Gene Expression Analysis and Whole Genome Sequencing Reveal the Potential Mechanism of Ciprofloxacin Resistance in a Salmonella Dublin Isolate.

There is a growing need to understand ciprofloxacin (CIP) resistance in less prevalent Salmonella serovars like Salmonella Dublin, which causes life-threatening conditions in both humans and animals. This study investigated potential factors contributing to CIP-resistance in a Salmonella Dublin isolate. The isolate was detected from an initial screening of 17 biobanked Salmonella isolates using the Kirby-Bauer disk diffusion (KBDF) method. The minimum inhibitory concentration (MIC) values of the identified CIP-resistant Salmonella Dublin isolate and a CIP-susceptible isolate of the same serovar were also obtained using the broth-dilution (BD) method. The two candidates were then challenged in 1/4 of their respective BD MICs for gene expression analysis, focusing on the acrAB efflux genes and the regulator genes marA, ramA, and soxS. Genomes of the isolates were also sequenced using the Oxford Nanopore sequencing platform, and then analyzed for mutations, antimicrobial resistance genes, and plasmids using ABRicate. The SWISS-MODEL server was used for protein modeling and comparison. For our results, the MIC values (KBDF; BD) for the CIP-resistant and CIP-susceptible Salmonella Dublin isolates were (1.5 μg/mL; 1.95 μg/mL) and (<0.125 μg/mL; 0.03 μg/mL), respectively. Both isolates had genes (mdtK, emrR, emrA, and emrB) notable for fluoroquinolone resistance, with the CIP-susceptible isolate also carrying the IncFII(S) plasmid. Expression of the acrA, acrB, ramA, and soxS genes was markedly higher in the CIP-resistant isolate, which also harbored an Asparagine (N) to Serine (S) mutation at position 868 in the GyrA protein. This mutation, however, caused no significant structural change. Despite reporting on a single CIP-resistant Salmonella Dublin isolate, our result highlights the potentially significant role of an efficient efflux system in contributing to CIP resistance in this isolate, even when no impactful mutations were identified.