Time-resolved toxicity study reveals the dynamic interactions between uncoated silver nanoparticles and bacteria

It is still unclear whether the toxicity of silver nanoparticles (AgNPs) can be attributed solely to the release of Ag⁺ or whether dissolved and nanoparticulate Ag act in parallel; this is due to the difficulty in distinguishing Ag⁺- from AgNP-effects. Also, AgNPs undergo changes during toxicity tests. This is the first study to investigate the influence of AgNP dissolution over time on viable counts at high time resolution and low cell density, avoiding the apparently reduced toxicity at higher cell densities identified in our study. Uncapped AgNPs were synthesized to avoid any interference from surface coatings. The transformations of AgNPs during storage were reduced. Lowering the concentration of AgNPs reduced their aggregation in Davis minimal medium (DMM). Also, AgNPs dissolved more slowly in DMM than in water. The minimum inhibitory concentrations (MICs) of Ag⁺ and AgNPs increased with cell density according to a power law, suggesting that binding to cells decreased effective concentrations. However, AgNPs acted as a reservoir of Ag, releasing new Ag⁺ to maintain the Ag stress. The toxicity of AgNPs was dominated by dissolved Ag. Combining controlled conditions, high time-resolution and low cell density, we could demonstrate different roles of ionic and nano Ag in bacterial death caused by AgNPs.

目前仍不清楚银纳米颗粒（silver nanoparticles，AgNPs）的毒性是否仅源于银离子（Ag⁺）的释放，亦或是溶解态银与纳米颗粒态银可共同发挥毒性作用；这一研究难点在于难以区分银离子与银纳米颗粒各自产生的生物效应。此外，银纳米颗粒在毒性测试过程中会发生形态转变。本研究为首次在高时间分辨率、低细胞密度条件下，探究银纳米颗粒随时间的溶解过程对活菌数的影响，规避了本研究中此前发现的、高细胞密度下毒性表观降低的干扰。本研究合成了无包覆层的银纳米颗粒，以避免表面涂层带来的实验干扰，同时降低了银纳米颗粒在储存过程中的形态转变。降低银纳米颗粒的浓度可减少其在戴维斯基本培养基（Davis minimal medium，DMM）中的聚集；此外，银纳米颗粒在戴维斯基本培养基中的溶解速率慢于纯水中。银离子与银纳米颗粒的最低抑菌浓度（minimum inhibitory concentrations，MICs）随细胞密度呈幂律增长，这表明菌体结合作用降低了二者的有效浓度。但银纳米颗粒可作为银的储存库，通过持续释放新的银离子以维持银胁迫环境；银纳米颗粒的毒性主要由溶解态银所主导。通过结合可控实验条件、高时间分辨率检测与低细胞密度实验体系，本研究明确了离子态银与纳米颗粒态银在银纳米颗粒引发的细菌死亡过程中发挥的不同作用。