The effects of propolis on the honey bee (Apis mellifera) immune system and mouthpart microbiome

Abstract

Honey bees (Apis mellifera) collect and apply antimicrobial plant resins to the interior of their nest cavity to form a lining called a propolis envelope. Previous studies show that exposure to a propolis envelope within the nest cavity resulted in reduced immune system activation in adult bees, however the mechanism for this reduction remained unclear. In Chapter 1, I tested the hypothesis that propolis exposure would reduce the general bacterial load (16S rRNA transcription) in and on honey bees, thereby reducing antimicrobial peptide (hymenoptaecin gene) expression by the honey bee innate immune system. The results showed that bees exposed to a propolis envelope in field colonies had significantly lower transcript levels of hymenoptaecin, but in contrast to previous studies had significantly greater 16S rRNA transcription, compared to bees in colonies without a propolis envelope. Bees held in cages had significantly greater hymenoptaecin expression and significantly greater 16S rRNA transcription compared to bees from colonies, suggesting that bees are exposed to different bacterial communities between colonies and cages. The consistent reduction in immune activation yet variable general bacterial loads upon propolis exposure, as seen in previous studies, suggests that there may be a relationship between propolis exposure and the abundance and diversity of specific bacterial species in particular microbial niches in and on the honey bee body. In Chapter 2, I hypothesized that the antimicrobial activity of a propolis envelope in bees from field colonies would influence the bacterial diversity and abundance of the worker mouthpart microbiome. The results of DNA sequencing revealed that the mouthparts of worker bees in colonies with a propolis envelope had significantly lower bacterial diversity and significantly higher bacterial abundance, compared to the mouthparts of bees in colonies without a propolis envelope. Based on the taxonomic results, the propolis envelope appeared to reduce pathogenic or opportunistic bacteria and to promote the proliferation of putatively beneficial bacteria on the honey bee mouthparts, thus reinforcing the core microbiome of the mouthpart niche. This work suggests that the mechanism for reduced immune system activation may be due to the antimicrobial properties of propolis reducing pathogenic and opportunistic bacterial species and promoting beneficial bacterial species in the mouthparts, which may affect disease transmission throughout the colony, thus promoting colony health and wellbeing. This relationships among honey bees, propolis, and microbes likely stems from their long evolutionary history together. The differences in bacterial loads between bees from field colonies and cages suggest that the antimicrobial properties of propolis, the community of microbes, and the individual immune response may vary according to the nest environment, availability of floral resources, and social and organizational behaviors of the bees within the colony.