Seasonally increasing parasite load is associated with microbiota dysbiosis in wild bumblebees.

Gut microbiota often influence host defense against infection, but this relationship is incompletely understood in wild bumblebees. These critical pollinators host a characteristic core gut microbiota, yet field studies have offered conflicting insights into its association with IMPORTANCE: The community of microorganisms in close association with an animal, its microbiota, can be important to its health. Understanding how microbiota composition relates to health and disease is an important goal with broad potential implications. Like most animals, bumblebees have a characteristic core gut microbiota. We have conducted a broad survey of bumblebees over 3 years to examine the interactions of microbiota composition with infection by an endemic trypanosomatid parasite. We found that the relative abundances of core microbes were inversely related to infection load, and that increased pathogen load was associated with the prevalence of novel microbes. These results are evidence of strong associations between bumblebees and their core microbiota and suggest a role in providing resistance to severe parasitism.