Abstract
Honey bees are highly prone to infectious diseases, causing colony losses in the worst case. However, they combat diseases through a combination of their innate immune system and social defence behaviours like foraging for health-enhancing plant products (e.g. nectar, pollen and resin). Plant secondary metabolites are not only highly active against bacteria and fungi, they might even enhance selective foraging and feeding decisions in the colony. Here, we tested six major plant terpenes and their corresponding acetates, characterizing six natural Thymus vulgaris chemotypes, for their antimicrobial activity on bacteria associated with European foulbrood. Comparison of the inhibitory activity revealed the highest activity for carvacrol and thymol whereas the acetates mostly did not inhibit bacterial growth. All terpenes and acetates are present in the nectar and pollen of thyme, with pollen containing concentrations higher by several orders of magnitude. The physiological response was tested on forager and freshly emerged bees by means of antennal electroantennography. Both responded much stronger to geraniol and trans-sabinene hydrate compared to carvacrol and thymol. In conclusion, bee-forageable thyme product terpenes (mainly from pollen) yield effective antibiotic activity by reducing the growth of bee disease-associated bacteria and can be detected with different response levels by the honey bees’ antennae. This is a further step forward in understanding the complex pathogen-pollinator-plant network.
Highlights
Bees of the subfamily Apinae are of major economic and agricultural importance for human nutrition and bee product business[1], and for the pollination service of wild plants to facilitate plant reproduction and maintain the natural food chain[2]
Some thyme species were used to discover the antibiotic potential of essential oils on honey bee parasites and pathogens; e.g. Andean thyme (Acantholippia seriphioides) against the bacterium Paenibacillus larvae;[14] Thymus satureoides, T. serpyllum and T. vulgaris essential oils against the fungus Ascosphaera apis[15]
A handful of studies tested the antimicrobial activity of plant secondary metabolites against P. larvae and against Melissococcus plutonius with sometimes very weak activity[23,24,25,26,27]
Summary
Bees of the subfamily Apinae (mainly Apis mellifera, Bombus sp.) are of major economic and agricultural importance for human nutrition and bee product business[1], and for the pollination service of wild plants to facilitate plant reproduction and maintain the natural food chain[2]. Disease-associated bacteria (Bacillus pumilus, Brevibacillus laterosporus, Enterococcus faecalis, Paenibacillus alvei and Paenibacillus dendritiformis), the so-called secondary invader, can be found in chronically diseased colonies and larval remains, and some of them have been attributed to increased disease symptoms and infection success[7,28,29]. For T. vulgaris six different chemotypes are known: thymol (T-type), carvacrol (C-type), geraniol (G-type), linalool (L-type), α-terpineol (A-type) and trans-sabinene hydrate (U-type)[33] These chemotypes are characterised by their dominant monoterpene alcohol which provides more than 50% of the respective essential oil. For most of the thyme essential oil single compounds, except for thymol and carvacrol, antibiotic activities are not well characterised. ® ® ® used by the beekeeping industry (Apiguard , ApiLife Var and Thymovar ) with thymol as the active ingredient
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