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Abstract
Monarda spp. are promising Lamiaceae due to essential oil (EO) composition and antimicrobial activity. To investigate these properties, Monarda didyma and M. fistulosa were grown for two years in two Italian sites for EO extraction (distillation), analysis (GC and GC/MS), and antimicrobial activity (microplate diffusion broth method). EO yield greatly increased from first to second year of cultivation (0.27 and 0.51% in 2013 and 2014, respectively), showing minor differences between M. didyma and M. fistulosa. Conversely, EO composition significantly varied between the two species and years. Of the fifteen major compounds identified, eight were shared by the two species. However, the amounts differed, especially in thymol (62 vs. 31% in M. didyma vs. M. fistulosa) opposed to α- and β-phellandrene (their average, 1% in M. didyma and 16% in M. fistulosa). An increase of thymol was observed between 2013 and 2014 in both species (average, +11%), reflecting juvenile plants showing only vegetative organs (2013) vs. fully flowered plants (2014). Despite such differences, EO’s from the two Monarda species exhibited a similar activity against pathogenic and non-pathogenic strains of fungi and bacteria from human, animal and plant source. In general, Monarda EO’s showed a lower minimum inhibitory concentration, i.e. a stronger activity, against pathogenic (Escherichia coli, Erwinia amylovora and Candida albicans) than beneficial microorganisms (Bifidobacterium animalis and Lactobacillus casei). The higher susceptibility of pathogenic microorganisms supports the use of Monarda EO’s as antimicrobial agents with a favourable profile of selectivity. Applications are envisaged in food preservation, plant protection and human health.