Exploring the Effect of the Composition of Three Different Oregano Essential Oils on the Growth of Multidrug-Resistant Cystic Fibrosis Pseudomonas aeruginosa Strains

Oregano essential oils (EOs) could represent interesting therapeutic strategies to treat multidrug-resistant (MDR) pathogens as Pseudomonas aeruginosa, responsible for respiratory infections in cystic fibrosis (CF) patients. There could be a great variability in EOs composition when extracted from different plant species. The aim of this study was to chemically characterize and to test EOs, extracted from Origanum compactum, Origanum vulgare and Origanum vulgare var. hirtum, for in vitro antimicrobial activity against a panel of twenty MDR P. aeruginosa strains isolated from CF patients. EOs main components were carvacrol (71.8-73.8-47.1%), thymol (1.6-2.3-21.5%), p-cymene (11.6-7.4-10.8%) and gamma-terpinene (1.7-3.1-8.4%). In general, the EOs showed inhibitory activity even at low concentration: 0.5% (v/v) OvEO and OhEO were able to inhibit the 80% of P. aeruginosa strains. Furthermore, the three EOs killed at least 75% of the strains at concentrations lower than 1% (v/v). Average MIC and MBC values were not significantly different. Similar levels of OEOs antimicrobial activities might be related to the fact that the main chemical class (i.e. carvacrol/thymol) is represented in quite similar percentages. Hence, the results of the present study shed light on a carvacrol/thymol-rich EO with a well-represented monoterpene hydrocarbons class as promising standardized antimicrobial herbal product.