Characterisation 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 the CTX-M-type extended-spectrum β-lactamase (ESBL) gene bla_CTX-M is mainly carried by antimicrobial resistance plasmids, Escherichia coli possessing chromosomally-located bla_CTX-M has been reported in previous studies. This study aimed to characterise the genetic structure of the chromosomally-located bla_CTX-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 bla_CTX-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 bla_CTX-M was confirmed in 22 (27.2%) of the 81 E. coli isolates examined; five and four location types of chromosomally-located bla_CTX-M-14 and bla_CTX-M-15 were determined, respectively. Among the 22 E. coli isolates, 15 (68.2%) possessed single chromosomally-located bla_CTX-M gene, probably due to single transposition of a plasmidic bla_CTX-M to the chromosome. In isolate N0057, the bla_CTX-M-15 transposition unit was transferred from a plasmid into two different chromosomal regions. In addition, 'recurrent' transposition of already existing chromosomally-located bla_CTX-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 bla_CTX-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 bla_CTX-M on the chromosome in CTX-M-type ESBL-producing E. coli.