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Commensal bacteria are necessary to clear pathogenic bacteria from intestinal infections

In a very exciting paper, this study showed that in a mouse model of intestinal bacterial infections using Citrobacter rodentium, commensal bacteria were required to clear the infection. The ability of the commensal bacteria to outcompete the invading C. rodentium pathogen was shown to be in part due to the competition for structurally similar carbohydrates.
This derived from observations that C. rodentium can be outcompeted by certain bacteria such as E. coli, but not other Bacteroides species. After demonstrating that both C. rodentium and E. coli both grew optimally on monosaccharides whereas certain Bacteriodes species grew equally well on mono- or polysaccharides, a sugar competitive growth test was designed. After infecting GF mice with C. rodentium, 21 days post-infection these mice were colonized with Bacteroides thetaiotaomicron. These mice were fed either simple sugar (only monosaccharides) or conventional diets. C. rodentium declined 200 fold in in mice fed the simple sugar diet but not the conventional diet. Crucially, there was no change in either diet in those mice not colonized with B. thetaiotaomicron, suggesting that when this bacterium cannot use polysaccharides it is forced to metabolically compete with C. rodentium for available monosaccharides. An interesting follow-up would be more comprehensive genetic approaches with E. coli/C. rodentium to see if one can alter this competitive relationship with C. rodentium.

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