Optimum conditions of killer toxins produced by Torulaspora delbrueckii and Wickerhamomyces anomalus and their action as antifungal agents

Abstract

Background This study was conducted to estimate the antifungal activity and efficiency of purified killer toxins (TK and WK) produced by Torulaspora delbrueckii and Wickerhamomyces anomalus as alternative drugs of resistant strains against several pathogenic fungi (Candida albicans, Aspergillus niger, and Aspergillus flavus). A specific focus was on the antifungal activity of such purified proteins by using a simple and economical tools under controlled conditions of fermentations. Results The results showed that the optimum conditions of these experiments were detected at 30 °C, 150 rpm, pH 4.5, 3 days incubation period and by using one culture medium (yeast malt extract broth with 2% SDS). The extracted crude toxin had an excellent effect on Candida albicans, Aspergillus niger, and Aspergillus flavus. The inhibition zone values were 35 and 30 mm for WK and TK, respectively, against Candida albicans. While the lowest values of inhibition zone were recorded at 25 mm for WK and 18 mm for TK against Aspergillus flavus. The WK killer toxins were in general more active against the three clinical strains compared to the TK that had the lowest inhibitory effect. Conclusion This study is conducted for the first time in Iraq to determine the optimum conditions for producing two types of killer toxins and testing their antifungal activity. The optimal killer activity was obtained at low pH and mesophilic temperature. The two yeast strains had a good role in the reduction of radial growth of tested pathogenic fungi

The antifungal capacity of a 681-membered collection of environmental yeast isolates

“…While we limited ourselves to specific screening conditions, which miss antifungal yeasts that target pathogens not represented in our panel or that require specific conditions for activity (e.g. high salt concentration, 107,108 a specific pH [77][78][79][80][81][82] or temperature [79][80][81][82] or a transcriptional activator 109 ), our workflow can be adapted to other pathogens and screening conditions that a future researcher is interested in. Specific growth or selection conditions required for continuous production of a killer toxin could explain why some of our killer yeast candidates lost the inhibitory activity along our study.…”

“…Activity was judged by a growth inhibition halo around the isolate (Figure 3A) to better visualize the presence of a growth inhibition halo. As most yeast killer toxins showed optimal fungicidal activity at acidic pH and ambient temperature [77][78][79][80][81][82] , we performed this first screening assay at pH 4.0 and 25 °C.…”

“…We note that our yeasts most likely have a different mode of action: micafungin inhibits fungal cell wall biosynthesis 125 while yeast killer toxins present a wider range of killing mechanisms. [41][42][43][44] Moreover, the conditions used for the different assays used an acidic pH for killer toxins; [77][78][79][80][81][82] while the optimal pH of micafungin is between 6.2 and 6.9. 126 Our killer yeast candidates targeted Candida species within 0.5 -2 µg/mL micafungin equivalents, being ≥2 µg/mL micafungin equivalents in the case of C. parapsilosis.…”

A Sustainable Alternative for Postharvest Disease Management and Phytopathogens Biocontrol in Fruit: Antagonistic Yeasts

“…Genetic studies in Saccharomyces cerevisiae have shown that the ability to produce killer toxins is cytoplasmically inherited and related to the presence of double-stranded linear RNA (dsRNA) plasmids, which are then encapsulated, forming non-infectious virus-like particles (VLP) within the cell cytoplasm [66]. All killer toxins are produced under acidic conditions, and their activity decrease with the increase in pH and temperature of the medium in which they are found-an increase in these variables is sufficient for the yeasts to stop producing them [67,68].…”