Synergistic activity of sub-inhibitory concentrations of curcumin with ceftazidime and ciprofloxacin against Pseudomonas aeruginosa quorum sensing related genes and virulence traits

Quorum sensing (QS) system in Pseudomonas aeruginosa may be an important target for pharmacological intervention. The present study aimed to investigate the synergetic activity of sub-MIC concentrations of curcumin (C) with ceftazidime (CAZ) and ciprofloxacin (CIP) against P. aeroginusa QS system. We determined the MIC and synergistic activity of C, CAZ and CIP against P. aeroginusa PAO1 using broth microdilution and checkerboard titration methods. The activity of sub-MIC (1/4 and 1/16 MIC) concentrations of C on the QS signal molecules was assessed using a reporter strain assay. The influence of sub-MIC of C, CAZ and CIP alone and in combination on motility and biofilm formation was also determined and confirmed by RT-PCR to test the expression of QS regulatory genes lasI, lasR, rhlI and rhlR. The addition of C decreased the MIC of CAZ and CIP. Curcumin showed synergistic effects with CAZ and additive activity with CIP. Treated PAO1 cultures in the presence of C showed significant reduction of signals C12-HSL and C4-HSL (P < 0.05). Sub-MIC concentrations (1/4 and 1/16 MIC) of C, CAZ and CIP alone and in combination significantly reduced swarming and twitching motilities and biofilm formation. Expression of QS regulatory genes lasI, lasR, rhlI, and rhlR using 1/4 MIC of C, CAZ and CIP alone and in combination was repressed significantly relative to untreated PAO1. Our results indicate that a combination of the sub-MIC concentration of C and CAZ exhibited synergism against P. aeroginusa QS system. This combination could lead to the development of a new combined therapy against P. aeruginosa.