Abstract
Purpose: Herein, an extended investigation of Tea tree oil (TTO) against a number of multi-drug resistant (MDR) microorganisms in liquid and vapor phases is reported. Methods: The activity of TTO was tested against methicillin-sensitive Staphylococcus aureus (MSSA), Escherichia coli, and clinical strains of methicillin-resistant S. aureus (MRSA), extended-spectrum beta lactamases producer carbapenem-sensitive Klebsiella pneumoniae (ESBL-CS-Kp), carbapenem-resistant K. pneumoniae (CR-Kp), Acinetobacter baumannii (CR-Ab), and Pseudomonas aeruginosa (CR-Pa). Minimal inhibitory/bactericidal concentrations (MIC/MBCs) and synergistic activity between TTO and different antimicrobials were determined. In the vapor assay (VP), TTO-impregnated discs were placed on the lid of a petri dish and incubated for 24 h at 37 °C. Results: TTO showed a potent bactericidal activity against all the tested microorganisms. TTO in combination with each reference antimicrobial showed a high level of synergism at sub-inhibitory concentrations, particularly with oxacillin (OXA) against MRSA. The VP assay showed high activity of TTO against CR-Ab. Conclusion: Evaluation of in-vitro activity clearly indicated TTO as a potential effective antimicrobial treatment either alone or in association with known drugs against MDR. Therefore, TTO could represent the basis for a possible role in non-conventional regimens against S. aureus and Gram-negative MDR. TTO in VP might represent a promising option for local therapy of pneumonia caused by CR-Ab.
Highlights
The emergence of multidrug-resistant (MDR) microorganisms represents a global challenge worldwide, since therapeutic options are limited, resulting in thousands of deaths [1]
Chemical Tea tree oil (TTO) characterization was performed throughout gas chromatographic/mass spectrometric (GC/MS) analysis both in the liquid and vapor phase (VP)
Gram-negatives with absence of bacterial growth at concentrations ranging from 0.25% to 0.5% v/v for E. coli, extended-spectrum beta lactamases (ESBLs)-CS-Kp, carbapenem-resistant Acinetobacter baumannii (CR-Ab), and carbapenem-resistant K. pneumoniae (CR-Kp) whereas a lower bactericidal activity (1% v/v) was found for E. coli, ESBL-CS-Kp, CR-Ab, and CR-Kp whereas a lower bactericidal activity (1% v/v) was found for carbapenem-resistant Pseudomonas aeruginosa (CR-Pa), in line with that observed in other studies and summarized by Carson et al, where most for CR-Pa, in line with that observed in other studies and summarized by Carson et al, where most bacteria were susceptible to TTO at concentrations of 1.0% or less and higher MICs were reported for bacteria were susceptible to TTO at concentrations of 1.0% or less and higher MICs were reported for organisms such as P. aeruginosa [13]
Summary
The emergence of multidrug-resistant (MDR) microorganisms represents a global challenge worldwide, since therapeutic options are limited, resulting in thousands of deaths [1]. Essential oils (EOs) are volatile, natural, fragrant liquids that can be extracted from different parts of the plants (especially leaves and flowers) presenting anti-inflammatory, antiviral, and antibacterial properties [3]. Given their antimicrobial activity broad-spectrum, together with the possibility of restoring antibiotic susceptibility [4,5], several efforts have been made to consider the use of EOs for the treatment of a wide range of infections, including those caused by MDR microorganisms [6]. EOs’ activity is commonly ascribed to the perturbation of cell membrane structural integrity, leading bacterial cell to death [7] and their potency varies with the type of microorganisms, Gram-positive bacteria being more susceptible than Gram-negatives [8,9,10,11].
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