In vitro inhibition of the bacterial honey bee pathogen <i>Melissococcus plutonius</i> with essential oils

European foulbrood, caused by Melissococcus plutonius (M. plutonius), is a significant bee disease that is difficult to treat due to the prohibition of antibiotics in most European countries. This study aimed to assess the antimicrobial activity of pine turpentine, cinnamon, lavender essential oils, and eucalyptol against M. plutonius and their potential synergistic effects with tetracycline hydrochloride. Agar diffusion and broth microdilution techniques showed that cinnamon and eucalyptol exhibited notable antimicrobial activity, with cinnamon showing the highest inhibition zone (18 mm) and eucalyptol (17 mm). The MIC values were 0.4% v/v for cinnamon and 1.56% v/v for eucalyptol, indicating the lowest concentrations effective against the bacteria. The addition of tetracycline did not enhance the activity of the essential oils. These findings suggest that cinnamon and eucalyptol have potential as alternative therapeutic agents against European foulbrood.