Silver nanoparticles (AgNP) undergo various transformations into different Ag species in the environment, which determines their toxicity in microorganisms. In aerobic condition, AgNPs release Ag+ that causes cell inactivation. Limited information is known about the AgNP-cell interaction in oxygen-free environment. Here we compared the transformation and antibacterial effects of AgNPs in aerobic and anaerobic environment. The bacterium Pseudomonas aeruginosa was relatively not susceptible to Ag+ or AgNP in anaerobic environment, indicated by near two orders of magnitude greater of anaerobic minimum inhibitory concentration (MIC) than the aerobic counterpart. In anaerobic environment, the dissolved Ag concentration decreased due to the reduction of Ag+. Electron microscopy images showed the formation of new AgNPs and aggregates, preferably on cell surface or associated with extracellular polymer substances (EPS) matrix. Accumulating AgNPs onto the cells could cause membrane damage, cytoplasm release or bacterial death. Meanwhile, EPS and cell lysate were very likely to bind AgNPs, facilitating the extensively assembling of AgNPs into large aggregates. This reduced the effective Ag exposure to cells and might contribute to the detoxification in anaerobic environment. Further, flow cytometry analysis quantified that bacterial membrane was largely intact under the treatment of AgNPs in anaerobic condition compared to the dose-response manner in aerobic condition.

中文翻译：

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有氧和厌氧环境中银纳米粒子的差异转化和抗菌作用。

银纳米颗粒（AgNP）在环境中经历各种转变为不同的Ag种类，这决定了它们对微生物的毒性。在有氧条件下，AgNPs释放Ag +，导致细胞失活。关于无氧环境中AgNP-细胞相互作用的信息有限。在这里，我们比较了有氧和无氧环境中AgNPs的转化和抗菌作用。铜绿假单胞菌在厌氧环境中相对不易受Ag +或AgNP的影响，这表明厌氧最小抑制浓度（MIC）比有氧对应物大近两个数量级。在厌氧环境中，由于Ag +的减少，溶解的Ag浓度降低。电子显微镜图像显示了新的AgNP和聚集体的形成，优选在细胞表面上或与细胞外聚合物（EPS）基质有关。AgNPs积累到细胞上可能会导致膜损伤，细胞质释放或细菌死亡。同时，EPS和细胞裂解物很可能结合AgNP，从而促进了AgNP广泛组装成大的聚集体。这减少了银对细胞的有效暴露，可能有助于在厌氧环境中进行排毒。此外，流式细胞仪分析定量表明，与需氧条件下的剂量反应方式相比，厌氧条件下在AgNPs处理下细菌膜基本完整。EPS和细胞裂解液极有可能结合AgNP，从而促进AgNP广泛组装成大的聚集体。这减少了银对细胞的有效暴露，可能有助于在厌氧环境中进行排毒。此外，流式细胞仪分析定量表明，与需氧条件下的剂量反应方式相比，厌氧条件下在AgNPs处理下细菌膜基本完整。EPS和细胞裂解液很可能结合AgNP，从而促进AgNP广泛组装成大的聚集体。这减少了银对细胞的有效暴露，可能有助于在厌氧环境中进行排毒。此外，流式细胞仪分析定量表明，与需氧条件下的剂量反应方式相比，厌氧条件下在AgNPs处理下细菌膜基本完整。