[Antibacterial activity of silver nanoparticles against multiple drug resistant strains]

The objective of the study was to assess the antimicrobial activity of silver nanoparticles (AgNPs) against multiple drug resistant strains.

Minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of AgNPs against three model microbes, namely Escherichia coli, Staphylococcus aureus, Candida albicans were measured by microdilution broth method. Time-kill curve within 24 h was made according to colony count method after three model microbes were treated with a series concentration of AgNPs. Post-antibiotic effect was tested by colony count method. Finally, we determined the antimicrobial efficacy against multiple drug resistant strains in biological safety laboratory grade 2 (BSL-2).

AgNPs with a diameter of 5 nm to 30 nm were synthesized by the biological method. The zeta potential was -19.5 mV. The time-kill curve of the three model microbes showed time-dependent antibacterial activity. The effect of AgNPs on E. coli and C. albicans after "antibiotic effect" increased with time, there was no obvious "post-antibiotic effect" on S. aureus. Both MIC values and MBC values of AgNPs for the three model microbes were between 1 μg/mL and 4 μg/mL. However, the MIC value of AgNPs for the three human multidrug-resistant strains was 6 μg/mL to 26 μg/mL and MBC value of AgNPs was 10 μg/mL to 32 μg/mL. The MIC values of AgNPs for 14 animal multi-drug resistant strains were between 4 μg/mL and 10 μg/mL, and the MBC values were between 8 μg/mL and 16 μg/mL. The MBC/MIC values of all the tested strains were less than 2.

AgNPs is a time-dependent antimicrobial agent with different "post-antibiotic effect", which can inhibit both human and animal-derived multi-drug resistant bacteria.