Global biogeography of airborne viruses in public transit systems and their host interactions.

Background: There is a diverse assemblage of microbes in air in built environments (BEs), but our understanding of viruses and their interactions with hosts in BEs remains incomplete. To address this knowledge gap, this study analyzed 503 metagenomes isolated from air samples from public transit systems in six global cities, namely Denver, Hong Kong, London, New York City, Oslo, and Stockholm. Viral genomes were recovered from samples via metagenomic binning, and viruses' taxonomy, functional potential, and microbial hosts were determined. The study also investigated correlations between virus and host abundances, the coevolution of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems and anti-CRISPR (Acr) proteins, and the potential impacts of auxiliary metabolic genes (AMGs) on hosts.

Results: Airborne viruses in global BEs exhibited biogeographical variations in diversity, composition, function, and virus-host interactions. Nearly half of the vOTUs analyzed were from the Caulimoviridae family, while 31.8% of them could not be taxonomically classified. Diverse functions were identified within the vOTUs, together with antimicrobial resistance genes with the potential to confer resistance to various antibiotics and antimicrobial agents. Strong correlations were observed between vOTU and host abundances, with clear distinctions between virulent and temperate viruses. However, there was limited co-evolution of CRISPR-Cas systems and Acr proteins, which was likely due to the oligotrophic and physical conditions in the BEs and the dominance of vOTUs with a virulent lifestyle. Phage-encoded AMGs appeared to have the potential to enhance host fitness. These findings highlight biogeographical variations in airborne viruses in BEs and that physical and oligotrophic conditions in BEs drive virus survival strategies and virus-host coevolution.

Conclusion: There are biogeographical variations in airborne viruses in BEs in global cities, as physical and oligotrophic conditions in BEs drive virus survival strategies and virus-host coevolution. Moreover, the characteristics of airborne viruses in BEs are distinct from those of viruses found in other, more nutrient-rich ecosystems. Video Abstract.