Investigating the genetics and virulence of European foulbrood, a bacterial pathogen of honey bees

European foulbrood (EFB), a disease of honey bee larvae (Apis mellifera), is caused by Melissococcus plutonius. This bacterium has a worldwide distribution, found wherever honey bees are kept. EFB outbreaks vary in severity, with treatment options of shook swarm, oxytetracycline antibiotic application and colony destruction available to manage this disease. M. plutonius bacterial isolates have previously been differentiated using a multi- locus sequence typing scheme (MLST), with a focus on tracing the source of new EFB outbreaks. A putative toxin gene, melissotoxin A, has been shown to correlate with virulence in previous larval infection studies. Whole genome sequencing of approximately 50 M. plutonius isolates suggests that current strain typing (ST) methods are likely insufficient to separate the most common isolates in the UK, such as ST3. Statistical analysis of the relationship between isolate possession of a gene, melissotoxin A, and type of treatment used on the EFB outbreak, showed a negative relationship between the toxin presence and incidence of colony destruction. Using laboratory reared honey bee larvae, M. plutonius virulence was tested at the strain level. Within these experiments, it was found that isolates possessing the melissotoxin A gene did not cause a significantly higher larval mortality rate, and expected virulence phenotypes, based on clonal complex type, wasn’t always observed for strains tested. Anecdotally, treatment success of EFB with OTC may vary dependent on strain type present in the infection. Isolates of M. plutonius were tested for OTC resistance in vitro by using minimum inhibitory concentration (MIC) assays and several antibiotic resistant strains were identified. A complete understanding of all aspects of EFB, and the causative bacteria M. plutonius, is essential to improve treatment strategy, and therefore honey bee health, in the future.