Microbial ecology of European foul brood disease in the honey bee (Apis mellifera)

European honey bees (Apis mellifera) are beneficial insects that provide essential pollination services for agriculture and ecosystems worldwide. Modern commercial beekeeping is faced with a variety of pathogenic and environmental stressors often confounding attempts to understand recent colony loss. European foulbrood (EFB) is a larval disease whose causative agent, Melissococcus plutonius, has received limited attention due to methodological challenges in the field and laboratory. Here we improve the experimental and informational context of larval disease with the end goal of developing an EFB management strategy. We sequenced the bacterial microbiota associated with larval disease transmission, isolated a variety of M. plutonius strains, determined their virulence against larvae in vitro, and explored the potential for probiotic treatment of EFB disease. The larval microbiota was a low diversity environment similar to honey, while worker mouthparts and stored pollen contained significantly greater bacterial diversity. Virulence of M. plutonius against larvae varied markedly by strain and inoculant concentration. In the context of EFB infection in vitro, our chosen probiotic strain did not have a significant effect on larval survival. We discuss the importance of positive and negative controls for in vitro studies of the larval microbiome and disease.