Fermentation is critical to winemaking. But sometimes the yeast that ferments the wine shuts down prematurely, and bacteria consume the remaining sugar and spoil the drink—a chronic problem called stuck fermentation. Researchers report in the 28 August issue of Cell that they have discovered a biochemical communication system behind this problem. Normally, when grape sugar is present, a biological circuit based in the yeast’s cell membrane blocks the fungus from consuming carbon sources other than the sugar, a process called glucose repression that allows people to use yeast for winemaking and baking. But sometimes, bacteria can send signals that trigger prions—infectious, misfolded proteins infamous for causing mad cow disease in humans—to replicate on the yeast’s cell membranes. The prions interfere with the glucose repression process, enabling the yeast to consume other carbon sources and slow down glucose metabolism—which makes fermentation inefficient. The researchers suspect a similar mechanism might explain type 2 diabetes in humans: Cells monitoring blood sugar levels start to change their metabolism and ignore the glucose signal, possibly due to prion infection, leaving the sugar circulating in the blood. Prions that induce mad cow disease in humans work in a different manner, the researchers say. They suggest that winemakers can avoid the problem of stuck fermentation by adding sulfur dioxide early when crushing the grapes to kill bacteria that induce prion growth in the yeast.