In ''Saccharomyces cerevisiae'', 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''. Rodriguez et al. (2011) reported that out of 1,114 strains of ''S. cerevisiae'' isolated from spontaneous wine fermentations, 38% of them were killer positive with most producing K2. Only 7% of produced the Klus toxin (no commercial wine yeast strains have been reported to produce the Klus toxin that we know of) <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>. 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>. 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>. One study from India reported that a wild ''S. cerevisiae'' strain caught from flowers killed another wild caught strain of ''Brettanoyces anomulus'', however, their methodology was not explicit and potentially not scientifically rigorous enough <ref>[http://nopr.niscair.res.in/handle/123456789/7735 Production and effect of killer toxin by Saccharomyces cerevisiae and Pichia kluyveri on sensitive yeasts and fungal pathogens. Dabhole, Madhusudan P, Joishy, K N. 2005.]</ref>. For example, this study did not use DNA fingerprinting to identify the wild yeast strains used in the study and instead relied on morphology and media selection, and they did not identify the type of toxin produced by the killer strain of wild ''S. cerevisiae''. They also reported that the ''B. anamulus'' strain did not ferment glucose, which is not typical for this species and indicates that it might have been misidentified.
Several strains of ''Saccharomyces eubayanus'' isolated from seeds from monkey puzzle trees in Patagonia, Argentina, were found to secrete a killer toxin that kills ''Brettanomyces'' and ''Pichia''. One strain was found to produce a lot of the toxin, which is called "SeKT". ''S. cerervisiae'' strains, including strains that are sensitive to the above toxins, are not sensitive to this toxin. Mazzucco et al. (2019) found that SeKT toxin produced by this one strain of ''S. eubaynus'' in a special growth medium designed to maximize the SeKT toxin production (WUJ medium, which is "ultrafiltered" apple and pear juice) inhibited a strain of ''B. bruxellensis'' to around 50% growth after 48 hours in a wine growth medium. It also inhibited ''Pichia guilliermondii'', ''Pichia manshurica'', and ''Pichia membranifaciens'' by 50-70%. Note that the toxin was applied directly to the ''Brettanomyces'' and ''Pichia'' species, and not in a co-fermentation setting. Since ''S. cerevisiae'' strains are not effected by the SeKT toxin, it has been proposed as a way to limit ''Brettanomyces'' and ''Pichia'' in wine fermentations <ref>[https://www.ncbi.nlm.nih.gov/pubmed/30671692?dopt=Abstract Production of a novel killer toxin from Saccharomyces eubayanus using agro-industrial waste and its application against wine spoilage yeasts. Mazzucco MB, Ganga MA, Sangorrín MP. 2019. DOI: 10.1007/s10482-019-01231-5.]</ref>.
A newly discovered toxin Various other yeast species have the ability to produce toxins that is related to effect a range of other yeasts (but generally not bacteria), including species from the K1 toxingenera ''Candida'', ''Cryptococcus'', ''Debaryomyces'', ''Hanseniaspora'', ''Hansenula'', ''Kluyveromyces'', ''Metschnikowia'', ''Pichia'', ''Ustilago'', ''Torulopsis'', ''Williopsis'', ''Zygosaccharomyces'', ''Aureobasidium'', called "K1-like" or K1L''Zygowilliopsis'', has been identified in and ''Saccharomyces paradoxusMrakia''<ref name="Buyuksirit">[http://waset. The ability for this species to produce this toxin is caused org/publications/9999528/antimicrobial-agents-produced-by-yeasts Antimicrobial Agents Produced by a virus that binds the the DNA of the yeast cells, and spread via horizontal gene transferYeasts. The K1L toxin has a pH optimum mostly between 4 T.5 and 5Buyuksirit, with no inhibitory activity at pH 5H. Kuleasan.5 2014. It is denatured at a temperature of 98°C]</ref><ref name="Stewart_2018" />. A screening For example, strains of this genetic change, called “K1-like Killer Toxin” (KKT) genes, in other yeasts showed that many other the yeast species can also produce toxins similar to the K1L toxin but slightly different in effect, including ''Kazachstania africanaCandida pyralidae''<ref name="Buyuksirit"></ref>, ''Naumovozyma castelliiWickerhamomyces anomalus'', ''Naumovozyma dairenensisKluyveromyces wickeramii'', ''Tetrapisispora phaffiiTorulaspora 