Efficacy of oral and rectal administration of human faecal microbiota transplant (FMT) in human microbiota-associated mouse models

The gut microbiome has gained significant interest due to its association with immune dysregulation, allergies, autoimmune conditions, metabolic disorders, and inflammation-associated malignancies. Understanding underlying mechanisms requires appropriate in vivo models, such as human microbiota-associated mouse models to study the microbiota-host interactions. This study compared the efficacy of oral and rectal administration of human faecal microbiota transplant (FMT) in C57BL/6J germ-free mice as these methods are often used interchangeably. Using 16S rRNA sequencing, we quantified colonisation efficacy in luminal and tissue samples from orally- (n=6) and rectally- (n=6) colonised mice. We detected 84 genera in the FMT sample, 17 of which were not transferred at all, while additional 7 genera were found exclusively in rectally-colonised mice. A significantly higher proportion of amplicon sequence variants (ASVs) (33% vs 26%; P < 0.05) and genera (32% vs 25%; P < 0.05) were absent in orally- compared to rectally-colonised group. Some taxa showed different relative abundances in human vs. mouse samples (e.g. Alistipes and Bacteroides relatively more abundant in mice while Faecalibacterium considerably decreased). Beta diversity analysis revealed greater similarity between FMT and tissue samples irrespective of the administration route, with distinct separation of the tissue and luminal samples. Overall, rectal administration yielded slightly better outcomes, with more overlapping genera and ASVs with FMT highlighting its potential advantages for microbiota establishment studies.