Nadeem O Kaakoush, Donald S Thomas, Mahmoud M Ruzayqat, David Lynch, Steven T Leach, Daniel A Lemberg, Andrew S Day, Hazel M Mitchell

Campylobacter concisus is a member of the oral microbiota that has been associated with the development of inflammatory bowel diseases. However, the role of the bacterium in disease etiology remains poorly understood. Here, we examine optimal conditions for the growth of C. concisus, and the pathogenic potential of this bacterium to human gastrointestinal cells from the upper tract. Further, the presence of C. concisus in the lower tract of Crohn's disease patients undergoing therapy is observed, and the associations of C. concisus with the abundance of other microbial taxa and compounds they produce are evaluated. C. concisus strains had the ability to tolerate moderate levels of acidity, adhere to and invade esophageal and gastric cells; however, these properties did not correlate with their pathogenic potential in intestinal cells. The presence of the bacterium in the lower gut of Crohn's disease patients was associated with an increased relative abundance of Faecalibacterium and Lachnospiraceae incertae sedis. Short chain fatty acids that can be produced by these microbial species did not appear to be responsible for this association. However, we identified genetic similarity between C. concisus and Firmicutes, specifically within aspartate and glutamate racemases. The potential pathogenesis of C. concisus in the upper gastrointestinal tract, and the responsiveness of the bacterium to therapy in a subset of CD patients warrant further investigation into whether this bacterium has a causal role in disease or its presence is incidental.