Atypical <i>Melissococcus plutonius</i> strains: their characteristics, virulence, epidemiology, and mysteries

Abstract

Melissococcus plutonius is a Gram-positive lanceolate coccus that is the causative agent of European foulbrood, an important bacterial disease of honey bee brood. Although this bacterium was originally described in the early 20th century, a culture method for this bacterium was not established until more than 40 years after its discovery due to its fastidious characteristics, including the requirement for high potassium and anaerobic/microaerophilic conditions. These characteristics were considered to be common to the majority of M. plutonius strains isolated worldwide, and M. plutonius was also thought to be genetically homologous or clonal for years. However, non-fastidious variants of this species (designated as atypical M. plutonius ) were very recently identified in Japan. Although the morphology of these unusual strains was similar to that of traditionally well-known M. plutonius strains, atypical strains were genetically very different from most of the M. plutonius strains previously isolated and were highly virulent to individual bee larva. These atypical variants were initially considered to be unique to Japan, but were subsequently found worldwide; however, the frequency of isolation varied from country to country. The background of the discovery of atypical M. plutonius in Japan and current knowledge on atypical strains, including their biochemical and culture characteristics, virulence, detection methods, and global distribution, are described in this review. Remaining mysteries related to atypical M. plutonius and directions for future research are also discussed.

Quantitative tyramine analysis method for <i>Apis mellifera</i> larvae infected with <i>Melissococcus plutonius</i>, the causative agent of European foulbrood

“…Yet, DAT561ΔpMP19 killed 95.2% of larvae by day 21 pg in the present study ( Supplementary Fig. 2 ), and previous studies also demonstrated approximately 80% bee brood death for this M. plutonius strain using conditions almost identical to those in the present study [ 13 , 14 , 17 ]. Therefore, EFB-affected larvae are probably not killed solely by the tyramine produced by M. plutonius .…”

“…European foulbrood (EFB) is a bacterial infectious disease of the honey bee brood [ 5 ]. In recent years, the increasing impact of EFB on beekeeping and agricultural industries in European and North American countries necessitates the development of effective control methods for this disease [ 3 , 6 , 7 , 9 , 16 , 17 , 18 , 20 , 21 ]. Although essential to this development, a detailed understanding of EFB pathogenesis remains to be elucidated.…”

“…The EFB causal agent is the Gram-positive lanceolate coccus, Melissococcus plutonius . Strains of this bacterium are typed into more than 40 sequence types (STs) by multilocus sequence typing [ 9 ], then further grouped into several genetically related populations, such as clonal complex (CC) 3, CC12, and CC13 [ 3 , 7 , 9 , 17 ] ( https://pubmlst.org/organisms/melissococcus-plutonius ). Of these, CC12 strains are highly virulent in bee larvae, causing significant damage to colonies used to pollinate blueberries in the US and Canada [ 6 , 17 , 18 , 21 ].…”

“… . In the present study, to investigate whether M. plutonius produces tyramine when it infects larvae, we selected an inoculation dose that would establish infection in almost all larvae, based on previous studies [ 13 , 17 ]. For inocula preparation, strain DAT561ΔpMP19 anaerobically cultured on KSBHI agar for 3–5 days at 35°C was suspended in sterile H 2 O at a final concentration of approximately 1.9–3.6 × 10 7 colony forming unit (CFU)/mL, then the suspension and artificial diet B’ ( Supplementary Table 1 ) were mixed at a ratio of 1:9 for inocula containing M. plutonius at 1.9–3.6 × 10 6 CFU/mL.…”

Probing the interaction of Paenibacillus larvae bacteriophage as a biological agent to control the american foulbrood disease in honeybee

“…Hence, the milk proteins precipitate, and then the mixtures become clear. The positive samples contain P. larvae, the causative bacterium of AFB disease, but M. pluton, the EFB causative bacterium, does not have this capability ( Takamatsu, 2023 ).…”