Effects of essential oils on foulbrood bacteria and honey bee workers (Apis mellifera) under laboratory conditions

Introduction American and European foulbrood diseases (AFB and EFB, respectively) result in considerable economic losses for beekeepers. Currently, no satisfactory methods are available for the treatment of either disease. There has been a recent surge of interest in the use of natural substances, such as essential oils (EOs), as a means of combating infections in apiaries. The objective of this study was to evaluate the in vitro antimicrobial activity of a number of EOs against the causative agents of AFB and EFB, namely Paenibacillus larvae and Melissococcus plutonius , respectively, and their safety towards bees. Methods The antimicrobial activity of 18 EOs was initially assessed in vitro against the two foulbrood bacteria by the spot-on-agar test, where the EOs were deposited directly onto the agar surface. The EOs demonstrating the most significant antimicrobial activity against one or both of the bacterial pathogens were then selected for further assessment of their toxicity towards the foulbrood bacteria by the resuzurin-based microdilution method and towards adult bees using the vapor exposure bioassay at one and three hours. Results The study revealed that oregano, juniper, sage, thyme, cinnamon, cumin, clove and black pepper were the most effective against the foulbrood bacteria. The results of the exposure bioassays demonstrated that bees exposed to those EOs exhibited greater tolerance to the vapor of cinnamon and oregano after one hour of exposure than the other EOs. As the duration of exposure increased to a period of three hours, an escalation in toxicity was observed. Discussion The preliminary findings of the present study suggest EOs as potential candidates for the development of new natural antimicrobial treatments against foulbrood diseases. However, further in vitro larval bioassays and field trials are needed to validate their clinical effectiveness and safety.