The link between the intestinal microbiota and IBD extends as far back as Sir Thomas Kennedy Dalziel’s original description of what has since become known as Crohn’s disease [1]. In 1913, Dalziel compared the pathological changes of his “chronic interstitial enteritis” with those of Johne’s disease in cattle and in doing so offered Mycobacterium avium subsp. paratuberculosis as the possible cause.