Antimicrobial activity of 4-hydroxyderricin, sophoraflavanone G, acetylshikonin, and kurarinone against the bee pathogenic bacteria Paenibacillus larvae and Melissococcus plutonius

The global number of bees has recently declined. One reason is pathogenic microorganisms including Paenibacillus larvae, which causes American foulbrood disease, and Melissococcus plutonius, which causes European foulbrood disease. The aim of this study was to identify new antimicrobial agents against foulbrood disease. Broth-microdilution assays with 20 natural compounds were used to detect the in vitro antimicrobial activity against foulbrood disease, and MTT assays were performed to determine cytotoxicity against a human cell line. 4-Hydroxydericin, sophoraflavanone G, acetylshikonin, and kurarinone were found to have antimicrobial activity against P. larvae and M. plutonius, with less cytotoxicity against HaCaT cells. In addition, these compounds have a synergic effect with tetracycline or macelignan, which could potentially be effective to control foulbrood disease of honey bees.