TiO2 Nanoparticles Trigger Gut-to-Gill Bacterial Translocation and Dysbiosis in Zebrafish

Titanium dioxide nanoparticles (TiO2-NPs) are widely produced and persist in aquatic ecosystems, yet their indirect effects on host–microbe interactions remain poorly defined. By using zebrafish (Danio rerio) as a sentinel species, this study investigated the effects of subchronic 5 mg/L TiO2-NP exposure. Dynamic light scattering was utilized to characterize the bimodal aggregates (peaks at 917 and 46,841 nm; surface charge: +22.08 mV) that define the environmental state of TiO2-NPs. Parallel 16S rRNA metagenomic profiling on Day 6, prior to mortality, revealed profound gut dysbiosis. A marked increase in Chao1 richness (p < 0.01), alongside a catastrophic 333-fold reduction in beneficial Cetobacterium and an 856-fold enrichment of pathogenic Mycobacterium, was observed. Beta-diversity and hierarchical clustering analyses revealed a striking convergence between gut and gill microbial signatures, supporting a gut-to-gill translocation model. These results suggest that TiO2-NPs exposure induces intestinal dysbiosis, facilitating opportunistic bacterial migration via internal (gut–blood–gill) or external (fecal–water–gill) pathways. This study identifies dysbiosis-driven secondary infection as a novel, overlooked mechanism of nanoparticle toxicity, necessitating a shift in ecological risk assessments toward host–microbe interactions.