The European honey bee Apis mellifera is used as the primary pollinator of a substantial fraction of human agriculture, responsible for pollinating a third of crops for direct human consumption. Given their disproportionate impact on food availability and nutritional quality for humans, it is essential to improve our understanding of factors influencing honey bee health. I contribute to the broader field of apicultural research by characterizing the biogeography and genetic diversity of Melissococcus plutonius, the bacterium responsible for European Foul Brood (EFB), an economically important disease affecting honey bee larvae. I also identify potential molecular mechanisms of M. plutonius pathogenesis. I carried out a large-scale survey of strains of this pathogen from across the United States employing multi-locus sequence typing (MLST) to classify strains. I used in-vitro honey bee larval rearing to explore differences in virulence between genetically distinct strains. Finally, I used comparative genomics of M. plutonius strains with different virulence profiles to identify putative genetic correlates of virulence. This work uncovered a dynamic landscape of M. plutonius strain diversity associated with EFB outbreaks, revealing a complex ecology of both prevalent and rare strains that contribute to outbreaks. Multiple MLST sequence types were present in most diseased colonies. Strain virulence was broadly correlated with MLST type and depended on both background genome and a mobile genetic element, the plasmid pMP19. These results offer insight into M. plutonius epidemiology, and the ecology of EFB outbreaks in the United States, providing context for informed pathogen management strategies in North American apiculture.
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