Occurrence of a novel class 1 integron harboring qnrVC4 in Salmonella Rissen

- This is the first known report of qnrVC family in Salmonella enterica.
- The qnrVC4 was co-transferred with other resistance genes via a novel plasmid-borne integron.
- This qnrVC4 cassette may be derived from the SI of V. cholera into a plasmid-borne integron.
- Transferable plasmid allowed the spread of this resistance gene to Enterobacteriaceae.

We described qnrVC4 in S. Rissen 166ANSS50, a swine isolate, which was detected in the study on quinolone resistance mechanisms of nontyphoidal Salmonella in Thailand. The isolate was found to harbor a ̴17-kb non-conjugative plasmid carrying qnrVC4 within 8.91 kb of a novel In4-like class 1 integron (In805). It contained the multi-drug resistance gene cassettes of qnrVC4-qacH4-aacA4-cmlA7-blaOXA-10-aadA1-dfrA14 and unusual 3′-CS of mobC-IS6100. This 1014-bp qnrVC4 cassette included with promoter (PqnrVC4: -35 TTGAGA and −10 TAGTCT) showed high homology with qnrVC4 in superintegron of V. cholerae O1 El Tor. The qnrVC4 recombinant plasmid resulted in 4-, 8-, and 16-fold increase in the MICs of nalidixic acid (2-8 μg/mL), ciprofloxacin (0.015-0.125 μg/mL), and norfloxacin (0.03-0.5 μg/mL), respectively. In addition, the backbone plasmid revealed a novel replicon belonging to the MOBQ1 group from the broad-host-range mobilisable IncQ1 plasmid RFS1010 based on relaxase sequences. This is the first known report of qnrVC in Salmonella enterica. The qnrVC4 gene was co-transferred with other resistance genes via a novel plasmid-borne In805. This allowed the spread of this resistance gene to Enterobacteriaceae.