Inhibition of Pseudomonas aeruginosa quorum sensing by subinhibitory concentrations of curcumin with gentamicin and azithromycin

Objectives: Pseudomonas aeruginosa quorum sensing (QS) circuits regulate virulence factors and coordinate bacterial pathogenicity. This study aimed to investigate the inhibitory activity of subinhibitory concentrations of curcumin with azithromycin and gentamicin against P. aeruginosa QS-related genes and virulence factors. Methods: The minimum inhibitory concentrations (MICs) and synergistic activity of curcumin with azithromycin and gentamicin against P. aeruginosa PAO1 were determined using broth microdilution and checkerboard titration methods, respectively. The activity of sub-MICs (1/4x and 1/16x MIC) of curcumin on the QS signal molecules was assessed using a reporter strain assay. The influence of sub-MICs of curcumin, azithromycin and gentamicin alone and in combination on motility and biofilm formation was also determined and was confirmed by RT-PCR to test the expression of the QS regulatory genes lasI, lasR, rhlI and rhlR. Results: Addition of curcumin drastically decreased the MIC of azithromycin and gentamicin. Curcumin showed synergistic effects with azithromycin and gentamicin. Treated PAO1 cultures in the presence of curcumin showed a significant reduction of signals C12-HSL and C4-HSL (P < 0.05). Sub-MICs (1/4x and 1/16x MIC) of curcumin, azithromycin and gentamicin alone and in combination significantly reduced swarming and twitching motilities as well as biofilm formation. Expression of QS regulatory genes lasI, lasR, rhlI and rhlR using 1/4x MIC of curcumin, azithromycin and gentamicin alone and in combination was decreased significantly compared with untreated PAO1. Conclusions: These results indicate that a combination of sub-MIC of curcumin with azithromycin and gentamicin exhibited synergism against P. aeruginosa QS systems. (C) 2017 International Society for Chemotherapy of Infection and Cancer. Published by Elsevier Ltd. All rights reserved.