Pathogenic bacterial taxa constitute a substantial portion of fecal microbiota in common migratory bats and birds in Europe.

Identifying the wildlife reservoirs of bacterial pathogens, spatially and temporally, is important for assessing the threats to human and the rest of the biosphere. Our objective was to study Europe-wide characteristics of the fecal microbiota of four highly mobile migratory vertebrates, that is, one bat ( IMPORTANCE: The understanding of gut microbiota composition and dynamics in wild vertebrate populations, especially in highly mobile vertebrates, birds and bats, remains limited. Our study sheds light on the critical knowledge gap in how common pathogenic bacterial taxa of fecal microbiota are in migratory bats and birds in Europe. We found out that bacterial genera having species pathogenic to human or animals constituted a substantial portion of the fecal microbiota in all the studied host taxa. Most importantly, we identified asymptomatic individuals that were dysbiotic with bacterial pathogen overgrowth. These previously unknown pathogen bloomers appear as potent Europe-wide transmitters of bacterial pathogens, which cause, for example, diarrhea and bacteremia in human. Our findings may contribute to better understanding of seasonal disease hotspots and pathogen spillover risks related to migratory vertebrates.