"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 [12]. Almost all ale and lager strains are sensitive to the toxins produced by killer strains [13][14]. 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 [15]. However, none of the toxins have been found to kill Brettanomyces [16]. 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 [15]. See Killer Wine Yeast for more information on these toxins"
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