Research Article Comparative Evaluation of the Inhibitory Effect of Some Essential Oils with Antibiotics against Pseudomonas aeruginosa Lobna El-Hosseiny, 1 Moustafa El-Shenawy, 2 Medhat Haroun, 3 and Fadhil Abdullah 3 1 Department of Environmental Studies, Institute of Graduate Studies and Research, Alexandria University, Egypt 2 Department of Food Microbiology, National Research Center, Dokki, Cairo, Egypt 3 Department of Biotechnology, Institute of Graduate Studies and Research, Alexandria University, Egypt Correspondence should be addressed to Lobna El-Hosseiny; lobnaelhosseiny@yahoo.com Received 29 June 2014; Revised 24 September 2014; Accepted 24 September 2014; Published 1 October 2014 Academic Editor: Branka Bedeni´ c Copyright © 2014 Lobna El-Hosseiny et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Gas chromatography/mass spectroscopy analysis was performed to identify the chemical components of three extracted essential oils including thyme, marjoram, and sage. e antibacterial activity of the extracted essential oils against Pseudomonas aeruginosa (ATCC 9027) was investigated using disc diffusion assay, either alone or in combination with standard antibiotics (piperacillin, cefepime, meropenem, gentamicin, and norfloxacin). Results showed that the studied oils exhibited a variety of activities against the tested bacterium. yme oil was the most active followed by marjoram oil, whereas sage displayed no activity towards the tested organism. yme oil enhanced the antibacterial activity of cell wall targeting antibiotics (piperacillin, cefepime, and meropenem) by more than twofold. Marjoram oil potentiated the activity of all the tested antibiotics except norfloxacin. Sage, despite its inactivity against pseudomonas, synergistically enhanced the activity of piperacillin, meropenem, and gentamicin. yme essential oil, containing thymol as a major component (33.6%), exhibited higher activity alone or in combination with antibiotics than marjoram which contained alcoholic terpenes or sage essential oil that contained 1,8-cineole as its major component (29%). e investigated oils, as natural bioactive agents, may be used to enhance the activity of antibiotics towards pseudomonas. 1. Introduction Among all of the bacterial resistance problems, Gram-nega- tive pathogens are particularly worrisome, because they are becoming resistant to nearly all drugs that would be consid- ered for treatment. e most serious Gram-negative infec- tions are health-care associated and the most common patho- gens are Enterobacteriaceae, Pseudomonas, and Acinetobacter [1]. Pseudomonas aeruginosa is one of the leading causes of nosocomial infections and is associated with high mortality. Infections caused by Pseudomonas are difficult to treat as the repertoire of useful antipseudomonal agents is limited. More- over, Pseudomonas exhibits remarkable abilities to acquire resistance to these agents [2]. Historically plants have enjoyed a tradition of use for their flavor enhancement characteristics and for their medicinal prop- erties. A growing body of evidence points to the antimicrobial potentials of plant extracts in an era of prevalence of antibi- otics resistance. It is reported that plant derived antimicrobi- als have higher minimum inhibitory concentrations (MICs) than bacterial and fungal produced antibiotics [3, 4]. is provides the rationale for research into the potential outcome of plant derived essential oils on the in vitro activity of anti- biotics. In view of the dearth, in the near future, of new options against resistant P. aeruginosa strains, plants provide a poten- tial source for therapeutically useful compounds especially from the perspective of their potentials in combination with antimicrobial chemotherapy. Accordingly, the current study investigated the antimicrobial enhancing effect of three dif- ferent essential oils, extracted from some medicinal plants, against the bacterium Pseudomonas aeruginosa, when com- bined with conventional antibiotics most commonly used in the treatment of such pathogen. Hindawi Publishing Corporation International Journal of Antibiotics Volume 2014, Article ID 586252, 5 pages http://dx.doi.org/10.1155/2014/586252