Effects of a novel anti-biofilm peptide CRAMP combined with antibiotics on the formation of Pseudomonas aeruginosa biofilms

The remarkable ability of Pseudomonas aeruginosa to form biofilms renders antibiotic treatments inefficient and therefore causing a wide variety of chronic infections. The quorum sensing (QS) system in P. aeruginosa plays a role in the regulation of genes controlling virulence factors and biofilm formation, which may be an essential target for pharmacological intervention. The present study aimed to investigate the synergistic activity of sub-MIC concentrations of CRAMP (a cathelicidin-related antimicrobial peptide) with fourteen antibiotics against P. aeroginusa biofilms. Finally, CRAMP's best synergistic activity combined with colistin at 1/4 MIC was screened by the checkerboard method and the calculation of the synergetic coefficient. It was confirmed by experiments on 6-well plates, displaying the most significant biofilm formation inhibition % (91.05%, calculated by OD value of biofilm biomass) and the best bactericidal activity of biofilms (2.77-log10 decrease). These data correlate with the confocal laser scanning microscopy (CLSM) images obtained for the biofilm. The combination also down-regulated the expression of QS regulated genes, resulting in inhibitory effects on QS-regulated virulence phenotypes (pyocyanin and rhamnolipid). These results indicate that a proposed method of combination therapy of CRAMP with colistin has the potential to serve as a more effective therapy for P. aeruginosa biofilm infection.