Molecular strategies for enhancing microbial production of xylitol

Xylitol, a five-carbon polyol, has been widely used as a sugar substitute in dietary, food, and pharmaceutical industries because of its unique properties such as anticariogenicity, tooth rehardening, and prevention of otitis and upper respiratory infections. Recently, its demand has increased because of the increasing health consciousness of consumers. Xylitol has been currently produced by chemical hydrogenation of xylose at commercial level. However, microbial production of xylitol is a promising approach involving action of microorganisms, as it is carried out in mild conditions, does not require pure xylose, and involves simpler downstream processing steps in contrast to chemical method. Many wild-type microorganisms are reported (e.g., bacteria, yeasts, and fungi) in the literature for xylitol production, among which yeasts are considered the best xylitol producers. Although environmental conditions such as carbon source, temperature, pH, inoculum, and aeration are known to affect xylitol accumulation, its productivity is reported to be considerably limited by factors such as cofactor regeneration, inefficient xylose transport system, and low-level expression of enzymes involved in metabolic pathways. This study aims at describing the diversity of microorganism used for microbial production of xylitol with main emphasis on molecular approaches applied at the cellular/molecular level for enhancing its yield and productivity.