Core gut symbionts protect bumble bees from infection by a common bacterial pathogen. Serratia_Core_Symbionts

Microbes shape the health of bumble bees, an important group of pollinators that includes many species of conservation concern. Most microbial research on bumble bees has focused on eukaryotic and viral pathogens or on the core gut microbiome. The latter refers to a community of host-specialized bacterial symbionts whose best-known function is protecting hosts against eukaryotic pathogens. However, bumble bees also harbor a third class of microbes: non-core gut bacteria, which are defined as being non-host-specific and variable among individuals. Understanding the functional role of non-core bacteria and how they interact with core symbionts could be important for bumble bee microbiome ecology and management. Here, we measured the prevalence of non-core bacteria in wild bumble bee workers (Bombus impatiens) and conducted lab experiments with gnotobiotic B. impatiens workers to examine factors shaping colonization by a focal non-core species (Serratia marcescens) and its consequences for bee health. We found that non-core bacteria, including Serratia, are highly prevalent in wild bumble bees, with roughly half of individuals harboring at least 10% non-core bacteria in their gut. Experiments showed that Serratia marcescens is more likely to colonize the gut when bees are young and the core gut microbiome is disrupted. Exposing bees with disrupted gut microbiomes to Serratia strongly reduced lifespan and reproduction, indicating that at least some widespread non-core bacteria include opportunistic pathogens. Overall, we provide evidence for protection against non-core bacterial pathogens as a previously undescribed function of the bumble bee core gut microbiome. Our work lays a foundation for monitoring and managing bumble bee gut health to limit core microbiome disruption and the invasion of bacterial pathogens.