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To say something alters a gene is misleading here. The expression of the gene is altered.
:''Editor's note: special thanks to Richard Preiss of [[Escarpment Yeast Laboratories]] for helping to interpret the science referenced this section.''
''P. damnosus'', as well as some other bacteria, have been shown to alter the expression of genes of in most, but not all, [[Saccharomyces]] cerevisiae strains (and other ''Sacch'' species and even perhaps [[Brettanomyces]]), in a way that changes how they ferment sugars, and essentially forms a symbiotic environment with the yeast. Normally ''Saccharomyces'' will only ferment glucose when glucose is present and ignores other sugars such as maltose and maltotriose 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 types of 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. Normally this gene is 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. In wild fermentation of grapes, the wild GAF+ ''Sacch'' strains thrived over the other types of fungi that were found on the wild grapes. This led the researchers of the referenced study to speculate that the GAF+ gene may play a role in preventing other fungi from thriving. It is thought that this benefits 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 its offspring. In winemaking, this is the cause of arrested wine fermentations due to the lower amount of alcohol produced. For example in the referenced study, the GAF- ''Sacch'' cells fermented grape must into a 12% ABV wine, and the GAF+ ''Sacch'' cells fermented the same wine must into an 8% ABV wine <ref name="cross-kingdom"></ref>. However, the implications of this in sour beer brewing are much different and have yet to be explored scientifically.