LAUSR

OBJECTIVES Although it has been regarded that the CTX-M-type extended-spectrum β-lactamase (ESBL) gene blaCTX-M is mainly carried by antimicrobial resistance plasmids, Escherichia coli possessing chromosomally-located blaCTX-M has been reported in previous studies. This study aimed to characterise the genetic structure of the chromosomally-located blaCTX-M transposition unit and its surrounding sequence in ESBL-producing E. coli isolated in a Japanese hospital. METHODS A total of 81 ESBL-producing E. coli isolates were studied. The existence of chromosomally-located blaCTX-M was confirmed by S1 nuclease-digested pulsed-field gel electrophoresis (PFGE) and Southern blot hybridisation and by sequencing analysis of the PCR-amplified DNA fragments. RESULTS Chromosomally-located blaCTX-M was confirmed in 22 (27.2%) of the 81 E. coli isolates examined; five and four location types of chromosomally-located blaCTX-M-14 and blaCTX-M-15 were determined, respectively. Among the 22 E. coli isolates, 15 (68.2%) possessed single chromosomally-located blaCTX-M gene, probably due to single transposition of a plasmidic blaCTX-M to the chromosome. In isolate N0057, the blaCTX-M-15 transposition unit was transferred from a plasmid into two different chromosomal regions. In addition, 'recurrent' transposition of already existing chromosomally-located blaCTX-M-14 to another chromosomal region was observed in isolates N0211, N0214, N01127, N1682 and N1753; consequently, these isolates possessed two copies of chromosomally-located blaCTX-M-14. CONCLUSION Considering that isolates N0211, N0214, N01127, N1682 and N1753 in which the 'recurrent' transposition event occurred were genetically related according to PFGE, these data suggest the possibility of accumulation of blaCTX-M on the chromosome in CTX-M-type ESBL-producing E. coli.