Prevalence of acquired fosfomycin resistance among extended-spectrum β-lactamase-producing Escherichia coli and Klebsiella pneumoniae clinical isolates in Korea and IS26-composite transposon surrounding fosA3.

OBJECTIVES: To investigate the prevalence of plasmid-mediated fosfomycin resistance determinants among extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae and their genetic environments.
METHODS: A total of 347 non-duplicate ESBL-producing E. coli (165) and K. pneumoniae (182) were collected. The fosfomycin MICs were determined by the agar dilution method according to CLSI guidelines. PCR was used to detect the plasmid-encoded fosfomycin resistance genes (fosA, fosA3, fosB and fosC2). For isolates harbouring plasmid-encoded fosfomycin resistance genes, sequence types (STs) were determined. The transformation experiment was performed using E. coli TOPO10 (Invitrogen, USA) as a recipient strain. With the plasmids from the transformants, plasmid replicon typing was performed and the nucleotide sequences adjacent to fosA3 were determined.
RESULTS: The susceptibility to fosfomycin was 92.9% in E. coli and 95.2% in K. pneumoniae. Of the 21 isolates non-susceptible to fosfomycin (8 E. coli and 13 K. pneumoniae), 7 (5 E. coli and 2 K. pneumoniae) isolates harboured fosA3 and all of them co-harboured bla(CTX-M-1group) or bla(CTX-M-9group). The STs of the isolates harbouring fosA3 were diverse (E. coli: ST1, ST1, ST533, ST2 and ST86; K. pneumoniae: ST11 and ST101). The plasmid replicon types of transformants co-harbouring bla(CTX-M-1group) and bla(CTX-M-9group) were IncF and IncN, respectively. By sequence analysis, we found the common feature that the fosA3 gene, connected to bla(CTX-M) via insertion sequences, was located between two IS26 elements oriented in the opposite direction, composing an IS26-composite transposon.
CONCLUSIONS: An IS26-composite transposon appears to be the main vehicle for dissemination of fosA3 in E. coli and K. pneumoniae of diverse clones.