Analysis of Efflux Pump Contributions and Plasmid-Mediated Genetic Determinants in Ciprofloxacin-Resistant Salmonella

This study aimed to explore the interactions among genetic determinants influencing ciprofloxacin resistance in <i>Salmonella</i>. Treatment with PAβN, an efflux pump inhibitor, resulted in a 4-32-fold reduction in the minimum inhibitory concentration (MIC) across all 18 ciprofloxacin-resistant <i>Salmonella</i> isolates. Notably, isolates without point mutations reverted from resistance to sensitivity. The efflux pump played a crucial role in resistance development, particularly in serovar Enteritidis, where PAβN treatment caused a more significant MIC reduction (16-32-fold) in five strains carrying the GyrA (Asp87Tyr) mutation, which initially exhibited high MICs (8 μg/mL). Several resistance genes were identified on transferable plasmids: <i>oqxAB</i> and <i>aac(6')-Ib-cr</i> were associated with IncF plasmids in <i>S.</i> Enteritidis, IncA/C plasmids in <i>S.</i> Typhimurium, and IncHI2 plasmids in <i>S.</i> Virchow. Additionally, <i>qnrS1</i> and/or <i>qepA</i> were carried by IncA/C plasmids in <i>S.</i> Thompson. Whole-genome sequencing revealed the presence of an <i>oqxAB</i> module integrated into the chromosomal DNA of <i>S.</i> Derby. Although the MICs of ciprofloxacin in transconjugants and transformants remained low (1-4 μg/mL), they exceeded the clinical breakpoint for susceptibility. These findings highlight the synergistic impact of efflux pumps and plasmid-mediated resistance mechanisms, contributing to the increasing prevalence of ciprofloxacin resistance and posing a significant threat to food safety.