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Many wine yeast strains are known to be "killer" yeast strains. In ''Saccharomyces'', killer strains produce toxins that kill sensitive strains. 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>. Many ale and lager strains are sensitive to the toxins produced by killer strains <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 yeast from other genra, such as ''Hanseniaspora spp.'', ''Kluyveromyces lactis'', ''Candida albicans'', ''Candida dubliniensis'', ''Candida kefir'' and ''Candida tropicalis'', and the K1, K2 and K28 killer strains of ''S. cerevisiae'' <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>. However, none of the toxins 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 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>.
This tends to create some fears for brewers who believe that the ''Sacch'' strains in their beer might still be alive. However, in a highly acidic sour beer, ''Saccharomyces'' tends not to live for extended periods of time. Even if some cells do, their activity would be next to none due to no available sugars left in the beer (other than priming sugar), and thus their contribution to the flavor development of the beer would be minimal to none. Fear of autolysis occurring due to killer strain activity is usually unfounded because most of the ''Sacch'' cells are left behind and the ability of ''Brettanomyces'' to use the acids and proteins that are released during whatever autolysis might occur in the bottle. Re-yeasting with wine yeast for priming has the additional advantage of not fermenting maltose or maltotriose, so unexpected attenuation from the wine yeast will not occur. Flavor impact by the wine yeast is also probably nil due to the small amount of fermentables provided by the priming sugar. Thus, there is little argument against re-yeasting with wine yeast at packaging time.
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