Gas chromatography/mass spectroscopy analysis was performed to identify the chemical components of three extracted essential oils including thyme, marjoram, and sage. The 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. Thyme oil was the most active followed by marjoram oil, whereas sage displayed no activity towards the tested organism. Thyme 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. Thyme 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%). The 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-negative pathogens are particularly worrisome, because they are becoming resistant to nearly all drugs that would be considered for treatment. The most serious Gram-negative infections are health-care associated and the most common pathogens 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. Moreover, 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 properties. A growing body of evidence points to the antimicrobial potentials of plant extracts in an era of prevalence of antibiotics resistance. It is reported that plant derived antimicrobials have higher minimum inhibitory concentrations (MICs) than bacterial and fungal produced antibiotics [3, 4]. This provides the rationale for research into the potential
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