The V3-V4 region of the bacterial 16S rRNA gene was amplified using the forward primer (5-ACTCCTACGGAGGCAGCA-3) and the reverse primer (5-GGACTACHVGGGTWTCTAAT-3). The amplified fragments were sequenced on the Illumina platform (Shanghai Panshine Biotechnology Co., Ltd., Shanghai, China) using a paired-end sequencer (NovNovaSeq 6000 SP Reagent Kit). The samples were amplified in a 25 L reaction volume using Q5? High-Fidelity DNA Polymerase (NEB, Ipswich, MA, cat.# M0491L), 10 mol/L of each primer, and 2 L of DNA template. The PCR reaction program was 98C for 30 s, 50C for 30 s, and 72C for 30 s. Primers were added to the template to synthesize DNA, and each cycle was completed for 25 cycles, during which many amplified DNA fragments were accumulated. Finally, the products were held at 72C for 5 minutes to complete the extension and then stored at 4C.