Synergistic effects of carbamazepine and antibiotic-resistant bacteria co-exposure on zebrafish gut microbiota, resistome, and host health.

The spread of antibiotic resistance in aquatic ecosystems poses a dual threat to environmental and public health. Pharmaceutical residues and antibiotic-resistant bacteria (ARB) frequently co-occur in surface waters and wastewater effluents, yet their interactive effects on host health and resistance dynamics of aquatic organisms remain poorly understood. We exposed zebrafish larvae from two genetically distinct strains (AB and Tübingen) to the widely detected pharmaceutical carbamazepine (CBZ) and ARB, individually and in combination, and assessed impacts on gut microbiota, resistome dynamics, and host health. In the Tübingen strain, Co-exposure significantly increased the abundance of multiple antibiotic resistance genes (ARGs), and triggered a 387-fold rise in the mobile genetic element intI1, indicating enhanced horizontal transfer potential. In contrast, the AB strain exhibited strong host transcriptional responses. Germ-free zebrafish confirmed CBZ-induced behavioral changes were microbiota-dependent. Correlation analyses revealed divergent strategies, with AB microbiota shifts linked to host gene expression and Tübingen shift to ARGs (both 128 correlations). Together, these findings demonstrate that co-exposure to a pharmaceutical and ARB can drive either microbial resistome expansion or host physiological dysfunction depending on host genetic background, highlighting that combined chemical-biological pollution poses complex risks that may be underestimated by single-strain assessments.