Self-medication and innate immunity in the honeybee Apis mellifera
Final Report Abstract
We could show that central European monofloral honeys process variable antimicrobial activity on EFB-disease associated bacterial strains, with nearly complete inhibition of most strains. Strongest activity was detected for buckwheat, sunflower and two honeydew honeys. Factors explaining the difference in antimicrobial activity might be di- and tri-saccharide sugar profiles, polyphenol quantities and antioxidant activity, which are all factors that differed strongly between honey types. For some honeys, we identified honey type-specific floral marker substances that, however, processed only low antimicrobial activity against the same bacterial strains. Nevertheless, we could show that floral marker substances might be of high importance for potential self- and allomedication within honey bee colonies. With the results of the marker substance growth inhibition assays, it was clear that there will be additional factors that contribute to the full antimicrobial activity of the honey. We showed that proteins, secreted by worker bees, are produced in equal composition and are quite stabile to guarantee constant food preservation within the hive. After having an improved cultivation and infection assay, we could show that M. plutonius virulence differs among sequence types, but larval survival does not increase with secondary infections. Putative virulence factors, that have to be studied in future studies, might explain the origin of strain variance. Feeding different types of honey to diseased honey bee larvae showed no major effect on larval survival, although we identified two honeys with strong impact on larval survival. The same effects were observed for the corresponding honey specific marker substances. Both honey and its marker substances had effects on enzyme activity of the larvae haemolymph and had varying effects on bacterial pathogen loads in the larvae’s gut. Two different approaches revealed that the innate immune system does not play a central role for host response towards infections and the development of the disease. RNAseq data showed that genes involved in larval development associated metabolic processes are of major importance and may explain metabolic stress and retarded larval development. In the last part of the project, we were the first who found dynamic changes of worker bee densities and distribution over time, in a semi-colony setting, specifically for M. plutonius-treated brood. All project results show that we have developed several new techniques to further investigate nutrition and feeding effects for different types of honey on honey bees (larvae and adult bees). Finally, with the current design of our studies, we could not prove multidimensional self-medication behaviour under colony conditions. However, we found evidences that honey, in particular its plant secondary metabolites, have beneficial but also adverse effects on diseased larvae.
Publications
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(2018) Comparative genomics and description of putative virulence factors of Melissococcus plutonius, the causative agent of European foulbrood disease in honey bees. Genes, 9(8): 419
Djukic M, Erler S, Leimbach A, Grossar D, Charrière J-D, Gauthier L, Hartken D, Dietrich S, Nacke H, Daniel R, Poehlein A
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(2018) The curious case of Achromobacter eurydice, a Gramvariable pleomorphic bacterium associated with European foulbrood disease in honeybees. Microbial Ecology, 75(1): 1-6
Erler S, Lewkowski O, Poehlein A, Forsgren E
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(2018) The terpenes of leaves, pollen, and nectar of thyme (Thymus vulgaris) inhibit growth of bee disease-associated microbes. Scientific Reports, 8: 14634
Wiese N, Fischer J, Heidler J, Lewkowski O, Degenhardt J, Erler S
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(2019) Dyeing but not dying: colourful dyes as a non-lethal method of food labelling for in vitro-reared honey bee (Apis mellifera) larvae. Journal of Insect Physiology, 113: 1-8
Ehrenberg S, Lewkowski O, Erler S
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(2019) The effect of diet on the composition and stability of proteins secreted by honey bees in honey. Insects, 10(9): 282.
Lewkowski O, Mureşan CI, Dobritzsch D, Fuszard M, Erler S
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(2019) Virulence of Melissococcus plutonius and secondary invaders associated with European foulbrood disease of the honey bee. MicrobiologyOpen, 8(3): e649
Lewkowski O, Erler S
