Aim/background: The emergence of drug-resistant pathogens has drawn attention on medicinal plants for potential antimicrobial properties. The objective of the present study was the investigation of the antimicrobial activity of five plant essential oils on multi-drug resistant Gram-negative bacteria.
Material and Methods: Basil, Chamomile Blue, Origanum, Thyme and Tea Tree oil were tested against clinical isolates of Acinetobacter baumannii (n=6), Escherichia coli (n=4), Klebsiella pneumoniae (n=7) and Pseudomonas aeruginosa (n=5) using the broth macrodilution method.
Results: The tested essential oils produced variable antibacterial effect, while Chamomile blue oil demonstrated no antibacterial activity. Origanum, Thyme and Basil oils were ineffective on P. aeruginosa isolates. The MIC and MBC values ranged from 0.12 -1.50% (v/v) for Tea Tree oil, 0.25-4% (v/v) for Origanum and Thyme oil, 0.50->4% for Basil oil and >4% for Chamomile Blue oil. Compared to literature data on reference strains the reported MIC values were different by 2SD, denoting less successful antimicrobial activity against multi-drug resistant isolates.
Conclusions: The antimicrobial activities of the essential oils are influenced by the strain origin (wild, reference, drug sensitive or resistant) and it should be taken into consideration whenever investigating the plants’ potential for developing new antimicrobials.
Key words: Essential oil; antimicrobial activity; multi-drug resistant; Klebsiella; Acinetobacter; Escherichia coli; Pseudomonas
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