Metagenome or environmental sample from human oral metagenome
Bacterial DNA was isolated from fecal and sputum samples as previous described. DNA libraries covering the V3-V4 hypervariable regions of the bacterial 16S-rDNA gene were constructed using the FastPfu Polymerase (TransGen, China) according to the manufacturers instructions. We used the primer set composed of 338F: 5  ACTCCTACGGGAGGCAGCAG - 3, and 806R: 5  GGACTACHVGGGTWTCTAAT - 3, which was designed to amplify the V3V4 hypervariable region. All PCR products were purified with an AxyPrep DNA Gel Extraction Kit (Axygen Biosciences, USA) and quantified using a QuantiFluor-ST (Promega, USA) according to the manufacturers instructions.