Metabolic Mechanism Investigation of Antibacterial Active Cysteine-Conjugated Gold Nanoclusters in Escherichia coli

Elucidating the metabolic mechanism of gold nanoclusters (AuNCs) in bacteria will play a pivotal role in bacterial detection and inhibition. A facile method to investigate the metabolic mechanism of AuNCs is demonstrated in this work. The cysteine-conjugated gold nanoclusters (Cys-AuNCs) were successfully prepared with orange-red fluorescence, high water solubility, and superior biocompatibility by one-pot green synthesis to determine bacterial metabolism. The suggested metabolic process was that Cys-AuNCs were metabolized by Escherichia coli, as verified through a decrease in the fluorescence intensity that was clearly detected at 30 min, indicating the breakage of cysteine on Cys-AuNCs, which was further confirmed via X-ray photoelectron spectroscopy (XPS) that was used to observe the decrease in the size of Cys-AuNCs after being metabolized. The metabolic kinetics of Cys-AuNCs was determined by fitting the change in the fluorescence of Cys-AuNCs as a function of incubation time with E. coli, in which the rate constant could be a useful indicator for detecting different bacteria. In addition, the death of E. coli was characterized by an increase in intracellular reactive oxygen species (ROS) through metabolism. After the metabolism of cysteine on Cys-AuNCs by E. coli, significant intracellular ROS generation was induced by the AuNCs that killed the bacterium due to its lack of the ROS scavenger, cysteine. Our work provides a potential rapid method for bacterial detection and inhibition.