Multidrug resistant Pseudomonas aeruginosa (Pa) is a major cause of severe hospital-acquired infections. Currently, polymyxin B (PMB) is a last resort antibiotic for the treatment of infections caused by Gram-negative bacteria, despite its undesired side effects. The delivery of drug combinations has been shown to reduce the required therapeutic doses of antibacterial agents and thereby their toxicity if a synergistic effect is present.
In this study, we investigated the synergy between two cyclic antimicrobial peptides, PMB and gramicidin S (GS), against different Pa isolates with a quantitative checkerboard assay using resazurin as a growth indicator. Among the 28 strains that we studied, 20 strains showed a distinct synergistic effect, represented by a fractional inhibitory concentration index (FICI) ≤ 0.5. Remarkably, several clinical Pa isolates that grew as small colony variants revealed non-synergistic effect as indicated by FICIs between > 0.5 and ≤ 0.70. In addition to inhibiting the growth of planktonic bacteria, the peptide combinations significantly decreased static biofilm growth compared with treatment with the individual peptides.
There was also a faster and more prolonged effect of PMB combined with GS compared with single peptide treatments on the metabolic activity of pre-grown biofilms. The results of the present study define a synergistic interaction between two cyclic membrane active peptides towards 17 multidrug resistant Pa and biofilms of strain PAO1. Thus, the application of PMB and GS in combination is a promising option for topical medication and the prevention of acute and chronic infections caused by multidrug resistant or biofilm forming Pa.