Molecular Characterization Of Cephalosporin-Resistant Salmonella Enteritidis St11 Isolates Carrying Bla(Ctx-M) From Children With Diarrhea

Salmonella Enteritidis is an important foodborne pathogen with high prevalence of resistance to cephalosporins, imposing a serious threat to public health. Therefore, a total of 162 Salmonella Enteritidis isolates collected from child patients in China from 2007 to 2017 were characterized for their resistance to cephalosporins and investigated the transmission characteristics of cephalosporin resistance gene. We found that 15 (9.26%) isolates were all resistant to cefalotin (minimum inhibitory concentration [MIC] >= 512 mu g/mL), ceftazidime (MIC 16-128 mu g/mL), ceftriaxone (MIC 64 to >= 512 mu g/mL), ceftiofur (MIC 64-256 mu g/mL), and cefotaxime (MIC 64 to >= 512 mu g/mL) with the possession of cephalosporin resistance genes bla(CTX-M-55) (n = 13), bla(CTX-M-101) (n = 1), and bla(CTX-M-153) (n = 1). Molecular typing further revealed that these 15 isolates belonged to sequence type ST11 and shared close pulsed-field gel electrophoresis patterns, suggesting the possibility of clonal spread in Salmonella Enteritidis interspecies. Furthermore, conjugation experiments were successfully performed in 13 of 15 isolates, and bla(CTX-M-55) was present on conjugative plasmids with sizes ranging from 54.7 to 173.4 kb. Compared with recipient Escherichia coli C600, transconjugants conferred elevated MICs for cephalosporins ranging from 2- to 2048-fold. The genetic structure surrounding of bla(CTX-M-55) gene in transconjugants were Delta ISEcp1-bla(CTX-M-55)-orf477 (n = 8) and ISEcp1-bla(CTX-M-55)-orf477 (n = 3), respectively. Taken together, bla(CTX-M) on the plasmids might contribute to cephalosporin resistance in Salmonella Enteritidis, and conjugative transfer of bla(CTX-M-55) might facilitate the spread of cephalosporin resistance in Salmonella Enteritidis. Hence, effective mitigation measurements are needed to reduce the threat caused by cephalosporin-resistant Salmonella Enteritidis to public health.