Synergistic antibacterial activity of gold nanoparticles caused by apoptosis-like death.

Metal nanoparticles are promising materials for the management of infectious diseases as known to have various antimicrobial activities in pathogenic micro-organisms. Among them, gold nanoparticles (AuNPs) are used in a wide range of fields such as photodynamic therapy, molecular diagnostics and drug delivery because of their unique physicochemical properties. However, little is known about the synergistic antibacterial activity and mechanism of AuNPs on pathogenic bacteria. Combinations of AuNPs and cefotaxime and ciprofloxacin showed synergistic interaction against all Salmonella species, however the combination with kanamycin exhibited no interaction. We determined that AuNPs and in combinations with antibiotics exert its antibacterial effect through bacterial apoptosis-like death. AuNPs caused collapse of intracellular divalent cation homeostasis, and conventional antibiotics caused accumulation of reactive oxygen species, which induced apoptotic hallmarks such as membrane depolarization, caspase-like protein activation, cell filamentation and phosphatidylserine externalization. The cation homeostasis disruption by AuNPs and the accumulation of reactive oxygen species by conventional antibiotics synergistically affected bacterial cell death and induced apoptosis-like death in Salmonella cells. The synergistic activity between AuNPs and antibiotics propose that the AuNPs are a potential antibacterial agent and adjuvant for antimicrobial chemotherapy.