The V3-V4 hypervariable regions of the bacterial 16S rRNA gene were amplified with the primers 338F (5 -ACTCCTACGGGAGGCAGCAG-3  ) and 806R (5 -GGACTACHVGGGTWTCTAAT-3) by a thermocycler PCR system (GeneAmp 9724, ABI, Foster, CA, United States).