Impact of <i>In Vitro</i> Fluoroquinolone-Induced Multidrug Resistance on the Fitness and Virulence of <i>Salmonella enterica</i> Serovar Enteritidis

The global rise in fluoroquinolone (FQ) resistance among <i>Salmonella</i> spp. has prompted the WHO to classify these strains as high-priority pathogens. However, the trade-offs between FQ resistance and bacterial fitness or virulence, along with the underlying mechanisms, remain insufficiently investigated. In this study, FQ-resistant <i>Salmonella enterica</i> strains were generated via artificial <i>in vitro</i> induction using ciprofloxacin (0.125-8 mg/L). We comprehensively characterized the resultant strains regarding multidrug resistance, biological fitness, pathogenicity, and genetic profiles. Results showed that induction via laboratory serial passage conferred multidrug resistance but imposed significant fitness costs, including flagella reduction, impaired motility, increased auto-aggregation, and enhanced biofilm formation. Notably, the resistant strains exhibited compromised adhesion and invasiveness, and animal models confirmed attenuated pathogenicity compared to the wild-type strain. Mechanistically, qPCR revealed significant downregulation of key virulence genes (e.g., <i>spvB, ssaV, sipB</i>, and <i>pipB</i>), while resequencing identified specific mutations and indels in virulence and drug resistance loci. These findings elucidate the phenotypic and genetic shifts associated with FQ-induced resistance, offering critical insights for predicting infection outcomes and refining antimicrobial strategies.