Humans in industrialized countries spend nearly 90% of their time indoors.  Human health and well-being is intimately connected to microbial communities in the built environment, for example as sources of pathogens, allergens, and human commensals., Although recent advances have been made in our understanding of the composition and function of the human microbiome, our understanding of the built environment microbiome is limited.  This limitation has made it difficult to assess the influence of the built environment as a potential source and dispersal vector for microbial populations present in the human microbiome.

In this study, we explore how building design influences the built environment microbiome.  Despite the potential role of ventilation in built environments as a dispersal vector and habitat for microbes, the impact of ventilation practices on the indoor microbial diversity and human health are currently unknown. To bridge this gap we address three questions: 1) What is the magnitude and variability of microbial diversity in the built environment? 2) Are patterns of microbial diversity in the indoor air environment fundamentally different from outdoor environments? 3) What is the influence of building design (especially sustainable or “green” design) on microbial abundance and diversity?

To answer these questions, we sampled microbial communities and environmental conditions in the air of a hospital, specifically contrasting air samples from mechanically-ventilated hospital rooms with those ventilated naturally, and contrasting indoor and outdoor environments. Cell density measurements and bacterial 16S gene sequences based on Sanger sequencing and pyrosequencing of airborne DNA were analyzed to determine the taxonomic and phylogenetic composition and diversity of the bacterial communities in these samples.

Results/Conclusions

Natural ventilation increased the taxonomic and phylogenetic diversity of airborne microbial communities in hospital rooms, and led to shifts in the overall composition of microbial communities, with several taxonomic groups present only in natural or mechanical ventilation treatments. Regardless of ventilation treatment, indoor airborne microbial communities were dominated by bacterial taxa shared with the human microbiome and with known roles in human health. We discuss the implications of our findings with regards to the effects of ventilation systems on the rare microbial biosphere and indoor-outdoor dispersal of microbes, and the potential effects of architectural design on microbial dispersal, diversity and human health.