Metagenomic sequencing
Fresh fecal samples were collected and immediately frozen at −80°C. Total fecal DNA was extracted using QIAamp Fast DNA Stool Mini Kit (Qiagen, Germany). After the detection of concentrations of DNA samples by Qubit 3.0 (Thermo Fisher Scientific, USA), the quality was further checked on a 1.5% agarose gel. Next, 100 ng DNA was cut into fragments with an average size of 350 bp using the NEBNext