Transmission of Chromosomal MDR DNA Fragment Encoding Ciprofloxacin Resistance by a Conjugative Helper Plasmid in .

Resistance to ciprofloxacin, a treatment choice for infections, has increased dramatically in recent years in particular in serotype Derby 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 a Derby isolate, namely Sa64, which could extract and fuse to a multiple drug resistance (MDR) DNA fragment containing two PMQR genes, , and located on the chromosome of the strain. This process led to the formation of a new 188 kb fusion plasmid, which could be then subsequently transmitted to recipient strain Escherichia J53. The chromosomal MDR DNA fragment was shown to be flanked by one copy of IS element 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 IS mediated homologous recombination. The role of IS on enhancing the efficacy of fusion and transmission of this chromosomal MDR DNA fragment was further proven in other strains. 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.