Vertical transfer of the resistome in an urban setting

The phyllosphere resistome is considered a key site for the transfer of both natural and anthropogenically selected antimicrobial resistance genes (ARGs) to humans. Consequently, the development of green building systems may pose an unexplored risk of ARG transfer from outdoor plants to indoor environments. To investigate this potential risk, we collected leaves from plants climbing up buildings at 1, 2, 4 and 15m above ground level and collected associated dust samples from adjacent windowsills. A total of 142 ARGs from 14 major ARG classes as well as 18 MGEs were detected. The relative abundance of ARGs and MGEs within the phyllosphere decreased with increasing height above ground level. The compositions of bacteria and fungi were account for the shift of ARGs individually and synergically. Fast expectation-maximization microbial source tracking (FEAST) showed a stable microbial community within the phylosphere. A strong correlation (Mantel test P< 0.01) existed between the shared bacterial or fungal OTUs and ARGs within rhizosphere soil, soil and phyllosphere suggesting that there is potential for microganisms originating from soil to be found within the phyllosphere of climbing plants at all sampled heights. These results demonstrate that outdoor climbing plants that form a green facade provide a potential pathway for the transfer of ARGs to the indoor environment.