Antimicrobial Activities of Essential Oils of Mint and Peppermint as Well as Some of Their Main Compounds

The oil of peppermint (Mentha x piperita L.) from the USA, a dementholized cornmint oil (M. canadensis L.), and a fraction of the dementholized cornmint oil were analyzed by GC-FID and GC/MS. The main constituents of the three samples analyzed were menthone (23.4%, 20.4% and 58.3%, respectively) and menthol (40.7%, 41.2% and 12.3%, respectively). The two oils, the cornmint oil fraction and seven mint oil compounds [(-)-menthol, (-)-menthone, (+/-)-menthyl acetate, 1,8-cineole, limonene, β-pinene and β-caryophyllene] were investigated for their antimicrobial effects against two Gram-positive bacteria (Staphylococcus aureus and Enterococcus faecalis), five Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Proteus vulgaris, Klebsiella pneumoniae and Salmonella sp.) and the yeast Candida albicans using a modifed agar dilution and agar diffusion method. Medium to high antimicrobial effects were found for both oils, the dementholized cornmint oil fraction, and the target-compounds against all Gram-positive bacteria, whereas against the Gram-negative bacteria and the yeast, one or more samples showed only weak or no activity.