Analysis of the phase-variable regulation of the capsular polysaccharides in the human symbiont Bacteroides

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Bacteroides fragilis strain YCH46 possesses master DNA invertases that regulate DNA inversions at multiple loci simultaneously. One is Mpi that mediates the on-off switching of seven promoter regions for capsular polysaccharide(PS) biosynthesis. Dispite the importance of PS synthesis for intestinal colonization, little is known about PS regulation in vitro and in vivo. While this organism may theoretically display up to 128 distinct polysaccharide combinations, the PS3 promoter was only on in greater than 12% of bacteria grown in vitro. Intestinal lamina propria lymphocytes from PS3-deleted B. fragilis mono-associated mice displayed decreased percentages of CD4^+ IL-4^+ T cells compared with control mice.