Tolerance of Kluyveromyces marxianus Under Acetic Acid-, Isoamyl Alcohol-, Hydrogen Peroxide-, and Ethanol-Induced Stress

Kluyveromyces marxianus is a yeast that can be used as a microbial factory. However, little is known about its response to stress conditions. This work evaluated the response of this yeast against ethanol, acetic acid, isoamyl alcohol, and hydrogen peroxide as stress agents. Cytotoxicity assays were performed to assess the residual viability using a direct method (CFU counting) and an indirect method based on the reduction in MTT. Then, fermentation kinetics were performed at IC30 and IC50 for each stress factor to evaluate the effect of moderate and intense stress. This work is the first report presenting IC50 values for ethanol (21.82 g/L), acetic acid (1.19 g/L), isoamyl alcohol (2.74 g/L), and hydrogen peroxide (0.09 g/L) in K. marxianus. The IC50 values for the indirect method are between 3.7 and 68% higher than those for the direct method. Hydrogen peroxide and ethanol were the stress agents showing the highest overestimations. The results presented here demonstrated the overestimation of cell viability by the indirect method. Direct CFU counting is an adequate method to determine yeast viability during toxicity studies of chemical compounds. It was also established that ethanol and hydrogen peroxide have the highest toxicity against K. marxianus ITD-01005 during fermentation at concentrations equivalent to IC30 and IC50 of each stress agent.