Determining Antibacterial Activity of 19 Medicinal Plant Extracts on Standard and Clinical Strains of Pseudomonas aeruginosa and Synergistic Activity of the Most Active Extract with Ceftazidime

Background: The emergence of multidrug-resistant (MDR) strains of P aeruginosa has become a serious concern today. This bacterium is resistant to a wide range of antimicrobial drugs. There are several ways to treat and control infections caused by MDR bacteria. One of these methods is to find active plant compounds with antimicrobial properties and antimicrobial combination of herbal plants with antibacterial agent with synergistic effects. The aim of the present study was to investigate the antimicrobial effects of 19 aqueous, ethanoic, and methanolic extracts of medicinal plants on standard and MDR strains of P. aeruginosa. At the end, the synergistic effect of the most active plant extract with ceftazidime was evaluated. Methods: Antimicrobial activity of medicinal plants was investigated using agar dilution method on five ceftazidime sensitive and 20 MDR clinical isolates of P.aeroginosa resistance to ceftazidime with an MIC range of 64-1024 μg/ml. After determining the best extract, its antimicrobial effect with ceftazidime was investigated by agar dilution using checkerboard assay. Results: Methanolic and ethanolic extracts of Quercus infectoria with concentration of 1000 μg/ml inhibits the growth of all standard and clinical strains of P. aeruginosa. The water extract of Quercus infectoria and methanolic and ethanolic extract of Eucalyptus galbie and Myrtus communis also reduced the growth at this concentration but did not inhibit the growth completely. The results showed that the synergistic effects of methanolic extract of Quercus infectoria and ceftazidime were significant and the MIC of the drug decreased from 1024 μg/ml to 4 μg/ml, and also the MIC of the extract was reduced to four folds of MIC. Conclusion: It was shown that the most effective extracts against Pseudomonas aeruginosa are methanolic and ethanoic extracts of Quercus infectoria, which have significant synergistic effects with ceftazidime.