Acetate-adapted Kluyveromyces marxianus isolates show improved lactose fermentation under acetate stress

Introduction: Kluyveromyces marxianus is a promising nonconventional yeast for industrial biotechnology, particularly for lactose-containing substrates, but acetate stress can inhibit growth and reduce process performance. In this study, adaptive laboratory evolution was applied to K. marxianus DSM 5422 under progressively increasing acetate concentrations, with lactose as the carbon source. Materials and methods: A serial batch selection experiment was performed with K. marxianus DSM 5422 during growth on lactose under progressively increasing acetate concentrations. Isolates from populations adapted to 50 mM acetate (EvoAc1) and 210 mM acetate (K. marxianus MSCL 1632) were compared with the parental strain during lactose fermentation without added acetate and in the presence of 40 or 100 mM acetate. Results: The acetate-adapted isolate MSCL 1632 showed the strongest performance under acetate stress. At 100 mM acetate, MSCL 1632 reached a net biomass yield of 0.072 ± 0.003 g/g lactose, compared with 0.027 ± 0.004 g/g lactose for DSM 5422. Endpoint biomass increased from 1.500 ± 0.187 g/L in DSM 5422 to 3.755 ± 0.198 g/L in MSCL 1632. Residual acetate was also significantly lower in MSCL 1632 than in DSM 5422 and EvoAc1 at 100 mM acetate, with Tukey-adjusted pairwise p-values of 3.30 × 10−5 and 2.40 × 10−5, respectively. Conclusions: These results show that acetate-adapted isolates, particularly MSCL 1632, performed better than the parental strain during acetate-stressed lactose fermentation. These findings suggest, but do not establish, that the improved tolerance may be associated with enhanced acetate utilization.