Honey bees are an essential part of entire ecosystems, and important pollinators for commercial crops worldwide. Their health has declined significantly due to parasites, bacterial and viral pathogens, as well as chemical stressors and lack of biodiversity. It has become clear that the honey bee microbiota plays an important role in their ability to digest pollen and the development of their immune system. Recently we identified a large number of novel honey bee gut associated bacteriophage genomes. Most of the proteins they encode were of unknown functions. Nevertheless, genomes of bacteriophages predicted to infect nearly every known dominant honey bee gut bacterium were identified. Of great interest was the discovery of bacteriophage encoded auxiliary metabolic genes, which implies the potential of these phages to influence microbial metabolism in the honey bee gut. In the current project we will characterize the bacteriophage population associated with honey bees from various countries in Europe. We will isolate honey bee gut bacteria in a high throughput manner, and subsequently use them to isolate bacteriophages. The genomes of isolated bacteria and bacteriophages will be characterized using sequencing and mass spectrometry approaches. Finally, these bacteria and bacteriophages will be used to inoculate microbiota-free worker bees with various combinations of bacteria and bacteriophages to study their influence on honey bee health, behavior and development.