Synergistic Effect of Membrane-Active Peptides Polymyxin B and Gramicidin S on Multidrug-Resistant Strains and Biofilms of Pseudomonas aeruginosa

Abstract

Multidrug-resistant Pseudomonas aeruginosa 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 undesirable 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 P. aeruginosa isolates, using a quantitative checkerboard assay with 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) of <0.5. Remarkably, several clinical P. aeruginosa isolates that grew as smallcolony variants revealed a nonsynergistic 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 when the combination of PMB and GS was used compared with single-peptide treatments on the metabolic activity of pregrown biofilms. The results of the present study define a synergistic interaction between two cyclic membrane-active peptides toward 17 multidrug-resistant P. aeruginosa and biofilms of P. aeruginosa strain PAO1. Thus, the application of PMB and GS in combination is a promising option for a topical medication and in the prevention of acute and chronic infections caused by multidrug-resistant or biofilm-forming P. aeruginosa. Pathogenic Gram-negative Pseudomonas aeruginosa possesses both intrinsic and adaptive resistance toward many currently available antibiotics and causes infections that are effectively untreatable (1-3). P. aeruginosa "superbugs" are resistant to fluoroquinolones, expanded-spectrum cephalosporins, carbapenems, aminoglycosides, and in a few cases, even polymyxins, a last-resort class of antibiotics used to treat P. aeruginosa infections (4-7). One important mechanism for the intrinsic antibiotic resistance of Gram-negative pathogens, especially multidrug-resistant (MDR) P. aeruginosa clinical isolates, is their ability to efflux antibacterial agents via tripartite efflux pumps located in the inner and outer membranes, a mechanism that limits the access of the drug to intracellular targets (8). However, the overexpression of the P. aeruginosa efflux pump protein MexAB was unable to confer resistance to the host defense peptides (HDPs) cathelicidin LL-37 and defensins (9). One way in which pathogens acquire adaptive resistance to positively charged antimicrobial peptides is to modify lipid A by substitution with aminoarabinose (6). The emergence of drug-resistant P. aeruginosa strains, which exhibit increased MICs even for polymyxin B (PMB) (10), requires the development...

Antimicrobial Peptides: Virulence and Resistance Modulation in Gram-Negative Bacteria

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The New Metall-Beta-Lactamase’s Inhibitor Efficacy in a Model System in Vitro

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Влияние ареницинов и других β-шпилечных АНТИмикробных пептидов на биоплёнки бактерий <i>Pseudomonas aeruginosa</i> PAO1

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