ABSTRACT
Objective To characterize the resistance mechanisms of a clinical Shigella sonnei strain harboring blaCTX-M-55 .Methods A double-disk synergy test was conducted to detect ESBL.Antibiotic resistance genes were determined by PCR followed by amplicon sequencing.Conjugation experiments were performed to verify the transferability of the plasmids carrying ESBL genes.The minimum inhibitory concentration values were tested using VITEK 2.The transposition unit was confirmed by DNA sequencer,and the transcriptional start site was identified using primer extension assay.Results Strain #1083 produced CTX-M-55,which was encoded by plasmid p1083-CTXM that could be transferred into E.coli through conjugation experiments to confer corresponding antibiotic resistance to the transconjugant #1083-EC600.The transposition unit mediating the transfer of blaCTX-M-55 was ISEcp1-blaCTX-M-55 -Δorf477.ISEcp1 offered strong promoter regions for the resistance genes,facilitating their expressions.Besides,the expressions were constant,not induced by antibiotics.Conclusion BlaCTX-M-55 on plasmids is the major resistance genes for strain #1083.Their expressions and spread are mediated by the insertion sequence ISEcp1.