Abstract
European foulbrood (EFB) caused by Melissococcus plutonius is an important bacterial disease of honeybee larvae. M. plutonius strains can be grouped into three genetically distinct groups (CC3, CC12 and CC13). Because EFB could not be reproduced in artificially reared honeybee larvae by fastidious strains of CC3 and CC13 previously, we investigated a method to improve experimental conditions using a CC3 strain and found that infection with a potassium-rich diet enhanced proliferation of the fastidious strain in larvae at the early stage of infection, leading to the appearance of clear clinical symptoms. Further comparison of M. plutonius virulence under the conditions revealed that the representative strain of CC12 was extremely virulent and killed all tested bees before pupation, whereas the CC3 strain was less virulent than the CC12 strain, and a part of the infected larvae pupated. In contrast, the tested CC13 strain was avirulent, and as with the non-infected control group, most of the infected brood became adult bees, suggesting differences in the insect-level virulence among M. plutonius strains with different genetic backgrounds. These strains and the improved experimental infection method to evaluate their virulence will be useful tools for further elucidation of the pathogenic mechanisms of EFB.
Highlights
European foulbrood (EFB) is an important bacterial disease in honeybee larvae; it has spread globally and is recognised as an economically important disease for apiculture
M. plutonius DAT606(CC3) against A. mellifera larvae under the experimental condition nos 1 and 2 (Table 1)
DAT606(CC3) is a fastidious strain of ST3, which belongs to CC3
Summary
European foulbrood (EFB) is an important bacterial disease in honeybee larvae; it has spread globally and is recognised as an economically important disease for apiculture. In experimental infection I, the expected final concentration of DAT606(CC3) in the Day 0 diet (1.4–1.7 × 107 CFU/ml) (Supplementary Table S1) was higher than that of the previous study (5 × 106 CFU/ml)[4], the survival rate of the DAT606(CC3)-infected group at day 5 pi (94.7%) was comparable to that of the control group (92.3%) (Fisher’s exact test, P = 1) (Fig. 1A).
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