Anti-Bacterial Activity of Four Distinct Propolis Extracts against P. larvae and M. plutonius; Etiological Agent of American and European Foulbrood Disease of Honeybees

Four different propolis samples obtained from different regions of Iran were evaluated for their antibacterial effects against the bacterial agents responsible for two important honeybee diseases. Paenibacillus larvae (P. larvae) and Melissococcus plutonius (M. plutonius), as the etiological agents of American foulbrood (AFB) and European foulbrood (EFB) diseases, were subjected to propolis ethanolic extracts in the agar well diffusion assay. The minimum inhibitory concentrations (MIC) and the minimum bactericidal concentrations (MBC) of the antibacterial effects of the samples against the two indicator organisms were determined by the microdilution technique using different concentrations of the propolis extracts. Finally, the synergistic antibacterial actions of the mixed propolis samples were determined, and their MIC and MBC values were recorded. A two-way analysis of variance was used to evaluate correlations among the diameters of the inhibition zones, the bacterial agents, and the propolis extracts. Based on our results, three of the propolis samples showed significant antibacterial effects against P. larvae and M. plutonius during the agar well diffusion assay. Furthermore, the antibacterial capacity of the propolis samples, when mixed in equal proportions, was significantly enhanced, as indicated by the obtained MIC and MBC values. Approximately, 0.02 mg/mL of mixed propolis samples was required for inhibiting the growth of both pathogens. A direct correlation was observed between propolis concentrations and their antibacterial activity. The results of the study are conclusive of the significant antibacterial actions of Iranian propolis samples against the etiological agents of the mentioned honeybee diseases, suggesting their probable use as a safe biological agent to control AFB and EFB diseases.