Cationic antibacterial metal nanoclusters with traceable capability for fluorescent imaging the nano–bio interactions

A thorough understanding of antimicrobial mechanism is of great importance for developing novel, efficient antibacterial agents. While cationic nanoparticles, such as metal nanoclusters (NCs), represent an attractive type of antibacterial nanoagents, their interactions with bacteria remains largely un-elucidated. Herein, we report the synthesis of cationic bovine serum albumin-protected AuAgNCs (cBSA-AuAgNCs), which exhibit both near-infrared (NIR) fluorescence properties and significant antimicrobial effects. With E. coli and S. aureus as the representative bacteria, we investigated the antimicrobial process of cBSA-AuAgNCs in real-time based on their intrinsic fluorescence properties via fluorescence imaging. Our results showed that these cBSA-AuAgNCs exert their antimicrobial effects primarily by attaching on the outer membrane of bacteria without obvious internalization, which is significantly different from the antibacterial process of negatively-charged metal NCs. Further mechanistic investigation showed that these cationic NCs will cause serious disruption to the bacterial membrane due to strong electrostatic interactions, which then leads to over accumulation of reactive oxygen species (ROS) that finally causes the bactericidal action. This study demonstrates the great potential of cationic luminescent metal NCs as novel, traceable antimicrobial agents, which also provides new tools for further understanding microbial interactions of nanomedicines.