Antimicrobial silver inhibits bacterial movement and stalls flagellar motor

Silver (Ag) has been gaining broad attention due to their antimicrobial activities and the increasing resistance of bacteria to commonly prescribed antibiotics. However, various aspects of the antimicrobial mechanism of Ag have not been understood, including how silver affects the motility of bacteria, a factor that is intimately related to bacterial virulence. Here we report our study on the antibiotic effects of Ag+ ions on the motility of E. coli bacteria using swimming and tethering assays. We observed that the bacteria slowed down dramatically when subjected to Ag+ ions, providing direct evidence showing that Ag inhibits the motility of bacteria. In addition, through tethering assays, we monitored the rotation of flagellar motors and observed that the tumbling frequency of bacteria increased significantly in the presence of Ag+ ions. Furthermore, the rotation of bacteria in the tethering assays were analyzed using hidden Markov model (HMM); and we found that Ag+-treatment led to a significant decrease in the tumbling-to-running transition rate of the bacteria, suggesting that the rotation of bacterial flagellar motors was stalled by Ag+ ions. This work provided a new quantitative understanding on the mechanism of Ag-based antimicrobial agents in bacterial motility.