PCR Amplification and Illumina MiSeq Platform Sequencing

PCR amplification of 16S rRNA gene and sequencing were performed by Majorbio Co., Ltd. (Shanghai, China). PCR amplification was carried out using the 338F/806R primer targeting V3-V4 regions of the bacterial 16S rRNA gene (338F 5′-ACTCCTACGGGAGGCAGCAG-3′ and 806R 5′-GGACTACHVGGGTWTCTAAT-3′) (Chu et al., 2015). PCR products were purified by the AxyPrep DNA Gel Extraction Kit (Axygen Biosciences, Union City, CA, United States), and quantified by QuantiFluor^TM-ST (Promega, United States), then sequenced (2 × 250 bp) on the MiSeq PE300 platform (Illumina, United States).