Objectives: To describe the molecular characterization of CTX-M ESBL (Extended-spectrum Beta-lactamase)- producing Salmonella co-resistant ciprofloxacin and cefotaxime from chicken carcasses in 2014 in Qingdao, China.
Methods: A total of 62 Salmonella isolates were confirmed as ESBLs by a combination disc diffusion test from 355 Salmonella isolates studied. Antimicrobial susceptibility was determined by the broth dilution method. CTX-M subtypes, Quinolone Resistance-determining Regions (QRDRs), Plasmid-mediated Quinolone Resistance (PMQR) determinants and Multi-locus Sequence Typing (MLST) were identified using PCR (Polymerase Chain Reaction) and sequencing. Plasmids were characterized by S1-nuclease PFGE, PCR-based replicon typing and transformation. Broth mating assays were carried out for all isolates to determine whether the CTX-M or PMQR marker could be transferred by conjugation.
Results: Of the 62 ESBL-producing Salmonella isolates, showing resistant to a multiple antimicrobials, 59 isolates were identified as S. indiana (ST17) followed by two S. enteritidis and one S. typhimurium. Moreover, all 62 ESBLs were subtyped to six blaCTX-M types, with blaCTX-M-123 being the most predominant, followed by blaCTX-M-65, blaCTX-M-130, blaCTX-M-14, blaCTX-M-24, and blaCTX-M-79. Amino acid substitutions in GyrA (S83F and D87N) and ParC (T57S and S80R) and the PMQR-encoding genes [aac(6′)-Ib-cr, oqxA, oqxB, qnrB, and qnrS] were detected in almost all tested isolates. All isolates contained one to three large plasmids and 8 replicon types were detected with IncHI1, IncHI2, IncA/C, IncFIIAS, and IncFrep replicon types being the predominating incompatibility groups. Twenty-six isolates demonstrated the ability to transfer their CTX-M and PMQR marker at different transfer rates.
Conclusion: The emergence of CTX-M, QRDR and PMQR were found in ciprofloxacin and cefotaxime coresistant Salmonella, especially S. indiana among chicken carcasses in Qingdao. To the best of our knowledge, this is the first report of blaCTX-M-123 and blaCTX-M-130 in Salmonella. The high prevalence and conjugative characteristics suggest that control strategies are necessary to limit the dissemination of these isolates through food chain.