Changes

Many wine yeast strains are known to be "killer" yeast strains (this term has been changed to "zymocidal" in science <ref name="Stewart_2018">[https://link.springer.com/chapter/10.1007/978-3-319-69126-8_10 "Killer (Zymocidal) Yeasts." Brewing and Distilling Yeasts. Graham G. Stewart. 2018.]</ref>). Unwanted killer strains of ''Saccharomyces'' have been known to cause stuck wine fermentations by killing off the sensitive wine strain that was pitched into the wine by the winemaker <ref name="Bajaj_2017">[https://link.springer.com/chapter/10.1007/978-981-10-2621-8_7 Biology of Killer Yeast and Technological Implications. Bijender Kumar Bajaj, Satbir Singh. 2017.]</ref>. In ''Saccharomyces'', killer strains are genetically determined to secrete toxins (in the form of extracellular proteins or glycoproteins) called 'mycocins' that kill sensitive strains(there is no evidence that these toxins affect humans). These killer strains are immune to their own toxin. The mycocin toxins can act on sensitive strains in a number of ways: by blocking DNA synthesis and preventing cell division, inhibiting the synthesis of beta-glucans (β-1,6-glucan) that a part of their cell wall formation, and by causing ions to leak through the cell wall. In low dosages, which is typical in the natural environment, toxin triggers active cell death ([http://www.biology-pages.info/A/Apoptosis.html apoptosis]), while large dosages cause necrotic cell killing ([https://en.wikipedia.org/wiki/Necrosis necrosis]). One study in wine found that the use of killer strains to out compete outcompete sensitive strains resulted in off-flavors from yeast autolysis <ref>[http://www.nature.com/nrmicro/journal/v4/n3/full/nrmicro1347.html Yeast viral killer toxins: lethality and self-protection. Manfred J. Schmitt & Frank Breinig. 2006.]</ref><ref name="Hatoum2012">[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3525881/ Rima Hatoum, Steve Labrie, and Ismail Fliss. 2012.]</ref>. Another study found that a lager strain that was genetically modified to secrete killer toxins was able to eliminate all cells of a sensitive ale strain within 24 hours of beer fermentation at a pitching rate of 99% sensitive ale strain to 1% killer lager strain, indicating that even a small amount of killer cells is enough to kill a larger population of sensitive cells <ref name="Stewart_2018" />. Ratios of killer cells to sensitive cells that have shown to completely eliminate or nearly eliminate the sensitive population on lab media includes 1:1, 1;10, and 1:100. In a starter of a sensitive strain that had 10% cells of a killer strain introduced, the viability of the sensitive strain was greatly reduced <ref name="Bajaj_2017" />. Neutral strains do not produce toxins, nor are they killed by them <ref>[https://books.google.com/books?hl=en&lr=&id=mvORN6OXHh4C&oi=fnd&pg=PA93&dq=Bussey,+H.+1981.+Physiology+of+killer+factor+in+yeast.+Adv.+Microb.+Physiol.+22:93-121&ots=jUY4T9NpgB&sig=aw-Y1um0KsDnGe6rRe5PTWIDYdI#v=onepage&q&f=false Advances in Microbial Physiology, Volume 22. Academic Press, Sep 15, 1981. Pg 94-95.]</ref>. Almost all ale and lager strains are sensitive to the toxins produced by killer strains <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1179271825434363/?comment_id=1179424538752425&offset=0&total_comments=5&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Bryan of Sui Generis Blog on MTF on Killer Factor for Saccharomyces. 11/16/2015.]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1973.tb03515.x/pdf Strains of Yeast Lethal to Brewery Yeasts. A.P. Maule and P.D. Thomas. 1972.]</ref>.

