Dietary florfenicol residues from eggs drive multidrug resistance in Salmonella and enrich intestinal antibiotic resistance genes

This study comprehensively investigates the molecular and ecological mechanisms by which florfenicol (FFC) residues drive multidrug resistance (MDR) in Salmonella and the intestinal microbiota. By integrating long-term in vitro induction with an in vivo mouse model of dietary exposure, we evaluated the risk of resistance development of FFC residues. In vitro results demonstrated that continuous FFC pressure rapidly evolved susceptible Salmonella into highly resistant strains, with minimum inhibitory concentrations escalating from 8 to ≥ 64 mg/L. Crucially, this process triggered broad cross-resistance to non-targeted antibiotics, including quinolones and β-lactams. Real-time quantitative polymerase chain reaction analysis confirmed this phenotype was primarily driven by the significant overexpression of transmembrane efflux pump genes (fexA, tetA) and the ribosomal protection gene optrA. At the microecological level, metagenomic sequencing revealed that FFC exposure induced a dramatic expansion of the intestinal resistome. Strategies for resistance converged on the comprehensive upregulation of ATP-binding cassette, resistance-nodulation-division, and major facilitator superfamily efflux pump families, alongside target modification mechanisms. Furthermore, FFC exposure significantly disrupted intestinal homeostasis, driving the microbial community toward a dysbiotic state dominated by opportunistic pathogens harboring abundant resistance genes, specifically Proteobacteria and Enterobacteriaceae. In conclusion, FFC acts as a potent stressor that not only induces MDR at the single-bacterium level through efficient efflux and target protection but also synergistically enriches broad-spectrum resistance genes and reshapes the host microbiome structural composition. These findings highlight the critical ecological risks of FFC residues in the food chain and underscore the need for strict residue control.