Structural characterization and properties of a novel acetylated extracellular polysaccharide from Rhodotorula graminis.

Rhodotorula yeasts are promising microbial cell factories, recognized for their intracellular production of lipids and carotenoids. Recently, research has expanded to extracellular metabolites, particularly exopolysaccharides and polyol esters of fatty acids. In this study, an exopolysaccharide isolated from Rhodotorula graminis was characterized using high-performance anion-exchange chromatography, size exclusion chromatography, and nuclear magnetic resonance. The results show that the polymer has a trisaccharide repeating unit of 1,4-linked a α-d-glucuronic acid, 1,2,3 linked α-d-mannose, and terminal β-d-galactose. Furthermore, the results suggest the presence of approximately two acetyl substituents per repeating unit and a molecular weight of ≥1.5 MDa. Rheological analysis of aqueous EPS solutions showed a non-linear increase in viscosity with concentration, and sponges were prepared by freeze-drying. Biocompatibility assays with human epidermal keratinocytes and Atlantic salmon cells indicated no cytotoxic effects under the tested conditions. The EPS also showed weak, concentration-dependent growth inhibition against Escherichia coli, Serratia marcescens, and Candida glabrata over the tested concentration range of 7.87-1007.5 μg·mL-1. Overall, the results indicate that this EPS is a structurally unusual Rhodotorula-derived polymer with interesting physicochemical properties and potentially good biocompatibility.