Antibacterial effects of propolis and brood comb extracts on the causative agent of European Foulbrood (Melissococcus plutonius) in honey bees (Apis mellifera)

Among a long list of parasites and pathogens that threaten the European honey bee (Apis mellifera), European Foulbrood (EFB) has become an urgent apiary disease, as epidemic outbreaks are becoming increasingly common. EFB is a bacterial disease of larval honey bees, caused by a gram-positive, anaerobic bacterium, Melissococcus plutonius. The most effective current treatment for EFB, oxytetracycline hydrochloride, can disrupt the bee microbiome, cause bee mortality and residues may persist in honey harvested for human consumption. In this study, we explore the efficacy of more sustainable bee-derived solutions, including propolis, honey comb, and brood comb ethanol extracts. Propolis has been shown to successfully inhibit the growth of a similar larval bee disease, American Foulbrood (AFB). Using a series of dilutions of these extracts, we determined the minimum inhibitory concertation (MIC) of each bee-derived product on M. plutonius, as well as two model bacterial species, Staphylococcus saprophyticus (gram-positive) and Escherichia coli (gram-negative). Overall, we found that propolis extract was most effective at inhibiting the growth of gram-positive bacteria, and that M. plutonius was also susceptible to honey comb (MIC = 16.00 mg/mL) and brood comb (MIC = 45.33 mg/mL) extracts, but at much higher concentrations than that of propolis (MIC = 1.14 mg/mL). As previously demonstrated for AFB, propolis is a promising natural defense against EFB, which could have important implications for hive management.