In vitro growth-inhibitory effect of plant-derived extracts and compounds against Paenibacillus larvae and their acute oral toxicity to adult honey bees

In total, 26 natural compounds of various chemical classes (flavonoids, alkaloids, terpenoids) and 19 crude extracts from selected plants were tested in vitro for antibacterial activity against three strains of P. larvae, the causal agent of American Foulbrood Disease of honey bees (AFB) by the broth microdilution method. Among the individual substances, sanguinarine (MIC 4 μg/ml), followed by thymoquinone, capsaicin, trans-2-hexenal and nordihydroguaiaretic acid (MIC 4–32 μg/ml) possessed the strongest antibacterial effect. In case of extracts, common hop (Humulus lupulus L.) and myrtle (Myrtus communis L.) methanolic-dichloromethane extracts exhibited the highest growth-inhibitory effect with MICs ranging from 2 to 8 μg/ml. Acute oral toxicity of the most active natural products was determined on adult honey bees, showing them as non-toxic at concentrations as high as 100 μg peer bee. Our study leads to identification of highly potent natural products effective against AFB in vitro with very low MICs compared to those reported in literature, low toxicity to adult honey bees and commercial availability suggesting them as perspective, low cost and consumer-acceptable agents for control of AFB.