Susceptibility of Paenibacillus larvae isolates to a tetracycline hydrochloride and Cinnamon (Cinnamomum zeylanicum) essential oil mixture

The antimicrobial activity of tetracycline hydrochloride (OTC) and cinnamon (Cinnamomum zeylanicum Nees) essential oil (CEO) was evaluated against six different isolates of Paenibacillus larvae (White), the causal agent of American Foulbrood (AFB) disease in honey bee colonies. The bacteria isolates were collected from different localities of Argentine. Minimal inhibitory concentration (MIC) in MYT broth by the tube dilution method was evaluated for each substance and for the combinations of both antimicrobials using Krogstad and Moellering technique in order to establish the possible synergistic effects between these substances. OTC mean MIC values were of 3.67 ± 1.80 μg/ml, while the mean MIC values obtained for CEO were of 41.67±19.17 μg/ml. An inhibitory synergetic effect between these substances was observed with FIC index < 1 on 50% of the on P. larvae isolates.