Abstract
Fluoroquinolone resistance in Salmonella has become increasingly prevalent in recent years. To probe the molecular basis of this phenomenon, the genetic and phenotypic features of fluoroquinolone resistant Salmonella strains isolated from food samples were characterized. Among the 82 Salmonella strains tested, resistance rate of the three front line antibiotics of ceftriaxone, ciprofloxacin and azithromycin was 10%, 39% and 25% respectively, which is significantly higher than that reported in other countries. Ciprofloxacin resistant strains typically exhibited cross-resistance to multiple antibiotics including ceftriaxone, primarily due to the presence of multiple PMQR genes and the blaCTX-M-65, blaCTX-M-55 blaCMY-2 and blaCMY-72 elements. The prevalence rate of the oqxAB and aac(6’)-Ib-cr genes were 91% and 75% respectively, followed by qnrS (66%), qnrB (16%) and qnrD (3%). The most common PMQR combination observable was aac(6’)-Ib-cr-oqxAB-qnrS2, which accounted for 50% of the ciprofloxacin resistant strains. Interestingly, such isolates contained either no target mutations or only a single gyrA mutation. Conjugation and hybridization experiments suggested that most PMQR genes were located either in the chromosome or a non-transferrable plasmid. To summarize, findings in this work suggested that PMQRs greatly facilitate development of fluoroquinolone resistance in Salmonella by abolishing the requirement of target gene mutations.
Highlights
Fluoroquinolone resistance in Salmonella has become increasingly prevalent in recent years
We reported the high prevalence of ciprofloxacin resistant Salmonella strains in food samples, most of which were found to harbor either only a single mutation in gyrA, or no mutation in both target genes
The resistance rate of the three most important front line antibiotics were respectively 10%, 39% and 25%, which is significantly higher than that reported in other countries
Summary
Fluoroquinolone resistance in Salmonella has become increasingly prevalent in recent years. The combination of PMQR such as oqxAB and a single target gene mutation, in particular in the gyrA gene, could possibly mediate development resistance to ciprofloxacin in Salmonella, and dramatically reduced the time required for the development of a resistance phenotype associated with generation of double gyrA mutations and single parC mutation[18]. This idea is supported by the observation of an increasing prevalence of different PMQR genes in various species of Enterobacteriaceae, and the emergence of ciprofloxacin-resistant E. coli and Salmonella strains carrying multiple PMQRs without target mutations[19,20]. The current situation warrants a need for continuous surveillance of the prevalent mechanisms of ciprofloxacin resistance in Salmonella in order to better understand the genetic background of this new category of resistant organisms
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