the V3-V4 region of the bacterial 16S rDNA was amplified using the primers 338F (ACTCCTACGGGAGGCAGCAG), 806R (GGACTACNNGGGTATCTAAT), and the polymerase chain reaction was performed in a 40 microL reaction system. The amplification products were purified using AMPure PB beads (Pacific Biosciences, CA, USA) and quantified using the QuantiFluor-ST (Promega, WI, USA). The PE libraries were constructed using the NEXTFLEX Rapid DNA-Seq Kit (Bioo Scientific, TX, USA).