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

Daniela Grossar,Verena Kilchenmann,Eva Forsgren,Jean-Daniel Charrière,Laurent Gauthier,Michel Chapuisat,Vincent Dietemann

doi:10.1080/21505594.2020.1768338

Daniela Grossar, Verena Kilchenmann + Show 5 more

Open Access

https://doi.org/10.1080/21505594.2020.1768338

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Abstract

ABSTRACT 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 co-infection 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.

Highlights

The Western honey bee (Apis mellifera L.) is an insect of major worldwide ecological and economic importance
We examined whether virulence was associated with M. plutonius multilocus sequence type (MLST), coinfection with P. alvei, differences in growth dynamics in a culture medium and presence of a putative toxin-coding gene [39]
The M. plutonius isolates collected in various regions of Europe belonged to six multilocus sequence types and two clonal complexes (CC 3 and CC 13, Table 1) of the typical form

Summary

Introduction

The Western honey bee (Apis mellifera L.) is an insect of major worldwide ecological and economic importance. Honey bees produce honey and wax and pollinate many economically important crops[1]. The recent loss of managed honey bee colonies in several regions of the world threatens the ecological services provided by this pollinator [2,3,4,5]. One of the most detrimental bacterial diseases affecting honey bees is European foulbrood (EFB). High numbers of clinical cases have been reported from Finland, France, Greece, Holland, Czechia and Italy [18,19,20], making EFB an economically important veterinary disease [18,21]

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