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ABSTRACTThree hybrid CTX-M β-lactamases, CTX-M-64, CTX-M-123, and CTX-M-132, with N and C termini matching CTX-M-1 group enzymes and centers matching CTX-M-9 group enzymes, have been identified. The hybrid gene sequences suggested recombination between blaCTX-M-15and blaCTX-M-14, the two most common blaCTX-Mvariants worldwide. However, blaCTX-M-64and blaCTX-M-123are found in an ISEcp1-blaCTX-Mtransposition unit with a 45-bp “spacer,” rather than the 48 bp usually associated with blaCTX-M-15, and 112 bp of IncA/C plasmid backbone. This is closer to the context of blaCTX-M-55, which has one nucleotide difference from blaCTX-M-15, on IncI2 plasmid pHN1122-1. Here, we characterized an IncI2 plasmid carrying blaCTX-M-15with a 45-bp spacer (pHNY2-1) by complete sequencing and also sequenced IncI2 plasmids carrying blaCTX-M-64(pHNAH46-1) or blaCTX-M-132(pHNLDH19) and an IncI1 plasmid carrying blaCTX-M-123(pHNAH4-1). pHNY2-1 has the same ISEcp1-blaCTX-M-IncA/C insertion as pHN1122-1, pHNAH46-1, and pHNLDH19, and all four plasmid backbones are almost identical. pHNAH4-1 (IncI1 sequence type 108 [ST108]) carries a transposition unit that includes a 2,720-bp fragment of the IncI2 backbone, suggesting ISEcp1-mediated transfer of blaCTX-M-IncA/C-IncI2 to an IncI1 plasmid. All three hybrid blaCTX-Mgenes may have resulted from recombination between blaCTX-M-14and blaCTX-M-15with a 45-bp spacer on an IncI2 plasmid. Five additional Escherichia coliisolates of different sequence types from different provinces, farms, and/or animals had blaCTX-M-64on a pHNAH46-1-like IncI2 plasmid and 9 had blaCTX-M-123on a pHNAH4-1-like IncI1 ST108 plasmid. Thus, epidemic IncI plasmids may be responsible for the spread of blaCTX-M-64and blaCTX-M-123between different animals and different locations in China.

ABSTRACTThe chimeric blaCTX-M-123gene was identified in two ceftazidime-resistant Escherichia coliisolates from animals in different Chinese provinces. Like other CTX-M-1/9 group hybrids (CTX-M-64 and CTX-M-132), the ends (amino acids 1 to 135 and 234 to 291) of CTX-M-123 match CTX-M-15 while the central part (122 to 241) matches CTX-M-14. blaCTX-M-123is carried on related, but not identical, ∼90-kb IncI1 plasmids in the two isolates, and one isolate simultaneously carries the group 1 blaCTX-M-55gene on an additional IncI2 plasmid.

ABSTRACTpHN1122-1 carrying blaCTX-M-55, from an Escherichia coliisolate from a dog, was completely sequenced. pHN1122-1 has an IncI2 replicon and typical IncI2-associated genetic modules, including mok/hok-finO-yafA/B, nikABC, and two transfer regions, traand pil, as well as a shufflon. blaCTX-M-55is found within a 3.084-kb ISEcp1transposition unit that includes a fragment of IncA/C plasmid backbone. pHN1122-1 and closely related plasmids were identified in other E. coliisolates from animals in China.

Here, the mechanisms of colistin heteroresistance (CHR) were assessed in 12 Escherichia coliisolates from swine in China. CHR was investigated by population analysis profile tests. CHR stability was studied by culturing isolates for five overnight incubation periods in colistin-free medium. Subsequently, the mcr-1gene and mutations in PmrAB, PhoPQ, and MgrB were screened in parental isolates and resistant subpopulations. Additionally, the expression levels of phoPQ, its target gene pagP, and its negative regulator gene mgrB, as well as pmrABand its target genes pmrHFIJKLMand pmrC, were determined by real-time relative quantitative PCR.