Antimicrobial Cluster Bombs: Silver Nanoclusters Packed with Daptomycin

Abstract

Integration of two distinctive bactericides into one entity is a promising platform to improve the efficiency of antimicrobial agents. We report an efficient antimicrobial hybrid formed through conjugating silver nanoclusters (AgNCs) with daptomycin. The as-designed antimicrobial hybrid (D-AgNCs) inherits intrinsic properties of both bactericides with an enhanced synergistic performance. In particular, the chemically integrated D-AgNCs showed improved bacterial killing efficiency over the physically mixed daptomycin and AgNCs (D+AgNCs). More interestingly, the as-designed D-AgNCs could effectively damage the bacterial membrane. Propidium iodide (PI) stain showed bacterial membrane damage in about 85% of the bacteria population after treatment with D-AgNCs through creation of larger pores on the membrane as compared to D+AgNCs, largely due to the localization of daptomycin within the hybrid structure. These larger pores facilitated the entry of the D-AgNCs into the cell and led to more severe DNA damage of the bacterial DNA as compared to D+AgNCs in genomic DNA PAGE analysis. TUNEL assay further depicted more bacterial DNA breaks induced by D-AgNCs. The RecA gene expression level was upregulated, suggestive of DNA repair activation. The strong induced DNA damage benefited from the localization of AgNCs in the core of the antimicrobial hybrid structure, which could generate localized high ROS concentration and work as a critical ROS reservoir to continually generate ROS within the bacterium. The continual bombardments by these ROS generators restrict the ability of the bacteria to now develop resistance against this.