High‐level resistance of Melissococcus plutonius clonal complex 3 strains to antimicrobial activity of royal jelly

Summary Melissococcus plutonius is the causative agent of European foulbrood of honey bee larvae. Among its three genetically distinct groups (CC3, CC12 and CC13), CC3 strains have been suggested to be more virulent at the colony level. Honey bee larvae are fed royal or worker jellies by adult bees, and these jellies exhibit antimicrobial activity. Since M. plutonius orally infects larvae via brood food, we herein investigated the resistance of M. plutonius to the antimicrobial activity of royal jelly (RJ). The results obtained revealed that M. plutonius strains were more resistant to RJ and its component, 10‐hydroxy‐2‐decenoic acid, than the other species tested. Moreover, among the M. plutonius strains examined, CC3 strains exhibited the strongest resistance to antimicrobial activity; they temporarily proliferated and survived for a long period in 50% RJ‐containing broth. However, resistance was not observed when freshly cultured bacteria were used, it was only detected after a preculture on agar media for five or more days, suggesting that, under certain conditions, CC3 strains change their physiological state to that which is advantageous for survival in brood food. This high‐level RJ resistance of CC3 strains may contribute to their virulence in the field.