Silver nanoparticle antibacterial efficacy and resistance development in key bacterial species

Silver nanoparticles (AgNPs) are widely used for their antibacterial properties due to the expected benefit of long-term antibacterial efficacy and low potential for bacterial resistance. This study examined the potential for three bacterial species, Staphylococcus aureus, Pseudomonas aeruginosa, and Acinetobacter baumannii, to develop resistance to AgNPs in comparison to ciprofloxacin, benzalkonium chloride, and silver nitrate. Antibacterial efficacy was measured by determining the minimum inhibitory and minimum bactericidal concentrations using a broth microtiter method, and resistance development was tested using stepwise increasing concentrations of antibacterial agents. Only P. aeruginosa was able to develop resistance to AgNPs while all three organisms developed resistance to ciprofloxacin, and none of them developed resistance to silver nitrate or benzalkonium chloride. The resistance was stable and appeared to derive from multiple mechanisms since one AgNP-resistant variant was cross-resistant to silver nitrate, but the other variant was not. The precise mechanisms of AgNP resistance are yet to be elucidated, but they might involve the ability of phenazine pigments produced by P. aeruginosa to limit the bacterium's exposure to silver. In order to reduce the possibility of bacteria developing resistance, AgNPs or their carrier medium should be engineered to ensure that an inhibitory concentration of silver is consistently maintained.