Indoor-outdoor microbial exchange: Quantifying the contribution of outdoor sources to indoor microbiota across seasons

Background: Bacteria and fungi are ubiquitous, suspended in the air, and have the potential to influence human health. However, the relationship between microbes in outdoor air, soil, and indoor air remains unknown. Methods: This study comprehensively analyzed the microbes of indoor air, outdoor air and soil during summer and winter in Nanjing, a hot summer and cold winter city in China. A total of 102 rooms, 544 culturable samples, and 406 sequencing samples were investigated. The Sloan neutral model (SNM) and the fast expectation-maximization microbial source tracking (FEAST) were used to analyze the relationship between indoor and outdoor microbial communities. Results: The impact of season on bacterial diversity differed from that on fungal diversity. Microbial diversities and community structures in indoor air, outdoor air, and soil were similar within the same season but differed between seasons. The fitting results of the SNM indicated that the stochastic assembly processes of both indoor and outdoor airborne bacteria were greater in summer than in winter, whereas for airborne fungi, the situation was reversed. The FEAST calculating results suggested that outdoor air serves as a more significant source than soil for indoor air microbiota both in summer and winter. Conclusion: This study provided a new insight into understanding the quantitative contributions of outdoor sources to indoor airborne microbiota. Future studies should include a wider range of building types and extend across multiple seasons to provide deeper insights into the quantitative relationship between indoor and outdoor microbiota.