A propolis-rich hive environment affects redox gene expression and microbiota at the individual and social level in honey bees

The honey bee worker gut microbiome plays a critical role in host physiology including immunity, oxidative state and disease susceptibility. Here we investigated the effects of propolis (collected tree resin mixed with beeswax) on size and composition of gut microbiota, and host worker immune and redox gene expression. Using full-sized colonies, we prompted workers to collect tree resin creating a naturally propolis-rich colony/hive environment. Sampling pre-marked adult worker bees at 9-days post emergence revealed significantly larger populations of bacteria in the worker gut, but little change in the taxonomic composition or relative structure of the gut microbiome compared to propolis-poor colonies. Although immune gene expression in the fat body did not differ significantly, we found an overall trend towards decreased expression of immune genes in propolis-rich colonies. The expression of both pro-phenol oxidase and catalase were significantly reduced in the worker fat body suggesting that propolis augments host redox balance in individual workers. Consistent with clinical trials on human subjects, increased propolis levels resulted in greater expression of superoxide dismutase from both social head glands and the individual worker fat body. Assuming that social head gland secretions contribute to social immunity and net redox activity throughout the social environment, our results are consistent with a companion paper reporting drastic increases in beneficial native bacteria and reduced microbial diversity on the mouthparts, a primary marker of social interaction. Our study suggests that propolis supplementation has a net positive effect on host-microbial health at both individual and social levels, and through the upregulation of internal and socially secreted enzymes, may aid in the detoxification of biocides prevalent throughout the pollination environment.