Antimicrobial potency of differently coated 10 and 50 nm silver nanoparticles against clinically relevant bacteria Escherichia coli and Staphylococcus aureus

Silver nanoparticles (nanoAg) are effective antimicrobials and promising alternatives to traditional antibiotics. This study aimed at evaluating potency of different nanoAg against healthcare infections associated bacteria: Gram-negative Escherichia colt and Gram-positive Staphylococcus aureus. A library of differently coated nanoAg of two different sizes (10 and 50 nm) were prepared using coating agents poly-L-Lysine (PLL), cetyltrimethyl-ammonium bromide (CTAB), citrate (CIT), polyvinyl-pyrrolidone (PVP), polysorbate 80 (Tween 80), and dioctylsodium sulfosuccinate (AOT). Stability evaluation by means of agglomeration and dissolution behaviour was performed for all nanoAg under conditions relevant for this study. Antibacterial properties of nanoAg were addressed by determining their minimal bactericidal concentrations (MBC) in delonised (DI) water to minimise the influence of silver speciation on its bioavailability. In parallel, AgNO3 was analysed as an ionic control. Studied nanoAg were efficient antimicrobials being remarkably more potent towards E. colt than to S. aureus (4 h MBC values for different nanoAg ranged from 0.08 to 5.0 mg Ag/L and 1.0-10 mg Ag/L, respectively). The toxicity of all nanoAg to S. aureus (but not to E. coli) increased with exposure time (4 h vs 24 h). 10 nm sized nanoAg released more Ag-ions and were more toxic than 50 nm nanoAg. Coating-dependent toxicity was more prominent for 50 nm nanoAg coated with Tween 80 or CTAB rendering the least toxic nanoAg. Obtained results showed that the antimicrobial effects of nanoAg were driven by shed Ag-ions, depended on target bacteria, exposure time and were the interplay of NP size, solubility and surface coating.