title: 3C3
design: Polymerase chain reaction (PCR) was used to amplify the V3-V4 region of the bacterial 16S rRNA gene using the primer pair 338F/808R. PCR amplicons were purified individually using Agencourt AMPure XP (Beckman Coulter, Inc., USA) and then pooled in equal amounts. According to the manufacturer's instructions, sequencing libraries were generated using the NEB Next Ultra II DNA Library Preparation Kit (New England Biolabs, Inc., USA).