Bacillus subtilis causes dissolution of ceria nanoparticles at the nano-bio interface

The cytotoxicity of ceria nanoparticles (NPs) against microorganisms was identified as a particle-specific toxicity in previous work since no release of Ce3+ ions was evident. Although the literature has demonstrated a reduction of ceria NPs after their interaction with bacteria, it remains unknown whether the reduction is followed by a release of Ce3+ ions from the particle surface. The present work aims to study whether there is dissolution of ceria NPs at the nano-bio interface and evaluate the possible ionic toxicity against bacteria. Our results suggest that rod-like ceria NPs could be reduced by Bacillus subtilis under planktonic conditions, then the Ce3+ ions adjacent to the surface oxygen vacancies would be chelated by the adsorption sites on the bacterial cell wall and extracted from the particles. The biosorption of the dissolved Ce3+ ions by bacteria is further confirmed by the Ce3+ desorption after the protonation of carboxyl groups on the cell wall. Therefore, we demonstrate for the first time the dissolution of ceria NPs at the bacterial surface as a result of the nano-bio interaction. These findings together reveal a combined mechanism for the toxicity of ceria NPs: particle-related delivery of Ce to the bacterial surface and ion-related toxicity following the dissolution. This insight would benefit our understanding of the mechanism underlying the cytotoxicity of ceria NPs.