Released Ag ions or/and Ag particles are believed to contribute to the cytotoxicity of Ag nanomaterials, and thus, the cytotoxicity and mechanism of Ag nanomaterials should be dynamic in water due to unfixed Ag particle:Agthorn ratios. Our recent research found that the cytotoxicity of PVP-Ag nanoparticles is attributable to Ag particles alone in 3 hr bioassays, and shifts to both Ag particles and released Ag+ in 48 hr bioassays. Herein, as a continued study, the cytotoxicity and accumulation of 50 and 100 nm Ag colloids in Escherichia coli were determined dynamically. The cytotoxicity and mechanisms of nanoAg colloids are dynamic throughout exposure and are derived from both Ag ions and particles. Ag accumulation by E. coli is derived mainly from extracellular Ag particles during the initial 12 hr of exposure, and thereafter mainly from intracellular Ag ions. Fe3thorn accelerates the oxidative dissolution of nano-Ag colloids, which results in decreasing amounts of Ag particles and particle-related toxicity. Nathorn stabilizes nano- Ag colloids, thereby decreasing the bioavailability of Ag particles and particle-related toxicity. Humic acid (HA) binds Agthorn to form Agthorn -HA, decreasing ion-related toxicity and binding to the E. coli surface, decreasing particle-related toxicity. HA in complex conditions showed a stronger relative contribution to toxicity and accumulation than Nathorn or Fe3thorn. The results highlighted the cytotoxicity and mechanism of nano- Ag colloids are dynamic and affected by environmental factors, and therefore exposure duration and water chemistry should be seriously considered in environmental and health risk assessments. (c) 2019 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.