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 mg/ml), followed by thymoquinone, capsaicin, trans-2-hexenal and
nordihydroguaiaretic acid (MIC 4–32 mg/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 8mg/ml. Acute oral toxicity of themost active natural products was
determined on adult honey bees, showing them as non-toxic at concentrations as high as
100 mg 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.