Genetic contexts related to the diffusion of plasmid-mediated CTX-M-55 extended-spectrum beta-lactamase isolated from Enterobacteriaceae in China

BACKGROUND: CTX-M-55 extended-spectrum beta-lactamases are being rapidly disseminated and transmitted in clinical practices around the world. The genetic contexts of the transferable plasmid-mediated bla(CTX-M-55) gene in Enterobacteriaceae were detected and characterized in this study. METHODS: Isolates were obtained from the First Affiliated Hospital of Zhengzhou University between September 2015 and March 2016. Based on polymerase chain reaction and BLAST analysis, resistance genes and genetic context of the bla(CTX-M-55) gene were investigated. Conjugation experiments and multilocus sequence typing were performed to demonstrate plasmid-mediated bla(CTX-M-55) transmission. RESULTS: Thirteen bla(CTX-M-55)-positive isolates of Enterobacteriaceae were obtained. Seven isolates were Escherichia coli, 3 were Klebsiella pneumoniae, 1 was Citrobacter freundii, 1 was Morganella morganii and 1 was Serratia marcescens. The bla(CTX-M-55) gene has not previously been identified from C. freundii and M. morganii. Four different bla(CTX-M-55) genetic contexts were identified, and all of them harbored ISEcp1 in the region upstream of bla(CTX-M-55) (in two cases, ISEcp1 was truncated by IS26, and in one case, it was truncated by IS1294), whereas ORF477 was detected downstream of the bla(CTX-M-55) gene from 12 of 13 strains. The novel genetic context of ISEcp1∆-bla(CTX-M-55)-∆IS903 was firstly detected the IS903 element which was identified downstream of bla(CTX-M-55). A conjugation assay revealed that all bla(CTX-M-55) plasmids were quickly and easily transferable to recipient E. coli, which then presented resistance to multiple antibiotics. CONCLUSIONS: Numerous bla(CTX-M-55)-positive strains were isolated in a short period of 7 months. The findings indicate that bla(CTX-M-55) was rapidly disseminated. The genetic context and conjugative transfer found in this study demonstrate that there is active transmission of bla(CTX-M-55) among strains of Enterobacteriaceae in China, which could give rise to an urgent global public health threat.