Lifestyle and Horizontal Gene Transfer-Mediated Evolution of , a Core Member of the Murine Gut Microbiota

is an abundant inhabitant of the intestinal mucus layer of rodents and other animals and has been suggested to be a pathobiont, a commensal that plays a role in disease. In order to gain insights into its lifestyle, we analyzed the genome and transcriptome of ASF 457 and performed physiological experiments to test traits predicted by its genome. Although described as a mucus inhabitant, has limited capacity for degrading host-derived mucosal glycans and other complex polysaccharides. Additionally, reduces nitrate and expresses systems for scavenging oxygen and reactive oxygen species , which may account for its localization close to the mucosal tissue and expansion during inflammation. Also of note, harbors a type VI secretion system and putative effector proteins and can modify gene expression in mucosal tissue, suggesting intimate interactions with its host and a possible role in inflammation. The genome has been shaped by extensive horizontal gene transfer, primarily from intestinal - and , indicating that horizontal gene transfer has played a key role in defining its niche in the gut ecosystem. Shifts in gut microbiota composition have been associated with intestinal inflammation, but it remains unclear whether inflammation-associated bacteria are commensal or detrimental to their host. Here, we studied the lifestyle of the gut bacterium , which is associated with inflammation in widely used mouse models. We found that has specialized systems to handle oxidative stress during inflammation. Additionally, it expresses secretion systems and effector proteins and can modify the mucosal gene expression of its host. This suggests that undergoes intimate interactions with its host and may play a role in inflammation. The insights presented here aid our understanding of how commensal gut bacteria may be involved in altering susceptibility to disease.