High molecular-weight polysaccharide contamination from yeast extract in semi-defined bacteriological media: Effects on exopolysaccharide production and purity

Exopolysaccharides (EPS) produced by lactic acid bacteria (LAB) and other microorganisms have attracted considerable interest due to their structural diversity and physicochemical properties, which makes them valuable across various industrial applications. To achieve high cell densities and maximize EPS yields, microorganisms are typically cultivated in nutrient-rich media containing yeast extract. However, yeast extract may contain high molecular weight polysaccharides that are not metabolized by the bacteria. This can lead to an overestimation of EPS yields and contamination of the bacterial EPS, potentially resulting in misinterpretation of their structure and biological activity. In this study, we demonstrate the presence of high molecular weight -mannan and {beta}-glucan in yeast extract in EPS isolates using both ultrafiltration and the commonly used trichloroacetic acid/ethanol (TCA/EtOH) precipitation method. These polysaccharides were characterized by size-exclusion chromatography, high-performance anion-exchange chromatography, and nuclear magnetic resonance spectroscopy. Their abundances were estimated to range from 10 to 50 mg/L in MRS medium, depending on the supplier of the yeast extract. The main contaminant identified was yeast -mannan. By cultivating L. rhamnosus GG (ATCC 53103) and L. pentosus KW1 and isolating their respective EPS, we illustrate how these yeast extract contaminants affect the structural interpretation of the EPS and that the contaminants can be completely removed by ultrafiltration of the growth medium prior to bacterial cultivation. In conclusion, we emphasize the necessity of stringent controls in the production and purification of microbial EPS, with particular attention to the chemical purity of medium constituents.