Core symbionts, age at inoculation and diet affect colonization of the bumblebee gut by a common bacterial pathogen.

Microbes shape the health of bumblebees, an important group of pollinators, including species of conservation concern. Most microbial research on bumblebees has focused on eukaryotic and viral pathogens or the core gut microbiome, a community of host-specialized bacterial symbionts that helps protect hosts against eukaryotic pathogens. Bumblebees also harbour a third class of microbes: non-core gut bacteria, which are non-host specific and vary among individuals. Understanding their functional role and how they interact with core symbionts is important for bumblebee ecology and management. We surveyed non-core bacteria in wild bumblebee workers (Bombus impatiens) and conducted laboratory experiments with gnotobiotic B. impatiens to examine factors shaping colonization by a focal non-core bacterium (Serratia marcescens) and its consequences for bee health. Non-core bacteria, including Serratia, frequently occur at high abundance in wild bumblebees, with roughly half of individuals harbouring at least 10% non-core gut bacteria. Experiments showed that Serratia marcescens better colonizes the gut when bees are inoculated early (within 1 day of adult emergence) and the core gut microbiome is disrupted. A mixed wildflower pollen diet facilitated the highest level of infection compared with two monofloral pollen treatments. We also provide evidence that Serratia is pathogenic: exposing bees with disrupted gut microbiomes to Serratia strongly reduced lifespan and, as a result, also reduced total reproduction. These results have three important implications: first, non-core bacteria are widespread in wild bumblebees, and some species are opportunistic pathogens. Second, the core gut microbiome plays a crucial role in protecting against these pathogens. Third, the timing of inoculation relative to bee age, as well as diet, is a key factor controlling bacterial pathogen colonization of the gut. Overall, these findings suggest that gut bacterial health could be an important target for monitoring and managing bumblebee health.