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. Each of ]</ref>. In addition, the identified species could kill at least one other type of yeast with its toxin, produced by ''Wickerhamomyces anomalus'' and was immune to its own toxin, but susceptable ''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 K1-like toxins from yeast can occur naturally in the wild on fruit and can have a negative impact on other yeast species. Differences flora that are found in the production same environment <ref name="Buyuksirit"></ref>. Strains of these K1-like toxins between 5 different ''Torulaspora delbrueckii'' have been shown to kill killer strains of ''PS. membranifacienscerevisae'' indicated that the toxins can be strain-specific(wine strains), rather than as well as to kill ''Pichia'' species-specific<ref name="Ciani_2016"></ref>. Using The occurrence of killer strains of yeast in the genetic relatedness between the different KKT geneswild is also wide spread. For example, out of 210 yeasts from various genera isolated from molasses, the researchers concluded that this family 13 of them were killer strains. Out of K1-like toxins originated outside 1,000 isolates of the various ''SaccharomycesCandida'' genusspecies isolated from human skin, 52 were killer strains. This research uncovered a new family Out of 65 strains of K1-like antifungal various yeasts isolated from fermented foods, soil samples, and spoiled fruits/vegetables, 12 were killer toxins amoung many species of yeast in the Saccharomycotina subphylum strains <refname="Bajaj_2017" />[https://journals.plos It has been hypothesized that toxin production is ubiquitous throughout nearly all genera of yeast; the more studies that have been done on a particular genus of yeast, the more likely it is that toxin production has been found by species and strains within that genus.org/plosgenetics/article?id=10.1371%2Fjournal.pgen.1009341 Fredericks LR Yeasts that produce toxins have been found on every continent and in every natural habitat of yeast, including leaf surfaces, leaf litter, Lee MDtree slime fluxes, Crabtree AMfruits, Boyer JMcactus stems and cladodes, Kizer EAinsect guts, Taggart NTmammal feces, et al. (2021) The Speciesleaf-Specific Acquisition cutting ant nests, lake water, ocean sediment, soil, wine, bakeries, and Diversification of a K1-like Family of Killer Toxins in Budding Yeasts of the Saccharomycotina. PLoS Genet 17(2): e1009341. https://doi.org/10.1371/journal.pgen.1009341]dairy products <ref name="Boynton_2019" /ref>.
Various other yeast species have A newly discovered toxin that is related to the K1 toxin, called "K1-like" or K1L, has been identified in ''Saccharomyces paradoxus''. The ability for this species to produce toxins this toxin is caused by a virus that effect binds to the DNA of the yeast cells, and spread via horizontal gene transfer. The K1L toxin has a pH optimum mostly between 4.5 and 5, with no inhibitory activity at pH 5.5. It is denatured at a range temperature of 98°C. A screening of other yeasts this genetic change, called “K1-like Killer Toxin” (but generally not bacteriaKKT)genes, including in other yeasts showed that many other species from can also produce toxins similar to the genera ''Candida''K1L toxin but slightly different in effect, including ''CryptococcusKazachstania africana'', ''DebaryomycesNaumovozyma castellii'', ''HanseniasporaNaumovozyma dairenensis'', ''Hansenula'', ''Kluyveromyces'', ''MetschnikowiaTetrapisispora phaffii'', and ''Pichiamembranifaciens''. Each of the identified species could kill at least one other type of yeast with its toxin, ''Ustilago''and was immune to its own toxin, ''Torulopsis'', ''Williopsis'', ''Zygosaccharomyces'', ''Aureobasidium'', ''Zygowilliopsis'', and ''Mrakia'' <ref name="Buyuksirit">[http://wasetbut susceptible to other K1-like toxins from other yeast species.org/publications/9999528/antimicrobialDifferences in the production of these K1-agents-produced-by-yeasts Antimicrobial Agents Produced by Yeasts. T. Buyuksirit, H. Kuleasan. 2014.]