Environmental interlopers in the bumble bee gut microbiome: Ecology and roles in bee health

The bee gut microbiome is generally regarded to have two modes. Gut microbes of solitary bees are generalists and environmentally transmitted. Gut microbes of eusocial corbiculate bees are host-restricted and vertically transmitted within colonies. Bumble bees, however, blur this line. Across multiple continents and Bombus species, field-collected workers are often found to have gut microbiomes missing some or all of the host-restricted “core” bacteria. In their place is an erratic assemblage of environmental bacteria. What allows these interlopers to colonize the bumble bee gut? Are they harmful? Or beneficial? And what is it about bumble bee life history that makes them prone to this microbiome shift? We have been using field surveys of B. vosnesenskii and lab experiments with gnotobiotic B. impatiens to address these questions. Our data show that environmental bacteria become more prevalent as colonies age, but it is the youngest individual bees that are most vulnerable to colonization. One of the common environmental bacteria reduces bee lifespan and reproduction and is socially transmissible. Other interloper species can either facilitate or inhibit colonization by the pathogen, showing how ecological interactions within the gut can affect host health. I will discuss why this core-to-environmental microbiome shift evolved in bumble bees; annual life history and tradeoffs between microbiome maintenance and social functions may be important ingredients. Finally, I will discuss the implications of environmental bacteria for bumble bee conservation.