Bacteroides-associated NAD⁺ depletion correlates with exacerbated radiation-induced colorectal injury and impaired mucosal proliferative capacity.

Baseline fecal microbiome composition differed between patients who developed mild versus severe radiation proctitis, with Bacteroidales enriched in severe RP and Firmicutes enriched in mild RP.

• 55 patients were prospectively profiled with pretreatment fecal microbiomes and metabolomes and stratified by outcome into mild versus severe RP.
• Bacteroidales order was enriched at baseline in patients who went on to develop severe radiation proctitis.
• Firmicutes phylum was enriched at baseline in patients who developed mild radiation proctitis.
• Profiling was performed prior to radiotherapy, establishing these as pretreatment microbial differences.

Multi-omics integration highlighted nicotinate/nicotinamide pathways as distinguishing severe from mild radiation proctitis.

• Integration of fecal microbiome and metabolome data pointed to nicotinate/nicotinamide metabolism as a key differentiating pathway.
• Severe RP was characterized by concomitant reductions in both fecal and tissue NAD+ levels.
• Microbial nicotinate/nicotinamide metabolism genes were enriched in the severe RP group.
• The primary microbial contributors to enriched nicotinate/nicotinamide metabolism genes were Bacteroides ovatus, B. xylanisolvens, and B. fragilis.

Fecal microbiota transplantation from severe-RP donors exacerbated radiation-induced colorectal injury and decreased colorectal NAD+ in mice.

• Mouse experiments used FMT from human severe-RP donors compared to mild-RP or control donors.
• Mice receiving severe-RP donor FMT showed worsened radiation-induced colorectal pathology.
• Colorectal NAD+ levels were decreased in mice receiving severe-RP donor FMT.
• These results were interpreted as supporting a causal role for the microbiota in injury severity.

Gavage with Bacteroides species worsened radiation-induced colorectal pathology and lowered NAD+ levels in mice.

• Direct gavage with Bacteroides (including B. ovatus, B. xylanisolvens, and/or B. fragilis) was performed in mouse models.
• Bacteroides gavage resulted in worsened colorectal pathology following radiation.
• NAD+ levels in colorectal tissue were lowered following Bacteroides gavage.
• These findings complemented the FMT experiments in supporting a causal microbial role.

NMN supplementation attenuated radiation-induced colorectal injury exacerbated by Bacteroides gavage.

• Nicotinamide mononucleotide (NMN) was administered as a supplementation intervention in mice with Bacteroides gavage and radiation.
• NMN attenuated the radiation-induced colorectal injury in this model.
• NMN reversed the injury effects in parallel with NAD+ restoration.
• NMN supplementation also reversed the effects of Bacteroides gavage on mitochondrial membrane potential, Lgr5+ stem-cell proportion, and proliferative indices.

Bacteroides gavage reduced mitochondrial membrane potential, decreased the Lgr5+ stem-cell proportion, and reduced proliferative indices in colorectal mucosa, associated with Wnt pathway modulation.

• Mechanistic analyses showed Bacteroides gavage reduced mitochondrial membrane potential in colorectal tissue.
• The proportion of Lgr5+ intestinal stem cells was decreased following Bacteroides gavage.
• Proliferative indices of colorectal mucosa were reduced after Bacteroides gavage.
• These effects were associated with modulation of the Wnt signaling pathway.
• NMN supplementation reversed all of these mechanistic effects alongside NAD+ restoration.

Radiation proctitis is a frequent complication of pelvic radiotherapy that compromises treatment delivery and patient quality of life.

• RP is described as a frequent complication of pelvic radiotherapy.
• The factors shaping injury severity were described as incompletely defined prior to this study.
• The study was designed to identify microbiome and metabolite factors associated with RP severity.
• The work was framed as providing insight into RP pathogenesis and suggesting microbiome- and metabolite-targeted therapeutic approaches.