design: The V6V8 region of the 16S rRNA gene was amplified using universal (degenerate) PCR primers 799F (acCMGGATTAGATACCCKG) and bac1193R (CRTCCMCACCTTCCTC). Amplification was performed using the HotStarTaq Plus Master Mix Kit (Qiagen, Germantown, MD, USA) under the following conditions: 94C for 3 min, followed by 28 cycles of 94C for 30 s, 53C for 40 s and 72C for 1 min, with a final elongation step at 72C for 5 min. PCR products were used to prepare DNA libraries following Illumina MiSeq DNA library preparation protocol.