16S sequencing of negative control samples

Three types of negative control samples were sequenced on several 16S rRNA gene tag sequencing runs. In “blank swab” samples, a sterile swab was opened from its package in the sequencing lab, and the full sequencing protocol was applied to the swab. In “blank extraction” samples, DNA extraction and all subsequent steps were carried out with no additional input material. In “blank library” samples, the extraction protocol was not applied; DNA-free water (UltraClean PCR Water, MO BIO Laboratories, Carlsbad, CA) was used as input to the post-extraction steps of the protocol, starting with library generation.