Updated project metadata. We compared the microbiota of paired mouse caecal contents and faeces by applying a multi-omic approach, including 16S rDNA sequencing, shotgun metagenomics, and shotgun metaproteomics. The aim of the study was to verify whether faecal samples are a reliable proxy for the mouse colonic luminal microbiota, as well as to identify changes in taxonomy and functional activity between caecal and faecal microbial communities, which have to be carefully considered when using stool as sample for mouse gut microbiota investigations.