Acetic Acid Stress Hampers Kluyveromyces marxianus Growth on Lactose

Kluyveromyces marxianus is a non-conventional yeast capable of efficiently metabolizing lactose, but acetate can inhibit its growth. Because K. marxianus strains differ physiologically, their tolerance to acetate also varies. Acetate tolerance was investigated in four K. marxianus strains grown on glucose, lactose, and an equimolar mixture of glucose and galactose. The inhibitory effect of 40 mM acetate on growth was evaluated at pH 4.5–6.0 by using acetate and citrate buffer systems. In lactose-containing media at pH 4.5, the strongest inhibition was observed in strain NCYC 2791, whose specific growth rate decreased from 0.51 ± 0.01 h−1 to 0.13 ± 0.01 h−1, while lag-phase duration increased from 10.11 ± 0.35 h to 21.09 ± 1.95 h. In contrast, DSM 5422 showed a smaller decrease in specific growth rate, from 0.54 ± 0.03 h−1 to 0.31 ± 0.06 h−1. NCYC 2791 also reached only OD600 = 0.18 after 45 h in acetate-containing lactose media, whereas the other three strains reached approximately OD600 = 0.6. The distribution of cytosolic and non-cytosolic β-galactosidase activity differed among strains, with the highest proportion of cytosolic activity in NCYC 2791 (80% of total activity). A significant positive correlation was found between the proportion of cytosolic β-galactosidase and the degree of growth inhibition
by acetate at pH 4.5 (Pearson r = 0.9967, p = 0.0033). These findings suggest that strain-dependent β galactosidase localization may be related to acetate tolerance, although this association should be interpreted cautiously because localization was not measured under acetate stress conditions.