Detection of chromosomal blaCTX-M-15 in Escherichia coli O25b-B2-ST131 isolates from the Kinki region of Japan

Escherichia coli O25b-B2-ST131 isolates harbouring blaCTX-M-15 are distributed worldwide. The blaCTX-M-15 transposition unit has often been found on plasmids and the genetic contexts have been examined; however, less is known about the frequency and contexts of the blaCTX-M-15 transposition unit on the chromosome. This study was performed to determine the chromosomal location of the blaCTX-M-15 transposition unit and to analyse the molecular structure of the region surrounding the blaCTX-M-15 transposition unit in E. coli O25b-B2-ST131 isolates. Twenty-two E. coli O25b-B2-ST131 strains harbouring blaCTX-M-15 that had been isolated from university hospital patients and nursing home residents in the Kinki region of Japan were examined. Inverse PCR (iPCR) targeting blaCTX-M-15 was performed to classify the molecular structure of the region surrounding the blaCTX-M-15 transposition unit. The isolates were classified into nine types (types A-I) considering the iPCR results; type A was the most prevalent type (13/22 isolates). Sequences of the iPCR-amplified DNA fragments showed that the blaCTX-M-15 transposition unit consisted of ISEcp1, blaCTX-M-15 and orf477Δ. A homology search of the obtained sequences showed that the blaCTX-M-15 transposition unit was inserted into different chromosomal regions in eight of the nine classified types. Although 21 of the 22 E. coli isolates possessed chromosomally located blaCTX-M-15 transposition units, clonal spread was not evident on pulsed-field gel electrophoresis (PFGE) analysis. Taken together, these data indicate that certain E. coli O25b-B2-ST131 strains harbouring chromosomal blaCTX-M-15 have emerged and spread in the Kinki region of Japan.