Effect of Mentha longifolia L. ssp longifolia essential oil on the morphology of four pathogenic bacteria visualized by atomic force microscopy

The chemical composition and the antibacterial activity of the essential oil of Mentha longifolia L. ssp longifolia are being investigated in the present work. The minimal inhibitory concentration (MIC) of this oil against four gram+ and gram-reference bacteria including Salmonella typhimurium LT2, Escherichia coli ATCC 35218, Microccus luteus NCIMB 8166 and Staphylococcus aureus ATCC 25923 was used to study the morphological alteration of bacterial wall cell visualized by atomic force microscopy (AFM). The chemical analysis of the essential oil showed the presence of 34 compounds. The most important ones were: menthol (32.51%), menthone (20.71%), pulegone (17.76%), 1,8-cineole (5.61%), terpineol-4 (4.87%) and piperitone (2.16%). The MIC for bacteria ranged from 0.19 to 1.56 mg/ml. We found that M. longifolia (Menthol chemotype) has a high antibacterial effect. Cell wall of the tested bacteria was damaged at MIC concentrations. This susceptibility is more accentuated in S. typhimurium and E. coli (rod bacteria), whereas the damage is less important in coccoid bacteria (S. aureus and M. luteus).