The university campus microbiome: ecological drivers of human–environment microbial interactions
Urbanization is rapidly reshaping human habitats, with built environments (BEs) becoming the primary spaces where people live, study, and work. These settings act as ecological filters that shape both environmental and human-associated microbiomes, therefore influencing human and environmental health. Among BEs, university campuses represent distinctive, high-density, and highly dynamic micro-urban systems, characterized by intense human traffic, heterogeneous indoor and outdoor spaces, and recurrent population mixing. Despite their relevance as semi-closed ecosystems of daily exposure, the reciprocal relationships between human microbiota (skin and gut) and campus-associated microbial communities remain remarkably underexplored. Here, we investigated the microbiomes of the University of Milano-Bicocca (Milan, Italy) by integrating skin and gut profiles from campus-attending students with microbial communities sampled from indoor and outdoor environments across two seasons. Using 16S rRNA gene sequencing, we assessed diversity patterns, community composition, source–sink relationships, and co-occurrence networks to identify ecological drivers of human–environment microbial connectivity.  Collectively, this study provides the first integrated characterization of skin, gut, and built-environment microbiomes within an urban university campus. Our findings highlight how lifestyle, urban exposure, and BE features jointly modulate human-associated microbial diversity. These results emphasize the role of built spaces as active mediators of microbiome composition and underscore the value of microbiome profiling as a tool to evaluate the selective pressures of modern living spaces and to inform healthier urban design.