Hexanic extract of Achyrocline satureioides : antimicrobial activity and in vitro inhibitory effect on mechanisms related to the pathogenicity of Paenibacillus larvae

Introduction: Paenibacillus larvae is a spore-forming bacillus, the most important bacterial pathogen of honeybee larvae and the causative agent of American foulbrood (AFB). Control measures are limited and represent a challenge for both beekeepers and researchers. For this reason, many studies focus on the search for alternative treatments based on natural products. Aim: The objective of this study was to determine the antimicrobial activity of the hexanic extract (HE) of Achyrocline satureioides on P. larvae and the inhibitory activity on some mechanisms related to pathogenicity. Material and methods: The Minimum Inhibitory Concentration (MIC) of the HE was determined by the broth microdilution technique and the Minimum Bactericidal Concentration (MBC) by the microdrop technique. Swimming and swarming motility was evaluated in plates with 0.3 and 0.5% agar, respectively. Biofilm formation was evaluated and quantified by the Congo red and crystal violet method. The protease activity was evaluated by the qualitative technique on skim milk agar plates. Results: It was determined that the MIC of the HE on four strains of P. larvae ranged between 0.3 and 9.37 µg/ml and the MBC between 1.17 and 150 µg/ml. On the other hand, sub-inhibitory concentrations of the HE were able to decrease swimming motility, biofilm formation and the proteases production of P. larvae .