13,703
edits
Changes
no edit summary
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 this toxin is caused by a virus that 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 temperature of 98°C. A screening of this genetic change, called “K1-like Killer Toxin” (KKT) genes, in other yeasts showed that many other species can also produce toxins similar to the K1L toxin but slightly different in effect, including ''Kazachstania africana'', ''Naumovozyma castellii'', ''Naumovozyma dairenensis'', ''Tetrapisispora phaffii'', and ''Pichia membranifaciens''. Each of the identified species could kill at least one other type of yeast with its toxin, and was immune to its own toxin, but susceptible to other K1-like toxins from other yeast species. Differences in the production of these K1-like toxins between 5 different strains of ''P. membranifaciens'' indicated that the toxins can be strain-specific, rather than species-specific. Using the genetic relatedness between the different KKT genes, the researchers concluded that this family of K1-like toxins originated outside of the ''Saccharomyces'' genus. This research uncovered a new family of K1-like antifungal killer toxins amoung many species of yeast in the Saccharomycotina subphylum <ref>[https://journals.plos.org/plosgenetics/article?id=10.1371%2Fjournal.pgen.1009341 Fredericks LR, Lee MD, Crabtree AM, Boyer JM, Kizer EA, Taggart NT, et al. (2021) The Species-Specific Acquisition 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]</ref>.
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>. They later demonstrated that using 1.0 mg/mL of saccharomycin with 25 mg/L of SO<sub>2</sub> in grape must fermentation completely eliminated ''B. bruxellensis'' <ref>[https://www.mdpi.com/2076-2607/9/12/2528#cite Branco P, Coutinho R, Malfeito-Ferreira M, Prista C, Albergaria H. Wine Spoilage Control: Impact of Saccharomycin on Brettanomyces bruxellensis and Its Conjugated Effect with Sulfur Dioxide. Microorganisms. 2021; 9(12):2528. https://doi.org/10.3390/microorganisms9122528]</ref>.
See also: