R. gnavus colonization ameliorated colitis in antibiotic-treated Lyz1−/− mice but exacerbated colitis in antibiotic-treated WT mice, suggesting that the R. gnavus-mediated immune response in vivo was dependent upon lysozyme processing as supported by in vitro data showing that lysozyme-processed R. gnavus elicited a range of inflammatory cytokines. This evidence concerns the gene LYZ and colitis.