Identification and Characterization of Conjugative Plasmids That Encode Ciprofloxacin Resistance in Salmonella

This study aimed to characterize novel conjugative plasmids that encode transferable ciprofloxacin resistance in Salmonella. In this study, 157 nonduplicated Salmonella isolates were recovered from food products, of which 55 were found to be resistant to ciprofloxacin. Interestingly, 37 of the 55 Cip^r Salmonella isolates (67%) did not harbor any mutations in the quinolone resistance-determining regions (QRDR). Six Salmonella isolates were shown to carry two novel types of conjugative plasmids that could transfer the ciprofloxacin resistance phenotype to Escherichia coli J53 (azithromycin resistant [Azi^r]). The first type of conjugative plasmid belonged to the similar to 110-kb IncFIB-type conjugative plasmids carrying qnrB-bearing and aac(6')-lb-cr-bearing mobile elements. Transfer of the plasmid between E. coli and Salmonella could confer a ciprofloxacin MIC of 1 to 2 μg/ml. The second type of conjugative plasmid belonged to similar to 240-kb IncH1/IncF plasmids carrying a single PMQR gene, qnrS. Importantly, this type of conjugative ciprofloxacin resistance plasmid could be detected in clinical Salmonella isolates. The dissemination of these conjugative plasmids that confer ciprofloxacin resistance poses serious challenges to public health and Salmonella infection control.