Isolation and Characterization of [GAR+] Yeasts in Commercial Stuck Fermented Wines
Minami Ogawa, Lucy Joseph, and Linda Bisson*
*Department of Viticulture and Enology, University of California,
Davis, CA 95616 (lfbisson@ucdavis.edu)
Stuck fermentations are a problem during winemaking in which sugar metabolism arrests or slows, resulting in high residual sugar and microbial instability. One factor that contributes to fermentation arrest is the inability of the yeast to adapt to its surrounding environment. This adaptive stress tolerance comes at the cost of reduced metabolic rates as Saccharomyces cerevisiae adjusts its metabolism to enable survival. One mechanism for altering metabolism in a heritable way is via the establishment of heritable protein states or prions. One such prion is the [GAR+] prion, often induced under stress conditions. Typically, Saccharomyces shows glucose repression in the presence of glucose. This prevents it from using other carbon sources when glucose is available. [GAR+] or prion-induced yeasts can use alternative carbon sources in the presence of glucose; as a consequence of loss of glucose repression, sugar consumption is reduced. The [GAR+] prion can be induced by the presence of bacteria.
To determine whether prion induction occurs under commercial wine fermentation conditions, 127 samples of arrested wines were obtained from 40 wineries. The yeasts from these samples were isolated and three to 16 single colonies were picked from each wine. The yeast colonies were screened for presence of [GAR+] on selective medium to detect the presence of the prion. Twenty-eight out of the 127 sampled wines exhibited yeast with [GAR+] characteristics. From 6.25 to 87.5% of the isolated colonies from these 28 wines expressed good growth on the prion-selective medium. To confirm the prion state, yeast cells were desiccated for a month to cure the [GAR+] state and validate that the phenotype enabling growth on the medium was due to the presence of the prion. This work demonstrates that prion induction can be observed under commercial production conditions.
Funding Support: American Vineyard Foundation
