As discussed in previous posts (see Reduced Diversity of Faecal Microbiota in Crohn's Disease, Localized Dysbiosis of lactobacilli and the Clostridium leptum Subgroup May Be Related to UC, Lack of Faecalibacterium prausnitzii Bacteria May Contribute to Crohn's), imbalances or reduced numbers of certain types of bacteria in the gut have been linked to Crohn's disease.A further example of this was demonstrated by researchers in regard to the Bacteroides fragilis bacteria. When the bacteria was not present in the gut and a pathogenic bacteria, in this case Helicobacter hepaticus, was introduced, intestinal inflammation was induced. The B. fragilis bacteria produces Polysaccharide A (PSA), which suppresses the inflammatory response of the immune system (by suppressing IL-17 production).
With hundreds of bacteria in the gut, it's unclear which ones are needed to have a "normal" balance. And it's also unclear what is "normal" based on genetic pre-disposition. Nonetheless, it does highlight the need for further investigation into how all of these bacteria interact and how exposure, diet, and other factors can influence the balance of those interactions.