design: Construction of a high-throughput sequencing library and Illumina-based sequencing using a MiSeq instrument were carried out by Genewiz (South Plainfield, NJ, USA). Amplicons were generated using a MetaVxTM Library Preparation kit (Genewiz, South Plainfield, NJ, USA). V3 and V4 regions of the bacterial 16S rDNA gene were amplified using forward primers containing the sequence CCTACGGRRBGCASCAGKVRVGAAT and reverse primers containing the sequence GGACTACNVGGGTWTCTAATCC. Sequencing was performed using a 2 x 150 paired-end (PE) configuration.