Novel Synergistic Activity of Quercus infectoria Gall Extract with Ceftazidime Against Standard and Multiple Drug Resistant Pseudomonas aeruginosa and Escherichia coli Isolates.

BACKGROUND: Multidrug resistance pathogens are important heath challenges. In this study, the antibacterial activity of 20 plant extracts was tested against standard as well as 20 multidrug-resistant (MDR) strains of Pseudomonas aeruginosa and Escherichia coli. The most active plant extract (Quercus infectoria) was selected for the synergistic activity assay. METHODS: Plant extracts were prepared by maceration using water, methanol and ethanol. The antibacterial activity of extracts was determined by both broth and agar dilution methods. The synergistic activity of QIG with ceftazidime (CAZ) was evaluated by checker board assay. Antioxidant activity was determined by colorimetric Ferric reducing antioxidant power (FRAP) assay. RESULTS: Only the methanol extract of QIG inhibited the growth of all the bacterial strains at a concentration of 1000 µg/mL. Other active extracts were Myrtus communis and Eucalyptusglobulus inhibiting the growth of most bacterial strains tested at 2000 µg/ mL. In checker board assay, the minimum inhibitory concentration (MIC) to both QIG extract and CAZ was reduced. The MIC of CAZ was reduced from 64-4096 µg/mL to 4 µg/mL for P. aeruginosa and to 16 µg/mL for E. coli isolates. CONCLUSION: The QIG extract exhibited potent antioxidant activity determined by FRAP assay. The result of this study showed a strong synergistic activity between QIC and CAZ on P. aeruginosa and E. coli. The activity within ethyl acetate-methanol (7:3) fraction indicates that the active components of the plant have a semi-polar nature and further work with this fraction may lead to understanding the mechanism of this synergistic activity.