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

Abstract

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. 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. 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