A new challenge in cystic fibrosis antimicrobial therapy: activity of electrochemically synthesized silver nanoparticles against planktonic and biofilm cells of bacterial pathogens

Introduction: Staphylococcus aureus and Pseudomonas aeruginosa are the most prevalent respiratory pathogens in Cystic Fibrosis (CF) however other species, such as Burkholderia cepacia and Stenotrophomonas maltophilia, can cause a severe decline in lung function due to their Multidrug-Resistant (MDR) phenotypes. Recently, several studies have reported silver nanoparticles (AgNPs) as a promising alternative to antibiotics because of their relevant bactericidal effects against a wide range of microorganisms. In the present study we evaluated, for the first time, the activity of a new AgNPs formulation against MDR strains isolated from CF patients. Materials and Methods: the AgNPs were synthesized by a new electronic device using an electrochemical method. The AgNPs were assayed, comparatively to tobramycin, against MDR strains of P. aeruginosa, S. maltophilia, B. cepacia, and S. aureus for: i) in vitro activity against planktonic cells (MIC and MBC, and time-killing assay by using micro- and macrodilution method, respectively); ii) in vitro activity against preformed biofilm when tested at 1x, 2x and 4xMIC (viable cell count); iii) in vivo cytotoxic potential (Galleria mellonella wax moth larva). Finally, morphological changes induced in P. aeruginosa biofilm were monitored by Transmission Electron Microscopy (TEM). Results: The AgNPs were particularly active against P. aeruginosa and B. cepacia planktonic cells (median MIC: 1.06 and 2.12 mg/ml, respectively) by a rapid, bactericidal and concentration-dependent effect. The AgNPs resulted particularly effective against P. aeruginosa and S. aureus biofilm causing a viability reduction ranging from 50% (1xMIC) to >99.9% (4xMIC). TEM showed that the AgNPs deconstruct extracellular matrix of P. aeruginosa biofilm and accumulate at the cell surface causing cell death secondary to membrane damage. Compared to tobramycin, the AgNPs showed comparable, or even better, activity against planktonic and biofilm P. aeruginosa cells. The AgNPs at concentrations effective against B. cepacia and P. aeruginosa were not toxic to G. mellonella larvae.