Antibiotic-Potentiating Effect of Some Bioactive Natural Products against Planktonic Cells, Biofilms, and Virulence Factors of Pseudomonas aeruginosa

Background. Pseudomonas aeruginosa is an opportunistic human pathogen that causes infections that are mediated by both virulence factor production and biofilm formation. In addition, many antibiotics are increasingly losing their efficacy due to the development of resistance. The screening of potentially bioactive natural compounds that have both antivirulence and antibiofilm activities to enhance antibiotic efficacy and reverse antibiotic resistance is a good strategy to overcome these issues. In this study, the antibacterial, antibiofilm, and antivirulence factor activities of some bioactive natural products in combination with conventional antibiotics were evaluated against clinical isolates of P. aeruginosa. Methods. The broth microdilution method was used to determine the antibacterial and antibiofilm activities. The checkerboard method was used to evaluate the combination interactions. Spectrophotometric and agar plate techniques were used to assess the effect of the combination on the pyocyanin production and the motility in P. aeruginosa ATCC 27853 strain. Results. Out of the eighteen combinations tested, ten exhibited synergistic effects against planktonic cells, seven against biofilm inhibition, and five against the eradication of mature biofilm of P. aeruginosa biofilm. The best synergistic effect was the association of amikacin and sinapic acid against planktonic cells ( $\text{FICI}=0.08$ ) with a 70-fold reduction in the MIC value of amikacin. The same combination showed significant synergistic inhibition of biofilm formation ( $\text{FICI}=0.1$ ) and biofilm eradication ( $\text{FICI}=0.15$ ) reducing the MBIC and MBEC of amikacin by 32-fold. Some selected synergistic combinations showed statistically significant differences ( $p<0.01$ or $p<0.001$ ) in the inhibition of virulence factors compared to the antimicrobials alone. Conclusion. In summary, this study revealed sinapic acid as an antibiotic adjuvant and antivirulence compound to overcome P. aeruginosa infections. This finding indicates that the combinations of amikacin plus sinapic acid, ceftazidime plus thymol, and norfloxacin plus curcumin could be considered promising candidates for the development of combination therapies targeting virulence factors against P. aeruginosa infections.