Opportunistic gill infection is associated with TiO(2) nanoparticle-induced mortality in zebrafish

The large amounts of engineered titanium dioxide nanoparticles (TiO(2)NPs) that have been manufactured have inevitably been released into the ecosystem. Reports have suggested that TiO(2) is a relatively inert material that has low toxicity to animals. However, as various types of NPs increasingly accumulate in the ocean, their effects on aquatic life-forms remain unclear. In this study, a zebrafish model was used to investigate TiO(2)NP-induced injury and mortality. We found that the treatment dosages of TiO(2)NP are positively associated with increased motility of zebrafish and the bacterial counts in the water. Notably, gill but not dorsal fin and caudal fin of the zebrafish displayed considerably increased bacterial load. Metagenomic analysis further revealed that gut microflora, such as phyla Proteobacteria, Bacteroidetes, and Actinobacteria, involving more than 95% of total bacteria counts in the NP-injured zebrafish gill samples. These results collectively suggest that opportunistic bacterial infections are associated with TiO(2)NP-induced mortality in zebrafish. Infections secondary to TiO(2)NP-induced injury could be a neglected factor determining the detrimental effects of TiO(2)NPs on wild fish.