Characterization of chromosomally-located bla CTX-M and its surrounding sequence in CTX-M type extended-spectrum β-lactamase-producing Escherichia coli isolates.

OBJECTIVES: Although it has been regarded that CTX-M type extended-spectrum β-lactamase (ESBL) gene, blaCTX-M , is mainly carried by antibiotic-resistance plasmids, Escherichia coli-possessing chromosomally-located blaCTX-M has been reported in previous studies. This study aimed to characterize the genetic structures of chromosomally-located blaCTX-M transposition unit and its surrounding sequence in E. coli isolates-producing ESBL which were obtained in a Japanese hospital.

METHODS: In this study, total of 81 ESBL-producing E. coli isolates were examined. Existence of chromosomally-located blaCTX-M were confirmed by S1 nuclease-digested pulsed-field gel electrophoresis (PFGE) and the Southern blot hybridization, and by sequencing analysis of PCR-amplified DNA fragments.

RESULTS: Chromosomally-located blaCTX-M was confirmed in 22 of the examined 81 E. coli isolates (27.2%), 5 and 4 location types of the chromosomally-located blaCTX-M-14 , and -15 were determined, respectively. Among the 22 E. coli isolates, 15 isolates (68.2%) possessed single chromosomally-located blaCTX-M , probably due to single transposition of plasmid blaCTX-M to chromosome. In case of isolate N0057, blaCTX-M-15 transposition unit was transferred from plasmid into two different chromosomal regions. In addition, "recurrent" transposition of already existed chromosomally-located blaCTX-M-14 to other 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 the isolates (N0211, N0214, N01127, N1682, and N1753) in which the "recurrent "transposition occurred were genetically related on PFGE analysis, our data suggest possibility of accumulation of blaCTX-M on chromosome in CTX-M type ESBL-producing E. coli.