Structure and Function Relationships of Exopolysaccharides Produced by Lactic Acid Bacteria

  • Author / Creator
    Chen, Xiaoyan
  • Lactic acid bacteria produce glycans that may be applied in food and pharmaceutical industries as prebiotics, food additives, to prevent pathogen adhesion, or to modulate the host immune system. The recent identification of the structure of galacto-oligosaccharides (GOS) preparations demonstrates that their functionalities are dependent on the chemical structures. This research aimed to investigate the relation between the structures of exopolysaccharides (EPS) produced by lactic acid bacteria and their functions in food and health applications. Enterotoxigenic Escherichia coli (ETEC) is a major cause of secretory diarrhea in piglets; ETEC colonizes to the intestinal mucosa by fimbriae and produces diarrheal toxins. Anti-adhesive properties of the bacterial glycans reuteran and levan and of the commercial glycans dextran and inulin were determined using a small intestinal segment perfusion (SISP) model. Quantitative PCR identified E. coli as the dominant organism in infected segments. The presence of autochthonous ETEC K88 was revealed by qPCR. Bacterial EPS significantly decreased adherent ETEC K88; however, this effect was not attributed to reuteran but to bacterial extracts produced by the reuteransucrase negative strain L. reuteri TMW1.656ΔgtfA. The enzymatic digestion of the anti-adhesive compound by DNase, RNase, lysozyme and mutanolysin, and the screening of heteropolysaccharides gene cluster implied that heteropolysaccharides produced by L. reuteri was a candidate for the anti-adhesive activity. EPS produced by lactic acid bacteria improve the texture and shelf life of bread. The effect of EPS on bread quality depends on the properties of EPS and EPS-producing strains. The construction of a heterologous expression system of dextransucrase and reuteransucrase, and site-directed mutagenesis of glucansucrases allowed the ex situ production of pure reuterans and dextran. Linkage type and molecular weight of enzymatically-produced glucans were determined by 1H-NMR and asymmetric flow-field-flow fractionation. L. reuteri TMW1.656 and L. reuteri TMW1.656ΔgtfA served as fermentation strains to remove confounding effects of bacterial metabolites. Bacterial and enzymatically produced reuterans had comparable effects on bread volume and crumb hardness. Reuteran with higher portion of α-(1→4) linkages and lower molecular weight was as efficient as dextran in enhancing wheat bread volume and texture. Overall, this study established a valuable model to elucidate structure-function relationships of glucans in baking applications. In general, this is the first study to demonstrate structure and function relationships of bacterial and enzymatically-produced reuterans in bread baking application. The structural determinants of bacterial EPS that prevent ETEC K88 adhesion needs to be confirmed with further study.

  • Subjects / Keywords
  • Graduation date
    Spring 2017
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.