Antimicrobial activity of essential oils toward clinical and environmental strains of the opportunistic pathogen of Cystic Fibrosis patients Burkholderia cepacia complex

The rapid development of multidrug resistance pathogens is among the most serious threat to public health and to successful antibacterial treatment. The genus Burkholderia includes a variety of species, with opportunistic human pathogenic strains, with high resistance to many classes of antibiotics. A major role in resistance could be played by multidrug efflux pumps belonging to the RND superfamily. Essential oils, secondary plant metabolites, possess antibacterial, antifungal and antiviral properties and have been screened worldwide as potential sources of new antimicrobial compounds, alternatives to synthesis antibiotics to treat infectious diseases. Essential oils major components are derived from terpenes and terpenoids. These components seem to be able to damage bacterial cell membrane and to render it more permeable and to deplete the proton motive force. In our study we have screened by disk diffusion method, the antimicrobial activity of 6 different essential oils: clove (Eugenia carophyliata), lavender (Lavandula hybrida), oregano (Origanum vulgare), rosemary (Rosmarinus officinalis), tea tree oil (Malaleuca alternifolia) and thyme (Thymus vulgaris) against 10 Burkholderia strains, both of clinical and environmental origin, and against 8 B. cenocepacia J2315 mutants impaired in RND transporters. Results demonstrated that oregano, thyme and clove essential oils exhibited considerable inhibitory effects against all the organisms under test even against ciprofloxacin resistant strains. Furthermore tests made on mutant strains demonstrated a high level of inhibitory activity by essential oils tested against the RND4 mutant, in agreement with previous studies that have shown that this efflux system is a major cause of multidrug resistance in Burkholderia. The MIC determination for oregano, thyme and clove essential oils against each strain should be an obligatory step in the research of new bacterial inhibitors for the improved control of infectious diseases.