A Comparative Study Of The Antibacterial Mechanisms Of Silver Nanoparticles And Silver Ion By Fourier Transform Infrared Spectroscopy

Silver is perhaps the oldest antibacterial agent known by human being. Historical evidence about the use of silver as an antibacterial agent can date back to ancient time. Today, in this antibiotic era, silver-based antibacterial agents are still very attractive antibacterial agents due to its capability to kill a broad spectrum of bacteria and its potential to fight against antibiotic resistant bacterial strains. Silver nanoparticles(AgNPs) and silver ion(Ag~+) are two commonly known silver-based antibacterial agents. The similarity and dissimilarity of the antibacterial mechanisms of AgNPs and Ag~+ has been the topics of great interest in recent years. In this study, we performed the first comparative study of the antibacterial mechanisms of AgNPs and Ag~+ on bacterial Escherichia coli(E. coli) using Fourier transform infrared(FTIR) spectroscopy. Through a thorough analysis of the FTIR spectra of E. coli after silver treatment in the spectral regions corresponding to protein, DNA, thiol group, lipopolysaccharide(LPS), and hydrocarbon, we were able to reveal a multifaceted antibacterial mechanism of silver at the molecular level for both AgNPs and Ag~+. Features of such mechanism include:(1) silver complexes with thiol group;(2) silver induces protein misfolding;(3) silver causes loss of LPS from bacterial membrane;(4) silver changes DNA conformation. Despite the similarities between AgNPs and Ag~+ with respect to their antibacterial mechanisms, we further revealed that AgNPs and Ag~+ display quite different kinetics for silver-thiol complexation and loss of LPS, with AgNPs displaying slow kinetics and Ag~+ displaying fast kinetics. At last, we proposed a hypothesis to interpret the observed different behaviors of AgNPs and Ag~+ when interacting with E. coli by considering the particle effect of AgNPs.