The fermentation of red and white grape musts into desirable wines involves numerous biological and chemical processes. Yeast are the primary microorganisms associated with wine fermentation, with their conversion of grape sugars to ethanol. However, the microbial ecology of vinification is much more complex, with bacteria playing a vital and often complex role. Both lactic acid and acetic acid bacteria (LAB and AAB) are intricately involved by imparting desirable and sometimes less favourable sensory qualities to wine.
Oenococcus oeni is the most important wine associated LAB due to its unique ability to survive in the hostile conditions of wine; including high ethanol concentrations (<15% v/v), high acidity (> pH 2.9) and limited nutrient availability. O. oeni’s major role is to conduct malolactic fermentation (MLF), the conversion of L-malic acid to L-lactic acid and carbon dioxide. However, MLF also confers microbial stability to wine, and contributes to sensory qualities of wine. O. oeni exhibits a diverse array of secondary metabolic activities, including formation of diacetyl, release of glycosidically bound volatiles, esterasesand metabolism of polysaccharides, which impact on the colour, aroma and mouthfeel properties of wine.
Many bacterial species display substantial intra-specific genomic diversity and this genetic variation potentially provides options to winemakers to shape the style of their wines. We have followed several O. oeni strains during MLF in red wine over five vintages and demonstrated their consistent ability to influence fruity berry sensory attributes. To ascertain the basis of this phenotypic difference, we have sequenced the genomes of 12 strains. Results indicated that the strains comprise a genomically-diverse group; for example, numerous insertions and deletions were present. Strains shared a core group of conserved ORFs, but up to 10% of the coding potential of any one strain was specific to that isolate. We were able to connect the acquisition of ‘glycosidase’ genes by an O. oeni strain with desirable phenotypic changes that impact on MLF influences on wine quality.