OXIDATIVE STRESS RESISTANCE OF YEASTS WITH BIOLOGICAL CONTROL POTENTIAL

A current demand for agriculture is the distribution of safe and agrochemical-free food. In this sense, the biological control has been reported as an effective alternative to the use of chemical fungicides for management of rot in postharvest fruits and vegetables. Among potential microbial antagonists, yeasts have been studied because they have many characteristics that make them suitable for biocontrol agents, such as simple nutritional requirements, ability to colonize dry surfaces for long periods, and non- production of allergenic spores. The effectiveness of the use of these agents is based on the efficiency of inhibition of the pathogen and resistance to different environmental stresses, such as oxidative stress. This type of stress is closely associated with an increase in the level of ROS (reactive oxygen species), such as hydrogen peroxide (H2O2) and superoxide anions (O2-•) that have negative effects on cells and tissues. The yeasts Torulaspora globosa (strain 5S55) and Trichosporon asahii (strain 3S44) showed potential in the control of the fungus Alternaria alternata, causing black rot in tomatoes, in previous in vitro tests. The objective of this study was to determine the resistance of these two yeasts strains to oxidative stress conditions. A viable cell count test was performed on liquid YEPD medium with concentrations of 0, 10, 20, 30 and 40 mM H2O2 in quintuplicates with an initial yeast inoculum of 106 cells/mL and incubation under agitation at 150 rpm at 25 °C for 96 hours. Every 12 hours, the yeast cell viability was verified by counting cells stained with methylene blue. The 5S55 strain presented a reduction of live cells at different concentrations, reaching mortality of 78, 77, 81 and 78% at 10, 20, 30 and 40 mM respectively, at 96 hours. The 3S44 strain presented for all concentrations de H2O2 an increase of living cells over time and less than 1% of mortality at the end of the 96 hours. Thus, the results indicate a great resistance by the 3S44 strain and low resistance to 5S55 to situations of oxidative stress. For the 5S55, strategies such as pre-adaptation to stress and development of formulations of biological product with the addition of anti-stress compounds are necessary to intensify their natural resistance.