Understanding the interaction of nanomaterials with biological systems has always been of high concem and interest.An emerging type of nanomaterials,ultrasmall metal nanoclusters (NCs,＜ 2 nm in size),are promising in this aspect due to their well-defined molecular formulae and structures,as well as unique physical and chemical properties that are distinctly different from their larger counterparts (metal nanoparticles).For example,metal NCs possess intrinsic strong luminescence,which can be used for real-time tracking of their interactions with biological systems.Herein,luminescent gold (Au) NCs were used as traceable antimicrobial agents to study their interactions with the bacteria and to further understand their underlining antimicrobial mechanism.It is shown for the first time that the Au NCs would first attach on the bacterial membrane,penetrate,and subsequently accumulate inside the bacteria.Thereafter,the internalized Au NCs would induce reactive oxygen species (ROS) generation and damage the bacterial membrane,resulting in the leakage of bacterial contents,which can finally kill the bacteria.Traceable Au NCs (or other metal NCs)provide a promising platform to study the antimicrobial mechanisms as well as other fundamentals on the interfacing of functional nanomaterials with the biological systems,further increasing their acceptance in various biomedical applications.