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''P. damnosus'', as well as many other bacteria (but not all bacteria), have been shown to alter the genes of most, but not all, [[Saccharomyces]] species (and perhaps [[Brettanomyces]]), including S. cerevisiae, in a way that changes how they ferment sugars, and essentially forms a symbiotic environment with the yeast. Normally, ''Saccharomyces'' will always ferment glucose and only glucose when glucose is present. It will ignore other sugars until the glucose is gone. Biologically speaking, when the presence of glucose in the yeast's environment shuts down the yeast's ability to ferment any other sugar besides glucose, this is called "glucose repression" <ref name="cross-kingdom">[http://weitzlab.seas.harvard.edu/files/weitzlab/files/2014_cell_jarosz.pdf Cross-Kingdom Chemical Communication Drives a Heritable, Mutually Beneficial Prion-Based Transformation of Metabolism. 2014. Daniel F. Jarosz, Jessica C.S. Brown, Gordon A. Walker, Manoshi S. Datta, W. Lloyd Ung, Alex K. Lancaster, Assaf Rotem, Amelia Chang, Gregory A. Newby,David A. Weitz, Linda F. Bisson, and Susan Lindquist. Cell. 2014 Aug 28;158(5):1083-93.]</ref>. ''Saccharomyces'' and ''Brettanomyces bruxellensis'' (it is currently not known if other ''Brett'' species other than ''B. bruxellensis'' have this ability) have a gene called "GAF+" that when expressed actually allows it to bypass this "glucose repression" and ferment the other sugars simultaneously, but it is generally not expressed except by a very small number of cells <ref>[http://www.cell.com/cell/abstract/S0092-8674(14)00974-X An Evolutionarily Conserved Prion-like Element Converts Wild Fungi from Metabolic Specialists to Generalists. Daniel F. Jarosz, Alex K. Lancaster, Jessica C.S. Brown, Susan Lindquist. Cell. Volume 158, Issue 5, p1072–1082, 28 August 2014]</ref>.
When ''P. damnosus'' lives together with ''Saccharomyces'', a chemical is produced by ''P. damnosus'' that essentially "turns on" the GAF+ gene in ''Saccharomyces''. Not only does ''Sacch'' then have the ability to ferment other sugars at the same time as glucose, but it produces less alcohol. Viability over time is also increased in ''Sacch'' cells that express this gene versus those that don't. With wild fermentation of grapes, the GAF+ ''Sacch'' strains also prevented other fungi from thriving. It is theorized that this creates benefits for both microorganisms, which are often found together in the wild during fermentation of fruit; the bacteria isn't killed by higher alcohol levels, and the yeast has a broader food source. Furthermore, once a ''Saccharomyces'' cell expresses the gene, it will continue to pass this gene onto it's offspring. In winemaking, this is the cause of arrested wine fermentations due to the lower amount of alcohol produced (in the referenced study, the GAF- ''Sacch'' cells fermented out a must to 12% ABV wine, and the GAF+ ''Sacch'' cells fermented out an 8% ABV wine using the same must) <ref name="cross-kingdom"></ref>. However, the implications of this in sour beer brewing are much different and have yet to be further explored.
==See Also==