Antibiotic synergy and mechanisms of action of the antimicrobial peptide glatiramer acetate against Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is the most common pathogen infecting the lungs of people with cystic fibrosis (CF), causing both acute and chronic infections. Intrinsic and acquired antibiotic resistance, coupled with the physical barriers resulting from desiccated CF sputum, allow P. aeruginosa to colonise and persist in spite of antibiotic treatment. P. aeruginosa is also subject to the wider, global issue of antimicrobial resistance. Glatiramer acetate (GA) is a peptide drug, used in the treatment of multiple sclerosis (MS), which has been shown to have moderate anti-pseudomonal activity. Other antimicrobial peptides (AMPs) have been shown to be antibiotic resistance breakers; potentiating activities of antibiotics when given in combination restoring and/or enhancing antibiotic efficacy. Among different antibiotics tested here, the efficacy of tobramycin (TOB) was significantly improved by GA in growth, viability, minimum inhibitory concentration (MIC)-determination and synergy analysis against reference P. aeruginosa. This was also the case for clinical strains from people with CF where GA significantly reduced the concentration of TOB required to inhibit P. aeruginosa compared with TOB-only. Investigating mechanisms of GA activity showed that GA resulted in significant disruption of outer membranes, depolarisation of cytoplasmic membranes and permeabilisation of P. aeruginosa and was the only agent tested (including cationic AMPs) to significantly affect all three; indicating potentiation by permeabilisation. Assays also showed that GA interacts with the lipopolysaccharides (LPS) of P. aeruginosa and this interaction may be the mechanism of potentiation of GA against P. aeruginosa. GA had moderate impact on expression of genes involved in P. aeruginosa LPS modification and on LPS modification, further linking GA activity to LPS interactions. GA is, therefore, a membrane active antibiotic resistance breaker of P. aeruginosa and a strong candidate for drug repurposing as an antibiotic adjuvant.