Complete nucleotide sequence of the IncI1 plasmid pSH4469 encoding CTX-M-15 extended-spectrum β-lactamase in a clinical isolate of Shigella sonnei from an outbreak in the Republic of Korea

An outbreak of extended-spectrum β-lactamase (ESBL)-producing Shigella sonnei infections occurred in a school for disabled children in Gyeongbuk Province, Republic of Korea, in 2008. Five students were affected. Pulsed-field gel electrophoresis (PFGE) analysis revealed that all of the ESBL-producing S. sonnei isolates belonged to the same clone, and nucleotide sequence analysis of ESBL genes revealed that they harboured bla(CTX-M-15). This is the first identification of bla(CTX-M-15) in Shigella spp. in South Korea. In this study, a plasmid carrying the bla(CTX-M-15) gene, designated pSH4469, recovered from a S. sonnei isolate responsible for the outbreak was characterised. Replicon typing and plasmid multilocus sequence typing (pMLST) analysis of plasmids in the outbreak strain identified that the bla(CTX-M-15) gene was located on an IncI1 incompatibility group plasmid of sequence type 16 (ST16). The complete nucleotide sequence of pSH4469 revealed that this plasmid is 91109bp and harbours 119 putative genes, including another antibiotic resistance gene (bla(TEM-1b)) that is often associated with the ISEcp1-bla(CTX-M-15)-orf477delta transposable unit. The plasmid consists of a large backbone with considerable homology to the pEK204 plasmid isolated from Escherichia coli in the UK, except for insertion of an IS66 element found in pEK204. These data demonstrate that IncI1 plasmids are used as a successful platform for efficient horizontal gene transfer, thereby resulting in the dissemination of CTX-M-type β-lactamases among Enterobacteriaceae.