Strain specificity in antimicrobial activity of silver and copper nanoparticles

The antimicrobial properties of silver and copper nanoparticles were investigated using Escherichia coli (four strains), Bacillus subtilis and Staphylococcus aureus (three strains). The average sizes of the silver and copper nanoparticles were 3 nm and 9 nm, respectively, as determined through transmission electron microscopy. Energy-dispersive X-ray spectra of silver and copper nanoparticles revealed that while silver was in its pure form, an oxide layer existed on the copper nanoparticles. The bactericidal effect of silver and copper nanoparticles were compared based on diameter of inhibition zone in disk diffusion tests and minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of nanoparticles dispersed in batch cultures. Bacterial sensitivity to nanoparticles was found to vary depending on the microbial species. Disk diffusion studies with E. coli and S. aureus revealed greater effectiveness of the silver nanoparticles compared to the copper nanoparticles. B. subtilis depicted the highest sensitivity to nanoparticles compared to the other strains and was more adversely affected by the copper nanoparticles. Good correlation was observed between MIC and MBC (r2=0.98) measured in liquid cultures. For copper nanoparticles a good negative correlation was observed between the inhibition zone observed in disk diffusion test and MIC/MBC determined based on liquid cultures with the various strains (r2=-0.75). Although strain-specific variation in MIC/MBC was negligible for S. aureus, some strain-specific variation was observed for E. coli.