Bacterial communities of wild bee species and the western honey bee ( <i>Apis mellifera</i> ) (Hymenoptera: Apoidea): Alpine insights

Wild bees are decreasing in species diversity and populations due to human impact. The abundance of the western honey bee (Apis mellifera L.) experiences an inverse trend, enhancing competition with wild bees and the probability of microbiome exchange. Addressing this exchange, we studied the gut microbiome composition of wild and honey bees, focusing on patterns indicating honey bee influence. Three solitary wild bee species (large scabious mining bee [Andrena hattorfiana F.], grey-backed mining bee (Andrena vaga Panzer), and European orchard bee [Osmia cornuta Latreille]) as well as bumble bees as representatives of eusocial wild bees (Bombus spp. Latreille) and honey bees were sampled in the Austrian Alps. Subsequent 16S ribosomal DNA sequencing revealed the composition of the bacterial communities. The bee groups differed concerning their bacterial composition, with honey bees having the least variation among individuals and a low number of exclusive bacterial taxa and bumble bees the highest bacterial diversity. High honey bee densities corresponded with lower bacterial diversity in wild bees and a higher bacterial similarity between wild and honey bees. Some bacterial taxa were found for the first time in the studied bee groups. Furthermore, the composition of bacterial communities differed between solitary and social bees. We found the first hints that high honey bee density negatively impacts wild bees through alterations of wild bee microbiomes. Future studies should focus on understanding microbiome transmission mechanisms and their consequences for wild bees. Suggestions on how to consider wild bee fitness are indispensable in halting the biodiversity crisis.