Utilization of Monosaccharides by Hungateiclostridium thermocellum ATCC 27405 Through Adaptive Evolution

Hungateiclostridium thermocellum ATCC 27405 is a promising bacterium with a robust ability to degrade lignocellulosic biomass complexes, including crystalline cellulose components, through a multienzyme cellulosomal system. In contrast, it exhibits poor growth on simple monosaccharides such as fructose and glucose. This phenomenon raises many important questions concerning its glycolytic pathways and sugar transport systems. Until now, the detailed mechanisms of H. thermocellum adaptation to growth on monosaccharides have been poorly explored. In this study, adaptive laboratory evolution was applied to train the bacterium on monosaccharides, and genome resequencing was used to detect the genes that had mutated during adaptation. RNA-seq data of the 1st-generation culture growing on either fructose or glucose revealed that several glycolytic genes in the EMP pathway were expressed at lower levels in these cells than in cellobiose-grown cells. After 8 generations of culture on fructose and glucose, the evolved H. thermocellum strains grew faster and yielded greater biomass than the nonadapted strains. Genomic screening also revealed several mutation events in the genomes of the evolved strains, especially in genes responsible for sugar transport and central carbon metabolism. Consequently, these genes could be applied as targets for further metabolic engineering to improve this bacterium for bioindustrial usage.