title: Antimicrobial activity of Eucalyptus camaldulensis essential oils and their interactions with conventional antimicrobial agents against multi-drug resistant Acinetobacter baumannii.
abstract: BACKGROUND
  Traditional herbal medicine has become an important issue on the global scale during the past decade. Among drugs of natural origin, special place belongs to essential oils, known as strong antimicrobial agents that can be used to combat antibiotic-resistant bacteria. Eucalyptus camaldulensis leaves are traditional herbal remedy used for various purposes, including treatment of infections. The aim of this study was to determine antimicrobial potential of two E. camaldulensis essential oils against multi-drug resistant (MDR) Acinetobacter baumannii wound isolates and to examine possible interactions of essential oils with conventional antimicrobial agents.
  
  
  METHODS
  Chemical composition of essential oils was determined by gas chromatography-mass spectrometry analysis (GC-MS). MIC values of essential oils against A. baumannii strains were estimated by modified broth microdilution method. The components responsible for antimicrobial activity were detected by bioautographic analysis. The potential synergy between the essential oils and antibiotics (ciprofloxacin, gentamicin and polymyxin B) was examined by checkerboard method and time kill curve.
  
  
  RESULTS
  The dominant components of both essential oils were spatulenol, cryptone, p-cimene, 1,8-cineole, terpinen-4-ol and β-pinene. The detected MICs for the E. camaldulensis essential oils were in range from 0.5 to 2 μl mL(-1). The bioautographic assay confirmed antibacterial activity of polar terpene compounds. In combination with conventional antibiotics (ciprofloxacin, gentamicin and polymyxin B), the examined essential oils showed synergistic antibacterial effect in most of the cases, while in some even re-sensitized MDR A. baumannii strains. The synergistic interaction was confirmed by time-kill curves for E. camaldulensis essential oil and polymyxin B combination which reduced bacterial count under detection limit very fast, i.e. after 6h of incubation.
  
  
  CONCLUSIONS
  The detected anti-A. baumannii activity of E. camaldulensis essential oils justifies traditional use of this plant. The proven E. camaldulensis essential oil synergistic interactions with conventional antibiotics could lead to the development of new treatment strategies of infections caused by MDR A. baumannii strains in the term of antibiotic dose reduction.
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Biology and Ecology, Trg Dositeja Obradovica 3, Novi Sad, Serbia. Electronic address: petar.knezevic@dbe.uns.ac.rs.
authors.forename: Petar
authors.initials: P
authors.lastname: Knezevic
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Biology and Ecology, Trg Dositeja Obradovica 3, Novi Sad, Serbia.
authors.forename: Verica
authors.initials: V
authors.lastname: Aleksic
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, Novi Sad, Serbia.
authors.forename: Natasa
authors.initials: N
authors.lastname: Simin
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, Novi Sad, Serbia.
authors.forename: Emilija
authors.initials: E
authors.lastname: Svircev
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Biology and Ecology, Trg Dositeja Obradovica 3, Novi Sad, Serbia.
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Petrovic
authors.affiliation: University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, Novi Sad, Serbia.
authors.forename: Neda
authors.initials: N
authors.lastname: Mimica-Dukic
mesh_terms: D040981:Acinetobacter baumannii / Q000187:drug effects*; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D000890:Anti-Infective Agents / Q000494:pharmacology*; D000081008:Bicyclic Monoterpenes; D001643:Bridged Bicyclo Compounds / Q000494:pharmacology; D002939:Ciprofloxacin / Q000494:pharmacology; D003511:Cyclohexanols / Q000494:pharmacology; D000081007:Cymenes; D004347:Drug Interactions / Q000502:physiology; D024901:Drug Resistance, Multiple, Bacterial / Q000187:drug effects*; D000077591:Eucalyptol; D005052:Eucalyptus / Q000737:chemistry*; D008826:Microbial Sensitivity Tests / Q000295:instrumentation; D039821:Monoterpenes / Q000494:pharmacology; D009822:Oils, Volatile / Q000494:pharmacology*; D010938:Plant Oils / Q000494:pharmacology*; D011112:Polymyxin B / Q000494:pharmacology; D013729:Terpenes / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D000890:Anti-Infective Agents; D000081008:Bicyclic Monoterpenes; D001643:Bridged Bicyclo Compounds; D003511:Cyclohexanols; D000081007:Cymenes; D039821:Monoterpenes; D009822:Oils, Volatile; D010938:Plant Oils; D013729:Terpenes; C007210:4-cymene; C010789:beta-pinene; C034019:terpinenol-4; D002939:Ciprofloxacin; D011112:Polymyxin B; D000077591:Eucalyptol
keywords: Acinetobacter baumannii; Antibiotics; Essential oils; Eucalyptus camaldulensis; Multi-drug resistance; Synergistic interactions (synergy)