Abstract
European foulbrood is a globally distributed brood disease affecting honey bees. It may lead to lethal infections of larvae and, in severe cases, even to colony collapse. Lately, a profound genetic and phenotypic diversity was documented for the causative agent Melissococcus plutonius. However, experimental work on the impact of diverse M. plutonius strains on hosts with different genetic background is completely lacking and the role of secondary invaders is poorly understood. Here, we address these issues and elucidate the impact and interaction of both host and pathogen on one another. Moreover, we try to unravel the role of secondary bacterial invasions in foulbrood‐diseased larvae. We employed in vitro infections with honey bee larvae from queens with different genetic background and three different M. plutonius strains. Larvae infection experiments showed host‐dependent survival dynamics although M. plutonius strain 49.3 consistently had the highest virulence. This pattern was also reflected in significantly reduced weights of 49.3 strain‐infected larvae compared to the other treatments. No difference was found in groups additionally inoculated with a secondary invader (Enterococcus faecalis or Paenibacillus alvei) neither in terms of larval survival nor weight. These results suggest that host background contributes markedly to the course of the disease but virulence is mainly dependent on pathogen genotype. Secondary invaders following a M. plutonius infection do not increase disease lethality and therefore may just be a colonization of weakened and immunodeficient, or dead larvae.
Highlights
Higher mortality rates compared to the noninfected controls (Cox regression mixed-effects model: p < .001), there was no significant difference between the treatment with M. plutonius only (M) and the treatments with an additional infection (E. faecalis: ME, P. alvei: MP)
The colony-forming units (CFUs) per larva used for infection varied between replicates, especially in E. faecalis treatments (Table S2), it did not correlate with larval mortality or weight, neither for M. plutonius infected larvae nor larvae treated with an additional infection
The environment of the colony has to be mentioned as an important factor which profoundly affects the course of the disease
Summary
Endangered or protected species were not used in this study. Experiments and observations conform with the laws of Germany in relation to animal protection. Larval weight was again lower for control larvae grafted from colony B (72.21 mg ± 31.96, mean ± SD) compared to colony A (101.25 ± 25.73) (MWU test: Z = 4.59, p < .0001) as well as overall treatment groups (MWU test: Z = 7.44, p < .0001) (Table 1). Higher mortality rates compared to the noninfected controls (Cox regression mixed-effects model: p < .001), there was no significant difference between the treatment with M. plutonius only (M) and the treatments with an additional infection (E. faecalis: ME, P. alvei: MP) (log-rank test pairwise comparisons, Bonferroni adjusted: M-ME p = .38, M-MP p = .91, MP-ME p = .38, Figure 2). Infection treatments resulted in significantly lower larval weight compared to controls (Kruskal–Wallis ANOVA: H = 16.88, p = .0007; posthoc multiple comparisons, Bonferroni adjusted: M-C p = .004, ME-C p = .02, MP-C p = .02, Table 2) with approx. The CFUs per larva used for infection varied between replicates, especially in E. faecalis treatments (Table S2), it did not correlate with larval mortality or weight, neither for M. plutonius infected larvae nor larvae treated with an additional infection
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