Study On The Synthesis And Antibacterial Mechanism Of Silver Nanoparticles

The extensive use of antibiotics has spreaded the drug-resistant pathogenic microorganism,which makes the development of new antibacterial materials an urgent task. Inorganic antibacterial agents have been developed and applied with properties such as high safety, persistence and capacity of killing drug-resistant bacterias. With the development of nanotechnology, Ag-based nanocomposites(Ag NPs) have become the hot spot in the study of new antibacterial agents. The antibacterial properties of Ag NPs are closely related to their physiochemical characteristics,but molecular antibacterial mechanism of Ag NPs is still not clear.In this study, A series of nano silver compounds with various physicochemical properties were prepared to investgate the antibacterial effect. Poly dimethyl diallyl ammonium chloride(PDDA) was selected as surfactant to prepare Ag NPs-P1(AP1),Ag NPs-P2(AP2) and Ag NPs-P3(AP3) by simple chemical reduction and the potentials on their surface increased gradually. Through bacterial inhibition zone test, turbidimetry and minimal bactericidal concentration analysis, the antibacterial activities were revaeled as Ag NPs-P3 > Ag NPs-P2 > Ag NPs-P1. PDDA and Graphene were used to synthetize Ag NPs-P3、Ag NPs-GE and Ag/AgCl/rGO with the catalysis capabilities in the order as Ag/AgCl/r GO > Ag NPs-GE > Ag NPs-P3. However, the strongest sterilization capability of these compounds was Ag NPs-P3 and Ag NPs-GE had the weakest capacity. These results suggested that the antimicrobial activities of nanoparticles could be enhanced by increasing the positive charge on their surface, but were irrelevant to their catalytic activities in vitro.Kidney was an important organs to maintain internal environment homeostasis,so we investigated the cytotoxicity of Ag NPs on renal tubule epithelial cells of human and mouse. The results revealed that the survive rate of HK2 or MRTEC cells was more than 86% after treating by 10 ppm nano-silver for 24 hours. While under such conditions, Staphylococcus aureus and Pseudomonas aeruginosa can be completely killed. So we established a scalded model of closed colony ICR mouse to analyze the therapeutic effect of nano-silver on the scald wounds. The wound recovery was significantly promoted by nano-silver and the amount of hydroxyl amino acid at wound areas was higher in nano-silver group compared with the control group. These results indicated the Ag NPs were high biosafety wound therapy reagents.To further study the role of free radicals during the antimicrobial progress of Ag NPs, the ROS scavenger N-Acetylcysteine(NAC) was found to completely abolish antibacterial activities of Ag NPs, but the ROS catalytic activities of Ag NPs does not correspond to their bactericidal activities. So, we hypothesize that cellular ROS induced by Ag NPs is mediated by special biological pathways. In order to reveal more about molecular mechanism during the sterilization of Ag NPs, 11 E.coli mutants resistant to Ag NPs or silver ion was identified with the help of mutagen. Exploring the characters of these mutants, the results showed that Ag NPs or Ag+ induced cellular ROS reduced in these mutants. However, the fold of ROS reduction does not corrlated to the Ag NPs resistance, which indicating reduction of Ag NPs induced ROS is a mechanism of bacteria to gain Ag NPs resistance and there is a ROS independed pathway of bacteria to gain Ag NPs resistance. Whole-genome sequencing and comparation are employed to reveal those specific mutations, and the detail moleclar mechanism of Ag NPs resistance necessitate further investigation.