The first genome sequencing was attempted in 2007. However, only a partial sequence (40% of whole genome) could be assembled (Woolfit et al., 2007). The first whole genome sequence was actually released in 2012 (Curtin et al., 2012a), i.e. 16 years after that of Saccharomyces cerevisiae. The strain whose genome was sequenced had been isolated from an Australian wine. A few months later, the whole genome sequence of the initially partially sequenced French B. bruxellensis strain, was also made publicly available (Piškur et al., 2012). Finally, the genome of one more strain, this time isolated from beer, was sequenced in 2014 (Crauwels et al., 2014). Strain variability, complexities and unusual characteristics of this yeast were evident form the former two sequenced genomes, with chromosome numbers ranging from 4 to 9 (depending on the strain) in comparison to the 16 found in all strains of S. cerevisiae. In addition, unlike what was originally assumed that B. bruxellensis was a haploid organism, it was demonstrated to rather exhibit an intricate ploidy with one strain being diploid and the other triploid. Moreover, in a recent study, the genome sequences of the three strains were compared in order to explore the genome plasticity and diversity among the different isolates. It was reported that the beer strain had a significantly altered genome sequence in comparison to the two wine strains. The study also revealed that the genomes of the two wine isolates, even though very different, were more similar to one another than compared to the beer isolate. In particular, 20 genes present in both wine strains are absent in the beer strain. These genes could possibly confer a specific adaption to living in wine (Crauwels et al., 2014).
These studies have already shed some light on the complexity and huge diversity observed among strains. The availability of the full genome sequences is a significant step forward that will certainly allow for more in-depth investigations to better comprehend morphological and physiological characteristics, as well as specific adaptations associated with B. bruxellensis.