Fourth Generation Cephalosporin Resistance Among Serovar Enteritidis Isolates in Shanghai, China Conferred by Harboring Plasmids.

In this study, we investigated the pattern of antimicrobial resistance in enterica serotype Enteritidis isolates in Shanghai, China from 2005 to 2014. We found the first isolates with resistance to the fourth-generation cephalosporin cefepime starting in 2010. Furthermore, we analyzed the epidemic characteristics and mechanisms of underlying cefepime resistance in Enteritidis isolates found from 2010. In total, 38 of 2,914 (1.30%) isolates were identified as cefepime-resistant Enteritidis (CRSE) isolates by Kirby-Bauer disk diffusion. Two isolates were from animal derived food sources; 36 isolates were from fecal samples of human patients with salmonellosis. Antimicrobial susceptibility testing using the agar dilution method revealed that all CRSE isolates showed additional resistances at least to ceftazidime, cefotaxime, and ampicillin. Additionally, pulsed-field gel electrophoresis (PFGE) profiles indicated that 89.47% of CRSE isolates also displayed similar PFGE patterns. Five types of β-lactamase genes, (100.00%, 38/38), (65.79%, 25/38), (52.63%, 20/38), (18.42%, 7/38), and (5.26%, 2/38) were detected by PCR and sequencing. Among genes, was the dominant type (84.21%, 32/38). Conjugation and transformation experiments along with plasmid replicon typing revealed that was located on plasmids of various replicon types with sizes ranging from 76.8 to 138.9 kb. Plasmid sequence analysis also showed that the gene was mobilized mainly by the IS- -ORF477 transposition unit and had its own IS-based promoter, which accelerated the expression and transmission of . Analysis of whole genome sequences (Illumina) of one selected transformant SH12G706-C showed high similarity of the carrying plasmid with the IncI1 plasmid backbone p628-CTX-M of detected in 2010 in China. The present study demonstrated that the gene mobilized by IS- -ORF477 was the main feature shared by CRSE isolates and seems to play an important role for transmission of cefepime resistance. The number of CRSE isolates is rising annually, and the strong dissemination ability of IS- -ORF477-harboring plasmids among different species represents an important threat to the therapeutic effectiveness of cefepime.