The inhibitory effect of colloidal silver nanoparticles on three bacterial strains and macrophages in a 24-hrs cell culture

Silver nanoparticles, the new generation of antimicrobial agents, have been employed in different applications. Eradication of bacterial strains such as Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa by nanosilver is considerable. In the other hands, macrophages, the most important immune cells in nanoparticles clearance in semi-open human body interfaces determine nanosilver fate.In this study, colloidal nanosilver toxicity on macrophages and three bacterial strains was assessed so as to determine effective antibacterial concentrations without toxic effect on macrophages for harmless usage of nanosilver.

The bactericidal effect of silver nanoparticles was assessed based on minimum inhibitory concentration (MIC) of nanoparticles. The effect of silver nanoparticles on macrophages measured based on MTT assay in vitro.

The MIC of colloidal nanosilver was equal to 1/56 part per million (ppm) for E. coli strain and S. aureus strain but the MIC of colloidal nanosilver was equal to 3/125 ppm for P. Aeruginosa. Both of these MICs and the corresponding MBCs were the same. Maximum concentration of colloidal nanosiver which did not kill macrophage cells completely after 24 h was 2 ppm but non-toxic maximum concentration of nanosilver for macrophage cells compared to the control group was 0.5 ppm.

Accordingly, concentrations of nanosilver higher than 2 ppm are toxic for macrophages and lower than 0.5 ppm concentration has not any antimicrobial effect. Therefore, precaution must be taken for the usage of large concentrations of nanosilver. Approaches to truths pertaining to complex nanosystems necessitate accomplishment of in vivo experiments and comparison of their result with that of in vitro.