Some like it hot: Facultative endothermy augments symbiont-mediated antiparasitic activity in honey bees

Temperature variation is known to affect metabolism and interspecific interactions within hosts, including those of their associated parasites and symbionts. However, the effect of temperature alters these interactions is not well understood. Facultatively endothermic honey bees (Apis mellifera) warm their colonies between 34°C and 38°C, reaching temperatures akin to most mammals and providing a perfect environment for their suite of highly conserved gut symbionts. By leveraging the thermal ecology of honey bees, we investigated how temperature influences the antiparasitic activity of Lactobacillus gut symbionts on Lotmaria passim, a ubiquitous trypanosomatid gut parasite. Our previous work utilized cell cultures to demonstrate that bacterial symbionts possess greater heat tolerance than parasites and chemically inhibit parasite growth via acid production in vitro. In vivo, our current work aims to further elucidate how temperature affects these interactions within the honey bee hindgut. Here we demonstrate the presence of differential pathogen loads in gnotobiotic bees co-inoculated with bacterial symbionts and parasites. Parasite infection intensity decreased in response to increasing temperatures typical of honey bee colonies and was significantly lower in the presence of bacterial symbionts. Combined with our in vitro work, our results suggest that the endothermic behavior of honey bees can potentiate the antiparasitic activity and protective effects of bacterial gut symbionts and implicate thermoregulation as both an integral aspect of core symbioses and gut symbiont-mediated antiparasitic defenses.