Preliminary Study On The Antibacterial Activity And Mechanism Of Ag Nanoparticles Against Escherichia Coli

Silver has been known as antibacterial metal and widely used in medical care since 16th centuries. As a result of the development of nanotechnology, physicochemical properties and biological applications of silver nanoparticles have attracted widespread attentions. The antibacterial activity of these unique structured silver nanoparticles was significantly increased. Silver nanoparticles have the incomparable superiorities to conventional antibiotics due to their effective broad-spectrum activity to bacteria and far lower probability to induce microorganism resistance.However, there are only a few studies focused on the antibacterial effects of silver nanoparticles. These available studies used different method for antibacterial activity evaluation, thus their results are difficult to evaluate by comparing. In the reported studies regarding antibacterial mechanism of silver nanoparticles, only phenomenological description and hypothesis have been mentioned, thus experimental data is urgently needed for supporting of the ideas.In this study, after characterization of the silver nanoparticles, we developed a rapid and sensitive method for measuring their antibacterial activities. Results showed that the diameter of the silver nanoparticles is around 3 nm with hydrodynamic size of about 17 nm, and the zeta potential is -35 mV. A sensitive period of the antibacterial effect of silver nanoparticles toward Escherichia coli was determined at 5 h, with a median lethal dose of 5 mg/L. In comparison, no antibacterial effect was observed using gold nanoparticles with similar polymer coating, indicating that the strong antimicrobial effect resulted from the core silver element of the synthesized nanoparticles.E. coli was used as a model microorganism, and antibacterial activities of silver nanoparticles under different conditions were investigated. The results showed that temperatures, pH, microorganism proliferation, and oxygen amount were related to the antibacterial activity of silver nanoparticles. A 50% of mortality was observed for E. coli by using 5 mg/L nanoparticles solution, however, the mortality changed to 35% under 23℃,3% under acid stress (pH 2.0),20% anaerobic, and 99% in physiological saline.To illuminate the antibacterial mechanism of silver nanoparticles, atomic force microscopy (AFM) was employed to analyze the morphology of cells after incubation with silver nanoparticles. A reactive oxygen species (ROS) determination kit was used to determine the antimicrobial effect generated by ROS, and antioxidant was used to suppress ROS. Data showed that E. coli cells became irregular and rifty after incubation with silver nanoparticles. The antibacterial effect was positively correlated with the concentration of ROS, antioxidant (acetylcysteine,10 mM) affected the antibacterial activity of silver nanoparticles (10 mg/L) and resulted in a decrease of E. coli mortality from 90% to 40%.Antimicrobial effect of silver nanoparticles against E. coli in food samples is important for its further application. The present research also investigated the antibacterial effect of silver nanoparticles in milk, juice, soya-bean milk and LB broth. Result showed that the mortality of E. coli in physiological saline, apple juice and LB medium were 99.5%,99.9% and 92.6%, respectively, and in broth, milk and soybean milk were 29.0%,15.1% and 21.8%, respectively. The results indicated that the antibacterial effect of silver nanoparticles was high in low-protein food matrix, whereas low in high protein food matrix.