Experimental evolution of Pseudomonas putida under silver ion versus nanoparticle stress

Whether the antibacterial properties of silver nanoparticles (AgNPs) are simply due to the release of silver ions (Ag+) or, additionally, nanoparticle-specific effects, is not clear. We used experimental evolution of the model environmental bacterium Pseudomonas putida to ask whether bacteria respond differently to Ag+ or AgNP treatment. We pre-evolved five cultures of strain KT2440 for 70 days without Ag to reduce confounding adaptations before dividing the fittest pre-evolved culture into five cultures each, evolving in the presence of low concentrations of Ag+, well-defined AgNPs or Ag-free controls for a further 75 days. The mutations in the Ag+ or AgNP evolved populations displayed different patterns that were statistically significant. The non-synonymous mutations in AgNP-treated populations were mostly associated with cell surface proteins, including cytoskeletal membrane protein (FtsZ), membrane sensor and regulator (EnvZ and GacS) and periplasmic protein (PP_2758). In contrast, Ag+ treatment was selected for mutations linked to cytoplasmic proteins, including metal ion transporter (TauB) and those with metal-binding domains (ThiL and PP_2397). These results suggest the existence of AgNP-specific effects, either caused by sustained delivery of Ag+ from AgNP dissolution, more proximate delivery from cell-surface bound AgNPs, or by direct AgNP action on the cell's outer membrane.

Automated summary

The study found that bacteria displayed significant differences in evolutionary mutations when treated with silver ions or silver nanoparticles, suggesting the presence of nanoparticle-specific effects potentially caused by sustained release of silver ions from nanoparticle dissolution, direct action of nanoparticles on the cell's outer membrane, or more proximate release of ions from cell-surface bound nanoparticles.