Friend or foe? Concentration of a commensal microbe induces distinct responses in developing honey bees exposed to field-realistic pesticide concentrations.

Commensal microbes play important roles in modulating host health through varied mechanisms. Enterococcus faecalis, a Gram-positive commensal bacterium found across a wide range of hosts, has the potential to benefit its host through probiotic, antimicrobial and detoxification properties. However, it can also cause adverse effects, disrupting the host's healthy microbial communities and responses to co-stressors. Its context-dependent impact on the health of the agriculturally important pollinator - Apis mellifera - has been sparsely explored. Here, we examined the effects on honey bee brood survivorship and development when exposed at different concentrations and when co-exposed with chemical stressors (acetamiprid, thymol, glyphosate, and a mixture of the three). We found high doses of E. faecalis significantly reduced larval survivorship and size of brood at multiple developmental stages. Conversely, we found that low doses of E. faecalis increased larval size when individuals were co-exposed to the pesticide mixture. We also found that glyphosate alone and the pesticide mixture reduced the mass of brown-eyed pupae. These results are the first to show the dual role of E. faecalis in honey bee health is dependent on the concentration of the microbe and the co-stressors that brood are exposed to.