Putative determinants of virulence in Melissococcus plutonius, the bacterial agent causing European foulbrood in honey bees

Melissococcus plutonius is a bacterial pathogen that causes epidemic outbreaks of European foulbrood (EFB) in honey bee populations. The pathogenicity of a bacterium depends on its virulence, and understanding the mechanisms influencing virulence may allow for improved disease control and containment. Using a standardized in vitro assay, we demonstrate that virulence varies greatly among sixteen M. plutonius isolates from five European countries. Additionally, we explore the causes of this variation. In this study, virulence was independent of the multilocus sequence type of the tested pathogen, and was not affected by experimental coinfection with Paenibacillus alvei, a bacterium often associated with EFB outbreaks. Virulence in vitro was correlated with the growth dynamics of M. plutonius isolates in artificial medium, and with the presence of a plasmid carrying a gene coding for the putative toxin melissotoxin A. Our results suggest that some M. plutonius strains showed an increased virulence due to the acquisition of a toxin-carrying mobile genetic element. We discuss whether strains with increased virulence play a role in recent EFB outbreaks

Population genetic diversity and dynamics of the honey bee brood pathogen Melissococcus plutonius in a region with high prevalence.

European foulbrood (EFB) is a honey bee brood disease caused by the bacterium Melissococcus plutonius. Large-scale EFB outbreaks have been reported in several countries in recent decades, which entail costly sanitation measures of affected apiaries to restrict the spread of this contagious pathogen. To mitigate its impact, a better understanding of the population dynamics of the etiological agent is required. We here used multi-locus sequence typing (MLST) to infer the genetic diversity and geographical distribution of 160M. plutonius isolates collected from EFB symptomatic honey bee colonies seven years apart. Isolates belonged to three clonal complexes (CCs) known worldwide and to 12 sequence types (STs), of which five were novel. Phylogenetic and clustering analyses showed that some of these novel sequence types have likely evolved locally during a period of outbreak, but most disappeared again. We further screened the isolates for melissotoxin A (mtxA), a putative virulence gene. The prevalence of STs in which mtxA was frequent increased over time, suggesting that this gene promotes spread. Despite the increased frequency of this gene in the population, the total number of cases decreased, which could be due to stricter control measures implemented before the second sampling period. Our results provide a better understanding of M. plutonius population dynamics and help identify knowledge gaps that limit efficient control of this emerging disease.This research was funded by the Swiss Federal Food Safety and Veterinary Office grant number 1.12.15, the University of Lausanne and Agroscope. EGH was funded by a BBSRC CASE studentship in partnership with Bee Disease Insurance and the National Bee Unit. GEB was funded jointly by a grant from BBSRC, Defra, NERC, the Scottish Government and the Wellcome Trust, under the Insect Pollinator Initiative (BB/I000801/1)

Under Review: Data Requirements and Method Development of a New Bee Risk Assessment Scheme for Plant Protection Product Registration

An up-to-date ecotoxicological risk assessment of plant protection products (PPPs) depends on the constant improvement of risk assessment methods and guidelines, and a thorough evaluation of their impacts. Here, we explain how the risk assessment of PPPs with regard to bees and the authorisation of PPPs is conducted in Switzerland. We further report the design and application of a new method to study homing flights of honey bees using the Radio Frequency Identification (RFID) technique. The new method allowed to address the effects of sublethal doses of two neonicotinoids, thiamethoxam and thiacloprid, on the flight capacities of honey bees. Currently, this study design is under evaluation in an international ring test, in which the Swiss Bee Research centre participates. It is the first test design focussing on sublethal effects of PPPs on honey bees and a draft method will be submitted to OECD to become an official test guideline in the near future. Potential shortcomings and ideas for refinements on the RFID test design are discussed