</ref><ref name="Stewart_2018" />. For example, like toxins between 5 different strains of the yeast species ''Candida pyralidaeP. membranifaciens'' <ref name="Buyuksirit"></ref>indicated that the toxins can be strain-specific, ''Wickerhamomyces anomalus''rather than species-specific. Using the genetic relatedness between the different KKT genes, ''Kluyveromyces wickeramii'', ''Torulaspora delbrueckii'' and ''Pichia membranifaciens'' have been found to produce toxin the researchers concluded that inhibits this family of K1-like toxins originated outside of the ''BrettanomycesSaccharomyces'' genus. This research uncovered a new family of K1-like antifungal killer toxins amoung many species of yeast in the Saccharomycotina subphylum <ref name="Ciani_2016">[https://wwwjournals.researchgateplos.netorg/publicationplosgenetics/301581233_Yeast_Interactions_in_Inoculated_Wine_Fermentation Yeast Interactions in Inoculated Wine Fermentationarticle?id=10.1371%2Fjournal.pgen. Maurizio Ciani1009341 Fredericks LR, Lee MD, Crabtree AM, Angela CapeceBoyer JM, Francesca ComitiniKizer EA, Laura CanonicoTaggart NT, Gabriella Siesto and Patrizia Romanoet al. 2016.]</ref>. In addition, the toxin produced by ''Wickerhamomyces anomalus'' (2021) The Species-Specific Acquisition and ''Williopsis markii'' have been found to inhibit Diversification of a wide range K1-like Family of spoilage and pathogenic fungi <ref name="Hatoum2012"></ref>. Killer strains Toxins in Budding Yeasts 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 ''SSaccharomycotina. cerevisae'' PLoS Genet 17(wine strains2), as well as to kill ''Pichia'' species <ref name="Ciani_2016"><: e1009341. https://ref>doi. The occurrence of killer strains of yeast in the wild is also wide spreadorg/10. For example, out of 210 yeasts from various genera isolated from molasses, 13 of them were killer strains1371/journal. Out of 1,000 isolates of various ''Candida'' species isolated from human skin, 52 were killer strainspgen. Out of 65 strains of various yeasts isolated from fermented foods, soil samples, and spoiled fruits/vegetables, 12 were killer strains 1009341]<ref name="Bajaj_2017" />. It has been hypothesized that toxin production is ubiquitous throughout nearly all genera of yeast; the more studies that have been done on a particular genus of yeast, the more likely it is that toxin production has been found by species and strains within that genus. Yeasts that produce toxins have been found on every continent and in every natural habitat of yeast, including leaf surfaces, leaf litter, tree slime fluxes, fruits, cactus stems and cladodes, insect guts, mammal feces, leaf-cutting ant nests, lake water, ocean sediment, soil, wine, bakeries, and dairy products <ref name="Boynton_2019" />.
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" />. Branco et al. (2017 and 2019) discovered several strains of ''S. cerevisiae'' that excrete a biocin toxin that is active against several other genera of yeast, including ''Brettanomyces bruxellensis''. The toxin is composed of peptides derived from the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which is a protein that serves many different roles in different species of microbes and animals. This toxin is produced by some strains of ''S. cerevisiae'' as they enter the stationary phase after primary fermentation. However, the amount of the toxin needed to inhibit ''B. bruxellensis'' was 10 times the amount that is produced naturally during fermentation. The researchers later genetically modified a strain of ''S. cerevisiae'' to over-produce the toxin, which they named "saccharomycin", at levels required to completely inhibit ''B. bruxellensis'' when co-pitched at a 1:1 ratio (10^5 cells/ml for both). This toxin was also reported to be highly active against ''Hanseniaspora guilliermondii'', ''Kluyveromyces marxianus'', ''Lactobacillus thermotolerans'' (inhibited at 250 μg/ml of toxin), while inhibition of ''Torulaspora delbrueckii'' and ''B. bruxellensis'' required very high amounts of the toxin (500 μg/ml and 1000-2000 μg/ml) <ref>[https://link.springer.com/article/10.1007%2Fs00253-016-7755-6 Antimicrobial properties and death-inducing mechanisms of saccharomycin, a biocide secreted by Saccharomyces cerevisiae. Patrícia Branco, Diana Francisco, Margarida Monteiro, Maria Gabriela Almeida, Jorge Caldeira, Nils Arneborg, Catarina Prista, Helena Albergaria. 2017. DOI: 10.1007/s00253-016-7755-6.]</ref><ref>[https://link.springer.com/article/10.1007/s00253-019-09657-7 Biocontrol of Brettanomyces/Dekkera bruxellensis in alcoholic fermentations using saccharomycin-overproducing Saccharomyces cerevisiae strains. Patrícia Branco, Farzana Sabir, Mário Diniz, Luísa Carvalho, Helena Albergaria, Catarina Prista. 2019.]</ref>.