title: enrichment
design: The primers 515F (5'- GTGCCAGCMGCCGCGG-3') and 907R (5'- CCGTCAATTCMTTTRAGTTT-3') were used in this study. The PCR conditions were as follows: 94C for 5min; 31 cycles of three steps: 94C for 30 s, 52C for 30s, and 72C for 45s; a final step at 72C for 10min. The PCR products were sequenced by Illumina PE2500 Miseq platform