In ''Saccharomyces'', four toxins have been identified: K1, K2, K28, and Klus, the first three of which can only kill other strains/species of ''Saccharomyces''. The Klus toxin has been found to kill all strains of ''S. cerevisiae'' (including those that produce the previous three toxins), as well as yeast from other genera, such as ''Hanseniaspora spp.'', ''Kluyveromyces lactis'', ''Candida albicans'', ''Candida dubliniensis'', ''Candida kefir'' and ''Candida tropicalis'' <ref name="Rodriguez">[http://aem.asm.org/content/77/5/1822.long A New Wine Saccharomyces cerevisiae Killer Toxin (Klus), Encoded by a Double-Stranded RNA Virus, with Broad Antifungal Activity Is Evolutionarily Related to a Chromosomal Host Gene. Nieves Rodríguez-Cousiño, Matilde Maqueda, Jesús Ambrona, Emiliano Zamora, Rosa Esteban and Manuel Ramírez. 2011]</ref>. HoweverThe K1 toxin is most active between a pH of 4.6 and 4.8, while K2 and Klus are active around a pH of 4.0 to 4.3 <ref name="Rodriguez"></ref>. The activity of the toxin is greatest during the log phase of growth, and decays during the stationary phase of fermentation <ref name="Buyuksirit"></ref>. Generally, none of the toxins secreted by killer strains of ''Saccharomyces'' have been found to kill ''Brettanomyces'' <ref>[http://www.scielo.org.za/scielo.php?pid=S2224-79042015000100010&script=sci_arttext&tlng=pt Non-Saccharomyces killer toxins: Possible biocontrol agents against Brettanomyces in wine? S. Afr. J. Enol. Vitic. vol.36 n.1 Stellenbosch. 2015.]</ref>. The K1 toxin is most active between One study from India reported that a pH wild ''S. cerevisiae'' strain caught from flowers killed another wild caught strain of 4''Brettanoyces anomulus'', however, their methodology was not explicit and potentially not scientifically rigorous enough <ref>[http://nopr.niscair.res.6 in/handle/123456789/7735 Production and effect of killer toxin by Saccharomyces cerevisiae and Pichia kluyveri on sensitive yeasts and 4fungal pathogens.8 Dabhole, Madhusudan P, Joishy, while K2 and Klus are active around a pH of 4K N.0 to 4 2005.3 <ref name="Rodriguez">]</ref>. The activity of For example, this study did not use DNA fingerprinting to identify the toxin is greatest during wild yeast strains used in the log phase of growthstudy and instead relied on morphology and media selection, and decays during they did not identify the type of toxin produced by the stationary phase killer strain of fermentation <ref name="Buyuksirit"></ref>wild ''S. cerevisiae''. They also reported that the ''B. anamulus'' strain did not ferment glucose, which is not typical for this species.

Various other yeast species have the ability to produce toxins that effect a range of other yeasts (but generally not bacteria), including species from the genera ''Candida'', ''Cryptococcus'', ''Debaryomyces'', ''Hanseniaspora'', ''Hansenula'', ''Kluyveromyces'', ''Metschnikowia'', ''Pichia'', ''Ustilago'', ''Torulopsis'', ''Williopsis'', ''Zygosaccharomyces'', ''Aureobasidium'', ''Zygowilliopsis'', and ''Mrakia'' <ref name="Buyuksirit">[http://waset.org/publications/9999528/antimicrobial-agents-produced-by-yeasts Antimicrobial Agents Produced by Yeasts. T. Buyuksirit, H. Kuleasan. 2014.]</ref><ref name="Stewart_2018" />. For example, strains of the yeast species ''Candida pyralidae'' <ref name="Buyuksirit"></ref>, ''Wickerhamomyces anomalus'', ''Kluyveromyces wickeramii'', ''Torulaspora delbrueckii'' and ''Pichia membranifaciens'' have been found to produce toxin that inhibits ''Brettanomyces'' <ref name="Ciani_2016">[https://www.researchgate.net/publication/301581233_Yeast_Interactions_in_Inoculated_Wine_Fermentation Yeast Interactions in Inoculated Wine Fermentation. Maurizio Ciani, Angela Capece, Francesca Comitini, Laura Canonico, Gabriella Siesto and Patrizia Romano. 2016.]</ref>. In addition, the toxin produced by ''Wickerhamomyces anomalus'' and ''Williopsis markii'' have been found to inhibit a wide range of spoilage and pathogenic fungi <ref name="Hatoum2012"></ref>. Killer strains of ''S. cerevisiae'' and other yeast can occur naturally in the wild on fruit and can have a negative impact on other flora that are found in the same environment <ref name="Buyuksirit"></ref>. Strains of ''Torulaspora delbrueckii'' have been shown to kill killer strains of ''S. cerevisae'' (wine strains), as well as to kill ''Pichia'' species <ref name="Ciani_2016"></ref>. The occurrence of killer strains of yeast in the wild is also wide spread. For example, out of 210 yeasts from various genera isolated from molasses, 13 of them were killer strains. Out of 1,000 isolates of various ''Candida'' species isolated from human skin, 52 were killer strains. Out of 65 strains of various yeasts isolated from fermented foods, soil samples, and spoiled fruits/vegetables, 12 were killer strains <ref name="Bajaj_2017" />.  Scientists have used genetic modification to create ''S. cerevisiae'' strains that produce various killer toxins that can assist in completing fermentation in the baking, wine, distillation, and beer making processes. These yeasts are able to inhibit undesired yeast contaminants, preventing various off-flavors and other unwanted characteristics in the finished products. Ale and lager strains that have been modified to release these toxins have reportedly retained the positive fermentation and flavor characteristics of the original strains <ref name="Bajaj_2017" />.