Residential Outdoor Air Microbiome: Exploring its environmental predictors, impacts on host microbiome, and associations with human health

Background: There is increasing evidence that residential vegetation is associated with improved human health. The biodiversity hypothesis proposes that greater environmental microbial diversity exposure improves human health through improved immune function. A potential mechanism to explain the relationship between vegetation exposure and improved human health is exposure to diverse ambient outdoor microbiomes. Objectives: We examined three types of associations: (1) between residential greenness and diversity of the outdoor ambient microbiome; (2) between ambient air microbiome and human host microbiomes; and (3) between the residential outdoor ambient microbiome and biomarkers of systemic inflammation in order to elucidate the mechanisms behind the observed association between residential greenspace exposure and improved human health. Methods: For this work, we collected residential outdoor ambient microbiome samples outside the residences of 80 study participants in the Raleigh-Durham-Chapel Hill metropolitan area. Paired vegetated land cover data within 500 m of participant residence, questionnaire data, human biological samples, and human saliva and gut microbiome samples were collected as part of a larger study that focused on the relationship between greenspace and human health. Next-gen DNA sequencing of the 16S rRNA gene isolated from the microbiome samples and DADA2 bioinformatic analysis were conducted to measure microbial diversity within (α-diversity, Shannon’s Index) and between samples (β-diversity, weighted UniFrac). Regression and Permutational Multivariate Analysis of Variance (PERMANOVA) analyses were used to assess statistical associations. Results: We observed a statistically significant positive association between residential outdoor ambient microbiome α-diversity and vegetated land cover and tree cover. We also observed a statistically significant difference in the β-diversity between air, gut, and saliva microbiomes and a positive but non-significant association between air and gut microbiome α-diversity indices. We further observed some differences in ambient bacterial composition in individuals with differing levels of systemic inflammation. Conclusions: Overall, this explorative research provides some novel information on potential pathways linking the green environment, the ambient microbiome, and human health. The interpretation of these results is limited primarily due to the small sample size; the findings warrant further study on the mediating role of the outdoor ambient microbiome in the relationship between greenspace and human health.