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

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 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.

Melissococcus plutonius Can Be Effectively and Economically Detected Using Hive Debris and Conventional PCR

“…The bacterium multiplies in the digestive tract of the infected larvae after ingestion [ 9 ], and they may die at any time between the 4th day and pupation [ 6 ]. Whether or not the infection is lethal to all larvae depends on its extent [ 4 , 9 , 10 ] and how virulent the bacterial strain is [ 11 , 12 , 13 , 14 ]. The molecular mechanisms of EFB infection are not currently understood [ 2 , 15 ].…”

Microbial Ecology of European Foul Brood Disease in the Honey Bee (Apis mellifera): Towards a Microbiome Understanding of Disease Susceptibility

“…Although various assays can detect larval pathogens in hives [ 56 , 58 ] there is much to be learned about the ecology and epidemiology of M. plutonius, the causative agent of European Foul Brood (EFB) disease. Based on broad sampling throughout the United Kingdom, M. plutonius is comprised of at least three genetically distinct groups [ 7 ] and virulence differs markedly by strain [ 30 , 31 , 32 ]. Melissococcus plutonius is often found in the guts of workers so may be easily transmitted among hives when they are placed in proximity [ 13 ].…”

“…Melissococcus plutonius is often found in the guts of workers so may be easily transmitted among hives when they are placed in proximity [ 13 ]. Recent increases in antibiotic resistance and incidence of EFB and EFB-like brood disease have spurred efforts to understand virulence including genetic sequencing, comparative genomic analysis, the contribution of secondary bacteria and targeted molecular biology [ 8 , 30 , 31 , 32 , 52 , 55 , 59 , 60 , 61 , 62 ]. Understanding the microbial ecology associated with M. plutonius will provide a functional context of disease progression and suggest potential solutions to mitigate disease.…”

“…Melissococcus plutonius virulence varies by strain [ 30 , 31 , 32 ], and is often detected in asymptomatic hives. Culturing M. plutonius as defined by cell morphology and 16S sequence does not guarantee strain virulence [ 29 , 52 , 55 ].…”

“…While AFB disease is overt, highly virulent, and caused by a singular bacterial species, the causative factor of EFB and EFB-like symptomology is less well known. With the recent increase and severity of EFB disease worldwide, the last few years has seen great progress elucidating the mechanisms of virulence [ 8 , 30 , 31 , 32 ]. Historically, EFB was considered an opportunistic disease, affecting stressed hives.…”