Close encounters of the type‐six kind: injected bacterial toxins modulate gut microbial composition

Bacteria of the phylum Bacteroidetes constitute a substantial portion of the human gut microbiota, including symbionts and opportunistic pathogens. How these bacteria coexist and provide colonization resistance to pathogenic strains is not well understood. In this issue of EMBO Reports , Hecht and colleagues describe a mechanism by which strains of Bacteroides fragilis compete with each other for an intestinal niche [1][Hecht AL, 2016]. Prompted by the observation that B. fragilis populations appear to be dominated by either commensal, non‐toxigenic strains, or by enterotoxigenic, potentially pathogenic strains, the authors investigated mechanisms of competition between these two subsets. In agreement with two recent studies [2][Wexler AG, 2016][3][Chatzidaki‐Livanis M, 2016], Hecht et al [1][Hecht AL, 2016] found that competition between B. fragilis strains is dependent on a type‐6 secretion system (T6 SS ) apparatus, secreted effectors, and immunity genes. They identify a T6 SS effector–immunity gene pair that enables a non‐toxigenic strain to competitively exclude enterotoxigenic B. fragilis , thus providing a proof of principle for the use of T6 SS ‐mediated killing as a therapeutic strategy to eradicate pathogenic strains.