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

Microbes shape the health of bumble bees, an important group of
pollinators including species of conservation concern. Most microbial
research on bumble bees 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. Bumble
bees also harbor 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
bumble bee ecology and management. We surveyed non-core bacteria in wild
bumble bee workers (Bombus impatiens) and conducted lab 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 bumble bees, with roughly half of
individuals harboring at least 10% non-core gut bacteria. Experiments
showed that Serratia marcescens better colonizes the gut when bees are
inoculated early (within one day of adult emergence) and the core gut
microbiome is disrupted. A mixed wildflower pollen diet facilitated the
highest level of infection compared to 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
bumble bees, 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, are key factors controlling bacterial pathogen colonization of
the gut. Overall, these findings suggest that gut bacterial health could
be an important target for monitoring and managing bumble bee health.