Thermal adaptation of Lactiplantibacillus plantarum by laboratory evolution does not enhance tagatose biosynthesis

D-Tagatose is recognized as a promising sugar alternative, but its biosynthesis is limited by poor thermodynamics of isomerization from D-galactose. Higher reaction temperatures favor tagatose production, but microbial whole cell conversion is limited by viability of cells at elevated temperatures. This work attempts to address this by engineering Lactiplantibacillus plantarum for growth at elevated temperatures towards development of a high-temperature galactose to tagatose isomerization biocatalytic system. Using adaptive laboratory evolution (ALE), a strain capable of growth at 43°C was isolated over 85 passages of increasing thermal stress. The adaptation required using a cycling approach, where cultures were cultured at permissive (37°C) between growth at elevated temperatures. At passage #85, the cells were able to grow at 43°C over multiple passages, indicating complete adaptation. They were also able to grow well at 37°C. Unfortunately, galactose to tagatose conversion reaction at 37, 50, and 60°C using an L-arabinose isomerase transformed strain revealed no benefit of using the evolved strain over the parental strain. Nevertheless, the ALE protocol and isolated strain may be of benefit for other applications.