Metallic and metal oxide nanoparticles exist in nature; they are also synthesized for various applications. Synthetic nanoparticles released into the environment are subject to transport and fate processes that depend on nanoparticle chemistry, size, morphology and environmental conditions. Interactions with bacteria are probable, including toxic effects and nanoparticle transformations. Our group is studying interactions of CdSe, TiO₂, and other metal oxide nanoparticles with bacteria cultured in liquid and solid media to simulate aquatic and soil environments. In experiments with Pseudomonas, toxicity is observed, as evidenced by longer lag times, slower growth rates, and lower yields, with the extent depending on nanoparticle chemistry, dose, and culture conditions. Generalizable stress responses are also observed, as indicated by dose-dependent shifts in virulence factor production. Nanoparticle breakdown, but also formation, are observed in some cases. When considering releases of engineered metallic nanoparticles into the environment, the spectrum of interactions with bacteria including toxicity and propagation of toxic effects to larger scales, nanoparticle breakdown, and constituent metal sequestration and mobilization are important to understand. Additionally, parallels to, and differences from, natural nanoparticle fate and transport trajectories are important contexts for defining special concerns of synthetic nanoparticles in nature.