Metagenomics 16S rRNA gene of the old mice: exposed-baseline
DNA extraction and purification were performed using the QIAamp DNA microbiome kit (Qiagen) under aseptic conditions. The concentration of purified DNA was determined using the QuBit High Sensitivity DNA quantification kit (Invitrogen) prior to sample freezing. The hypervariable V1-V2 region of the 16S rRNA   gene was amplified using 27F universal and 338R primers. Samples were sent to the Genewiz platform (Azenta) for 250 bp paired-ends sequencing with Illumina MiSeq.