Gut Transplants from Bees Fed an Antipathogenic Pollen Diet Do Not Confer Pathogen Resistance to Recipients

Pollinators are threatened by diverse stressors, including microbial pathogens such as Crithidia bombi. Consuming sunflower pollen dramatically reduces C. bombi infection in the bumble bee Bombus impatiens, but the mechanism behind this medicinal effect is unclear. We asked whether diet mediates resistance to C. bombi through changes in the gut microbiome. We hypothesized that sunflower pollen changes the gut microbiome, which in turn reduces Crithidia infection. To test this, we performed a gut transplant experiment. We fed donor bees either a sunflower pollen treatment or buckwheat pollen as a control treatment and then inoculated recipient bees with homogenized guts from either sunflower-fed or buckwheat-fed donor bees. All recipient bees were then fed a wildflower pollen diet. Two days after the transplant, we infected recipients with C. bombi, and 2 days later, we provided another donor gut transplant. To quantify infection, we performed both fecal screens and dissections of the recipient bees. We found no significant differences in C. bombi infection intensity or presence between bees that received sunflower-fed microbiomes versus buckwheat-fed microbiomes. This suggests that sunflower pollen's effects on pathogen resistance are not mediated by gut microbiota.