16S ribosomal RNA sequencing
16S ribosomal RNA sequencing of the V3-V4 region was performed by PCR. Sequencing library was generated and quality was assessed by Quant-iT PicoGreen (Invitrogen, USA). Then the library was sequenced on Illumina MiSeq Platform (Illumina, San Diego, CA), which generated about 300 bp double-end reads with over 50000 readings per sample and base mass was greater than 30.