How do honey bees handle their stress? A focus on their gut microbiota and immune system. (Apis mellifera subsp. capensis)

ENGLISH ABSTRACT: Gut microbial symbionts have recently been shown to play roles in ensuring overall host health, a hot topic in honey bee research. Honey bees harbour a stable, core bacterial community in the gut, suggested to play a role in host health homeostasis, metabolic functioning, immune regulation, and food degradation. This gut microbiota provides a unique opportunity to observe the effects of common stressors on honey bees. Extrapolating the relationship of host-gut microbiota and immune system from higher hosts, we examined the effects of two common honey bee stressors; the indirect fungicide contamination and nutrient limitation. Honey bee colonies were exposed to the fungicide chlorothalonil and limited to only a single pollen food source, respectively. Effects of these treatments were observed through shifts in their gut microbiota using Automated Ribosomal Intergenic Spacer Analysis (ARISA). The immune response of honey bees was examined through gene expression levels of three immune genes, namely; immune deficiency (imd), prophenoloxidase (proPO), and spaetzle. The longevity of the honey bees was monitored through expression levels of vitellogenin (Vg). Overall colony metadata was also taken to observe changes in colony productivity. Both treatment groups were compared to an untouched, negative control group and a positive control group infected with Paenibacillus larvae. Both the fungicide and nutrient limited treatments showed no significant effect on the hindgut microbial communities but showed significant effects on the midgut communities. These treatments caused downregulation in the energy expensive Imd pathway, vital in the production of Anti-Microbial Peptides (AMPs), an invaluable defence against microbial pathogens. The phenoloxidase pathway was upregulated, ensuring a higher activity of the encapsulation and melanisation process, perhaps to compensate for the observed reduction in activity in the other immune pathways. Both treatments showed no significant effect on the gut-immune communicating Toll-like pathway. Honey bees within the nutrient limited group showed reduced colony productivity, probably as a result of delayed foraging, observed using Vg expression levels. Overall the treatments tested in this study significantly reduced the immune system of honey bees, opening the colonies up to potential secondary infections. This study does not provide any reason to discontinue the current beekeeping practices tested here, but attention should be paid to prevent the possibility of infection of colonies under similar conditions as a result of reduced immune system.