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Essential Oils and Their Vapors as Potential Antibacterial Agents against Respiratory Tract Pathogens

Increasing appearance of antibiotic-resistant pathogens, which could be one of the major causes of respiratory tract infections, has again drawn attention to natural substances and alternative treatments. Therefore, the aim of the present study was the antibacterial evaluation of cinnamon bark, clove, thyme, citronella, peppermint, Scots pine, and eucalyptus essential oils (EOs) against respiratory tract pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa with in vitro vapor phase (VP) and tube dilution (TD) techniques. The chemical and percentage compositions of the EOs were determined by GC-FID and GC-MS analysis. Among the EOs, cinnamon bark was the most effective against all the investigated pathogens (MIC: 31.25–125 μL/L) in the VP assay, but clove oil presented the best inhibition against MRSA in liquid medium (MIC: 0.1 mg/mL). Thyme oil also showed antibacterial activity against MRSA and the antibiotic-sensitive strain of P. aeruginosa in both methods. In higher concentration, we found that peppermint oil was effective only in vapor form; contrarily, eucalyptus oil was more efficient in liquid medium. Surprisingly, Scots pine did not show any activity in our test systems. These results suggest that EOs could be promising solutions for the problem of antibiotic resistance due to their multiple composition and complex mode of action. However, more in vivo studies are necessary to calculate the effective dose of EOs in patients and determine their possible side effects and toxicity.