Issue link: http://winesandvines.uberflip.com/i/513570
June 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 59 W I N E M A K I N G was thought to be due to the production of inhibitors or competition for nutrients. However, we discovered that these bacteria are efficient at inducing the establishment of the [GAR + ] prion in wine strains. 7 This prion, like other pri- ons, is a protein-conformational change that is inherited by progeny cells dur- ing cell division; thus, once cells in the population have changed to establish the prion, subsequent generations will also be in the prion state without need for the continuation of conditions leading to establishment of the prion. 5 The [GAR + ] prion changes yeast fer- mentation rates by reducing the expres- sion level of sugar transporters in the membrane, thus slowing fermentation. 7 This slower fermentation enables bac- teria to proliferate at the same time as the yeast. This represents a novel way by which cells can fundamentally alter metabolism and adapt to new environ- ments without needing to make a per- manent change in their DNA. That this change is driven by the microbial com- munity explains many aspects seen in sluggish fermentations where the impact of the presence of the bacteria seems to be delayed until a later stage of fermen- tation. We have discovered commercial wine strains that rapidly induce the prion, as induction was shown to occur at a rapid rate in five of 11 genetically unre- lated commercial wine strains evaluated. The prevalence of this ability to rapidly induce this state suggests the prion state plays an important role in survival dur- ing wine fermentation. Assessment of the impact of Acetobacter orientalis, Acetobacter ghanensis, Acetobacter malorum and Gluconobacter cerinus on yeast fermentation In 2013, we asked wineries to submit samples of difficult-to-ferment juices for analysis. All of these fermentations contained acetic acid bacteria, showed an accumulation pattern for acetic acid that is atypical of Acetobacter aceti, and arrested before acetic acid levels had become inhibitory. From these wines we were able to isolate four novel species of acetic acid bacteria: Acetobacter orientalis, Acetobacter ghanensis, Acetobacter malorum and Gluconobacter cerinus. In 2014, we assessed the ability of each of these organisms to impact the yeast fermentation negatively. Two strains, A. ghanensis and A. malorum, showed direct inhibition of yeast fermentation on plates but without producing the high concen- trations of acetic acid found with A. aceti infection of wine. The inhibitor in these cases is as yet unknown, and we hope to identify the inhibitory compound or compounds generated by these strains in future research. It is important to identify the inhibitor. Not all acetic acid bacterial strains form the inhibitor, so any screening of the juice prior to fermentation for the pres- ence of these organisms may not be effec- tive in determining if the strain present is actually inhibitory or not. Assessing the risk of the presence of these organ- isms for an impact on yeast fermentation may necessitate directly screening for the inhibitory molecule or for the genes involved in its production. These acetic acid bacteria seem to be confined to specific batches of wine and are apparently not readily spread to other fermentations or wines in the winery in contrast to what is seen with A. aceti. There are multiple possible explanations for this observation. For example, the inhibitor may be pro- duced early in the wine at the juice stage and may not become observed as toxic until ethanol has accumulated in the fer-