Essentials oils of some Mentha spp. and their relation with antimicrobial activity against Paenibacillus larvae, the causative agent of American foulbrood in honey bees, by using the bioautography technique

Essential oils of three mint species (Mentha aff. arvensis, Mentha aff. rotundifolia and a probably feral hybrid of Mentha spp.) were investigated for their antimicrobial properties against Paenibacillus larvae (White). The bioactivity of the oils was compared using the combination of in vitro techniques such as microdilution, agar dilution and bioautography. The chemical composition of the oils was analyzed by GC/MS. Using the bioautography assay, menthol, menthone, menthofuran and piperitone oxide were found to be responsible for the antimicrobial activity of these oils. A quantitative structure-activity relationship (QSAR) model was developed for four terpenoids with significant antimicrobial activity using Hyperchem 8.0 and Gaussian 03 software. The QSAR approach leads to a better understanding of the structural properties of these terpenoids which are responsible for bioactivity. The present work reports the first systematic study about the use of QSAR properties to correlate antimicrobial activity of natural substances against P. larvae.