Genetic investigation and characterization of killer toxins secreted by non-Saccharomyces yeasts

[摘要] ENGLISH ABSTRACT: In the current study, two isolates showing killer activity against several wine yeast species in aprevious study were identified to strain level and found to belong to the yeast species Candidapyralidae. The identified yeast strains and a Kluyveromyces wickerhamii yeast strain used as acontrol exhibited killer activity against B. bruxellensis known for its spoilage characteristics inred wine, and against several strains of the genus Brettanomyces on white and red grape juicemedium. The killer yeasts inhibited neither the growth of S. cerevisiae nor that of the lactic acidbacteria Oenococcus oeni and Lactobacillus plantarum strains. Yeasts are reported to secretekiller toxins, which can play a role in yeast microbial interactions under winemaking conditions.The C. pyralidae strains were found to secrete two novel killer toxins, designated CpKT1 andCpKT2. These killer toxins were stable and active under winemaking conditions, pH 3.5 - 4.5and temperature ranges between 15 and 25°C. Ethanol and sugar concentrations found duringwinemaking did not affect the activity and stability of these killer toxins. Although, the killertoxins differed with regards to their biochemical and environmental stability and activity, theywere found to have a similar mode of action. The killer toxins induced a fungistatic effect onB. bruxellensis sensitive cells in addition to binding to the cell wall of the sensitive cells, inducingcell surface and plasma membrane damage as did the Kwkt killer toxin secreted byK. wickerhamii. According to the author's knowledge this is the first report on the identification ofnovel killer toxins secreted by C. pyralidae strains isolated from a wine environment as well asthe identification of the mode of action of killer toxins on B. bruxellensis cells. This indeedprovides great research scope in this field.The exoproteomes consisting of the killer toxins Kwkt, CpKT1 and CpKT2 revealed thepresence of exo-glucanases and glucosidases, respectively. The enzymes KwExg1 (exoglucanase)and KwSun4 (glucosidase) retrieved from K. wickerhamii's exoproteome wereidentified as the potential toxins, but their killer activity could not be confirmed. These findingssuggest that hydrolytic enzymes possess killer activity, as previously reported in literature.However, further investigation is needed to identify the killer toxins characterized in this study.

[发布机构] Stellenbosch University