Social network cycle motifs and gut microbiome strain-sharing

Microbes are commonly transmitted through human social interactions, yet little is known about how higher-order social network structures shape microbial circulation. Here, we analyze network cycles, i.e., closed loops of individuals, in both friendship networks and microbial networks, across 1,787 individuals from 18 isolated Honduran villages. Using strain-level resolution, we construct species-specific microbial networks, study their cyclic structure, and compare them with the social networks in the same population. Cycles were strongly over-represented relative to degree-preserving randomized networks in both social and microbial networks for most species, indicating that microbial transmission frequently occurs within recurrent and clustered groups of hosts. However, the overlap between microbial and social cycles varies substantially across species and individuals, and regression analyses identify a small subset of species whose cyclic sharing patterns are associated with social cycle participation. Notably, several anaerobic species show negative associations, suggesting reliance on repeated local exposures or shared environments only partially aligned with social ties. Consistent with this, many species exhibit niche-like transmission pathways independent of social network structure. Together, these findings show that microbial sharing networks exhibit rich higher-order organization that only partially mirrors human social networks and that reveals that network cycles can provide a useful framework for understanding how repeated exposure might contribute to microbial spread.