Molecular characterization of IncFII plasmid carrying bla_NDM-5 in a Salmonella enterica serovar Typhimurium ST34 clinical isolate in China

The detection rate of carbapenem-resistant nontyphoidal Salmonella (NTS) is relatively low. However, carbapenem-sensitive and carbapenem-resistant Salmonella enterica serovar Typhimurium (S. enterica serovar Typhimurium) were isolated from a clinical outpatient within a span of 10 days, named 1104-65 and 1104-75. We aimed to reveal the mechanism of carbapenem resistance in S. enterica serovar Typhimurium isolates in this case. The resistance phenotype of S. enterica serovar Typhimurium was analyzed by the VITEK-2 Compact equipment and Kirby-Bauer disc diffusion method. Whole-genome sequencing was used to analyze the serotype, ST type, genetic relationship, resistance genes, plasmid replicon, the genetic environment of bla(NDM-5), and the characteristics of IncFII plasmid carrying bla(NDM-5) in S. enterica serovar Typhimurium. The transferability of the IncFII plasmid carrying bla(NDM-5) was verified by the conjugation assay and PCR. The serotypes of both NTS are S. enterica serovar Typhimurium, belonging to ST34. Single nucleotide polymorphisms analysis showed that they were the same clone. A conjugative IncFII plasmid pIncFII-NDM5 with transferability was detected in isolate 1104-75, which harboring the bla(NDM-5) gene was the primary mechanism responsible for mediating carbapenem resistance in S. enterica serovar Typhimurium. The genetic environment of bla(NDM-5) on pIncFII-NDM5 is "IS26-Delta IS3000-IS5-Delta ISAba125-bla(NDM-5)-ble(MBL)-trpF-dsbC -cutA-IS26", which was confirmed as a novel structure not previously reported in the NCBI database. Although the mechanism is rarely reported in NTS, the prevalence of IncFII plasmid pIncFII-NDM5 will pose a great threat to the clinical treatment of S. enterica serovar Typhimurium. Meanwhile, the coexistence of bla(CTX-M-55), qnrS1, bla(NDM-5), and mph(A) in S. enterica serovar Typhimurium ST34 warrants additional attention.