Antimicrobial Activity of Rosemary Essential Oil and its Interaction with Common Therapeutic Antibiotics on some Gram Positive and Gram Negative Bacteria

Rosemary has long been regarded as one of the antimicrobial agents in traditional medicine. Today, due to the increasing resistance of microbial strains to antibiotics, numerous problems have arisen in medical and pharmaceutical sciences. This study was performed to evaluate antimicrobial activity of Rosemary essential oil (REO) on some important human pathogenic bacteria.

Five pathogenic bacterial strains including Escherichia coli, Pseudomonas aeruginosa and Salmonella typhi as gram negative bacteria and Staphylococcus aureus and Listeria innocua as gram positive bacteria were selected. Antimicrobial activity of REO in vitro conditions on the selected pathogens was determined by disk diffusion agar, well diffusion agar, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). Furthermore, the interaction of REO with common therapeutic antibiotics was assessed.

The results showed that based on disc diffusion agar method, the inhibition zone diameter of REO for Staphylococcus aureus, Salmonella typhi, Listeria innocua, Escherichia coli and Pseudomonas aeruginosa were 14.60, 10.30, 12.50, 12.10 and 12.20 mm, respectively. A synergistic effect was revealed in combination of REO and chloramphenicol antibiotic against Salmonella typhi, Listeria innocua, and Pseudomonas aeruginosa, while this impact has been shown to be antagonistic for Escherichia coli and Staphylococcus aureus. Furthermore, synergistic interactions between REO and gentamicin antibiotic were found for all tested strains and the MIC for Staphylococcus aureus, Salmonella typhi, Listeria innocua, Escherichia coli and Pseudomonas aeruginosa were determined as 20.10, 18.60, 20.20, 18.00 and 19.10 mm, respectively. The minimum inhibitory concentration of REO for all tested bacteria in this study was 12.50 mg/ml.

Based on results of this study, it was found that REO has an acceptable antibacterial activity against the examined pathogenic strains. Therefore, REO may be used as a natural preservative in food products to control the growth of pathogenic microorganisms.