Antagonistic Activity of Three Newly Isolated Yeast Strains from the Surface of Fruits

Three Metschnikowia pulcherrima strains isolated from white grapes (strains designated SG1 and SG2) and cherries (strain CPM1) from Romania were tested for antagonistic activity against natural and reference yeasts, fungi and E. coli. The best results were obtained for SG2 strain against the following microbes isolated from natural environments: yeasts Candida tropicalis OT4 and Candida albicans Cc, and fungal strains Botrytis cinerea BCS and BCF1. Using 2 % sodium hydrogen carbonate and 1 or 2 % calcium chloride, antimicrobial activities of strains SG1 and SG2 were maintained or even enhanced, while strain CPM1 was less influenced. Antimicrobial tests revealed four types of colonies and halos of strains SG1, SG2 and CPM1 reflecting differences in iron immobilization under similar growth conditions. Antifungal activity and the level of pigment production increased at 25 °C, and the largest diffusible pigment area was obtained by strain SG2, which altered Botrytis conidia germination and induced modifications of mycelium. Competition for iron seems to be the major mechanism of antagonistic action of our strains, which is reduced/absent under iron chloride addition. Lowering of antifungal activity by heating or proteolytic treatment indicates that a protein might also be involved. The inhibitory activity of yeast supernatants was less important, suggesting a cell-related mechanism. When combined, strains SG1 and SG2 had synergistic activity, which was reduced in the presence of CPM1 due to possible production of an inhibitory compound.