Dysbiosis-associated gut bacterium Ruminococcus gnavus varies at the strain level in ability to utilize key mucin component sialic acid

Ruminococcus gnavus is a prevalent human gut commensal bacterium with known roles in intestinal mucus degradation, including by catabolism of the terminal mucin sugar sialic acid. While R. gnavus is not considered a pathogen, overabundance of this species is correlated with Inflammatory Bowel Disease (IBD), and its sialic acid metabolism may play a role in the dysbiotic state. Interestingly, liberation of mucin-bound sialic acid by R. gnavus yields the distinct product of 2,7-anhydro-N-acetylneuraminic acid (2,7-anhydro-Neu5Ac), in contrast to other known mucin-degrading bacteria, which generate Neu5Ac. This prompted us to look for 2,7-anhydro-Neu5Ac metabolism proteins in the genomes of 77 R. gnavus clinical isolates. We found that 2,7-anhydro-Neu5Ac metabolism is sporadically distributed in this species with respect to phylogeny and strain origin. We measured sialic acid-dependent growth of 12 sequenced isolates, finding that the presence of 2,7-anhydro-Neu5Ac catabolism proteins was predictive of growth on this substrate. Our analysis also uncovered "partial" 2,7-anhydro-Neu5Ac catabolism pathways in two R. gnavus strains, which we determined constitute the canonical Neu5Ac catabolism pathway, previously unreported in this species. These results reveal a notable diversity of sialic acid catabolism across the R. gnavus species, an essential consideration for further investigations into the importance of this metabolism in mucin degradation and in roles of R. gnavus in IBD and other gut dysbioses.