The influence of secondary metabolites on the bacterial colonization of the phyllosphere of four aromatic species of the Mediterranean region was studied for the determination of total bacterial populations (TBP) and populations of ice nucleation active bacteria (INA). The aromatic plants used were lavender (Lavandula angustifolia), rosemary (Rosmarinus officinalis), Greek sage (Salvia fruticosa), and Greek oregano (Origanum vulgare subsp. hirtum), all growing in neighboring sites. Lavender was heavily colonized by bacteria, whereas rosemary, sage, and oregano were poorly colonized. The differences in bacterial colonization were related to the plants' content of secondary metabolites and their antimicrobial activity, as recorded in the in vitro bioassays. Lavender had the lowest amount of surface phenolics, the lowest concentration of essential oil, and the overall weakest antibacterial activity. Among the epiphytic bacteria, ice nucleation active ones were not detected on oregano and sage leaves but were found in extremely low numbers on those of rosemary and lavender. For this reason, these aromatic plants were further studied regarding their effect against two INA bacteria, Pseudomonas syringae and Erwinia herbicola. Minimum inhibitory concentrations and minimum bactericidal concentrations were estimated for the essential oils and for their main constituents under different bacterial populations. The antibacterial effect of Labiatae aromatic plants against INA bacteria not only explains the scarce presence of the latter on their leaves but may have applications in agriculture as a frost-control method for sensitive crops.
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