Transmission of chromosomal MDR DNA fragment encoding ciprofloxacin resistance by a conjugative helper plasmid inSalmonella

Abstract

Resistance to ciprofloxacin, a treatment choice forSalmonellainfections, has increased dramatically in recent years in particular in serotypeSalmonellaDerby with most of strains carrying chromosome-encoded multiple plasmid-mediated quinolone resistance (PMQR) genes. In this work, we discovered a conjugative plasmid, pSa64-96kb, in aSalmonellaDerby isolate, namely Sa64, which could extract and fuse to a multiple drug resistance (MDR) DNA fragment containing two PMQR genes,aac(6')-Ib-cr, andqnrS2located on the chromosome of theSalmonellastrain. This process led to the formation of a new 188 kb fusion plasmid, which could be then subsequently transmitted to recipient strain EscherichiacoliJ53. The chromosomal MDR DNA fragment was shown to be flanked by one copy of IS26element at each end and could be excised from the chromosome to form circular intermediate, which was then fused to pSa64-96kb and form a single plasmid through IS26mediated homologous recombination. The role of IS26on enhancing the efficacy of fusion and transmission of this chromosomal MDR DNA fragment was further proven in otherSalmonellastrains. These findings showed that dynamic interaction between specific chromosomal fragment and plasmids may significantly enhance resistance development and transferability of mobile resistance-encoding elements among bacterial pathogens.