title: In vitro antimicrobial activity of five essential oils on multidrug resistant Gram-negative clinical isolates.
abstract: OBJECTIVE
  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 multidrug resistant Gram-negative bacteria.
  
  
  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 minimum inhibitory concentration (MIC) and minimum bactericidal concentration values ranged from 0.12% to 1.50% (v/v) for tea tree oil, 0.25-4% (v/v) for origanum and thyme oil, 0.50% to >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 multidrug 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.
authors.affiliation: Department of Microbiology, Faculty of Medicine, School of Health Sciences, University of Ioannina, Ioannina, Greece.
authors.forename: Hercules
authors.initials: H
authors.lastname: Sakkas
authors.affiliation: Department of Microbiology, Faculty of Medicine, School of Health Sciences, University of Ioannina, Ioannina, Greece.
authors.forename: Panagiota
authors.initials: P
authors.lastname: Gousia
authors.affiliation: Department of Hygiene and Technology of Food of Animal Origin, School of Veterinary Medicine, Faculty of Health Sciences, Aristotle University of Thessaloniki, Thessaloniki, Greece.
authors.forename: Vangelis
authors.initials: V
authors.lastname: Economou
authors.affiliation: Department of Chemistry, Laboratory of Analytical Chemistry, School of Sciences, University of Ioannina, Ioannina, Greece.
authors.forename: Vassilios
authors.initials: V
authors.lastname: Sakkas
authors.affiliation: Department of Microbiology, Faculty of Medicine, School of Health Sciences, University of Ioannina, Ioannina, Greece.
authors.forename: Stefanos
authors.initials: S
authors.lastname: Petsios
authors.affiliation: Department of Microbiology, Faculty of Medicine, School of Health Sciences, University of Ioannina, Ioannina, Greece.
authors.forename: Chrissanthy
authors.initials: C
authors.lastname: Papadopoulou
keywords: Acinetobacter; Escherichia coli; Klebsiella; Pseudomonas; antimicrobial activity; essential oil; multidrug resistant