Antimicrobial activity of Eucalyptus camaldulensis essential oils and their interactions with conventional antimicrobial agents against multi-drug resistant Acinetobacter baumannii Petar Knezevic a,n , Verica Aleksic a , Natasa Simin b , Emilija Svircev b , Aleksandra Petrovic a , Neda Mimica-Dukic b a University of Novi Sad, Faculty of Sciences, Department of Biology and Ecology, Trg Dositeja Obradovica 3, Novi Sad, Serbia b University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, Novi Sad, Serbia article info Article history: Received 10 July 2015 Received in revised form 2 December 2015 Accepted 4 December 2015 Available online 6 December 2015 Keywords: Acinetobacter baumannii Multi-drug resistance Eucalyptus camaldulensis Essential oils Antibiotics Synergistic interactions (synergy) abstract Ethnopharmacological relevance: 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 es- sential oils against multi-drug resistant (MDR) Acinetobacter baumannii wound isolates and to examine possible interactions of essential oils with conventional antimicrobial agents. Materials and 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 es- timated by modified broth microdilution method. The components responsible for antimicrobial activity were detected by bioautographic analysis. The potential synergy between the essential oils and anti- biotics (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-ci- neole, 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 com- pounds. 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 de- tection limit very fast, i.e. after 6 h 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. & 2015 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Acinetobacter baumannii has emerged as one of the most threatening pathogens on a global scale, particularly in health care institutions. The hospital acquired A. baumannii infections prolong the length of hospital stays and subsequently increase health care costs. Its clinical significance was propelled at the beginning of this century as a result of its remarkable ability to up-regulate existing genes or acquire resistance determinants, making it one of the organisms threatening usability of the conventional antibiotics (Peleg et al., 2008). The main factors most likely contributing to the persistence of A. baumannii in the hospital environment are resistance to desiccation and a high level of resistance to disin- fectants and antimicrobial drugs (Fournier et al., 2006; Smith et al., 2007). Significant increase in mortality was noted for A. baumannii Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jep Journal of Ethnopharmacology http://dx.doi.org/10.1016/j.jep.2015.12.008 0378-8741/& 2015 Elsevier Ireland Ltd. All rights reserved. n Corresponding author. E-mail address: petar.knezevic@dbe.uns.ac.rs (P. Knezevic). Journal of Ethnopharmacology 178 (2016) 125–136