Human colon bacteria show substrate dependent hierarchical preference to dietary fibers, with structure determining rank
Y. E. TUNCIL (1), Y. Xiao (2), N. Porter (2), B. Reuhs (3), E. Martens (2), B. Hamaker (3) (1) Purdue University, West Lafayette, IN, U.S.A.; (2) University of Michigan Medical School, , U.S.A.; (3) purdue university food science, west lafayette, IN, U.S.A..

The human colonic microbiota is significantly affected by its food supply. Accordingly, there has been an increasing interest to modify its composition via dietary fibers (DF), as a way to improve health. However, in order to achieve a predicted manipulation, it is essential to understand how colonic bacteria respond to different DFs. Here, we aimed to understand the strategies of <i>Bacteroides ovatus </i>(<i>Bo</i>) and <i>B. thetaiotaomicron</i> to utilize different glycans presented as a mixture. We performed a series of time course assays in which both bacteria were individually grown in a media containing different DFs: amylopectin (AP), arabinan, chondroitin sulphate, pectic galactan, polygalacturonic acid, and rhamnogalacturonanI (RGI) of which they both are capable of degrading. Remaining substrates were measured over time as well as gene expression profiles. Both bacteria utilized certain glycans before others, but with different priorities indicating that bacterial species show species-specific hierarchical preference to DFs. Co-culturing of these organisms in the same mixture revealed that hierarchical preferences of human gut symbionts are preserved even in competitive environments. Their different glycan priorities mediate their stable coexistence. Additionally, a hypothesis was tested whether molecular structure of a glycan affects its place in the hierarchy. To test this, we repeated the hierarchical study for <i>Bo</i> by substituting AP with a structurally simple starch analog [maltohexaose (MH)]. AP was used after RGI by <i>Bo</i> when AP was included in the mixture, whereas MH was used before RGI, so the utilization of RGI by <i>Bo</i> was delayed in the presence of MH, showing that bacteria species’ preferences of a particular glycan can be changed by manipulating its chemical structure. Our results provide essential information how bacteria species respond to different glycan structures. 

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