Abstract
Resistance to ciprofloxacin, a treatment choice for Salmonella infections, has increased dramatically in recent years in particular in serotype Salmonella 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 Salmonella Derby isolate, namely Sa64, which could extract and fuse to a multiple drug resistance (MDR) DNA fragment containing two PMQR genes, aac(6’)-Ib-cr, and qnrS2 located on the chromosome of the Salmonella strain. This process led to the formation of a new 188 kb fusion plasmid, which could be then subsequently transmitted to recipient strain Escherichia coli J53. The chromosomal MDR DNA fragment was shown to be flanked by one copy of IS26 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 IS26 mediated homologous recombination. The role of IS26 on enhancing the efficacy of fusion and transmission of this chromosomal MDR DNA fragment was further proven in other Salmonella 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.
Highlights
Non-typhoidal Salmonella represents the primary bacterial causes of food-borne gastroenteritis throughout the world (Gomez et al, 1997)
Our surveillance project on ciprofloxacin resistance in Salmonella in food samples collected from the wet markets and supermarkets in Shenzhen, China, allowed us to identify a Salmonella isolate that could transfer its phenotype of ciprofloxacin resistance via conjugation
Double and single mutations in gyrA and parC genes associated with ciprofloxacin resistance phenotypes in salmonella were shown to be consistent with previously reports (Chen et al, 2007)
Summary
Non-typhoidal Salmonella represents the primary bacterial causes of food-borne gastroenteritis throughout the world (Gomez et al, 1997). Three patterns of PMQR determinates have been described to date: (1) the Qnr types, which are mainly classified into seven different families and encoded pentapeptide duplicate proteins binding to DNA gyrase through imitating double stranded DNA, avoiding the binding of fluoroquinolones to gyrase; (2) the aac(6’)-Ib-cr, a modified aminoglycoside acetyltransferase that could hydrolyze fluoroquinolones; and (3) the efflux pumps QepA and OqxAB. Detection of these PMQR genes in Salmonella remains rare before 2009, with the first detection of qnrA and qnrS genes being reported in Europe (Gunell et al, 2009)
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