16S gene library construction, quantification, and sequencing

Samples were delivered to Sangon BioTech (Shanghai, China) for library construction using a universal Illumina adaptor and index. Before sequencing, quality control was performed using a bioanalyzer (Agilent 2100, Santa Clara, CA, USA). Depending on the coverage requirements, all libraries can be pooled in a single run. Equimolar proportions were pooled according to the concentration of the purified amplicon. The PCR products were sequenced on an Illumina MiSeq platform (Illumina MiSeq, San Diego, CA, USA), following the manufacturer’s recommendations.