Phenotypic and Genetic Characteristics of blaIMP-6-Harboring Enterobacterales Isolates Lacking blaCTX-M-2 in Japan

Carbapenemase-producing Enterobacterales (CPE) is a global threat. IMP-6, a prevalent carbapenemase in western Japan, is mostly disseminated via CTX-M-2 extended-spectrum β-lactamase (ESBL) co-producing Enterobacterales. However, the existence and characteristics of Enterobacterales harboring blaIMP-6 without blaCTX-M-2 remain unclear. We analyzed the phenotypic and genetic characteristics of clinical blaIMP-6-harboring Enterobacterales isolates, focusing on those lacking blaCTX-M-2. Overall, 220 blaIMP-6-harboring isolates collected from 76 Japanese hospitals between 2014 and 2021 were characterized by antimicrobial susceptibility, presence of CTX-M-type ESBLs, plasmid incompatibility, plasmid transfer experiments, and genome sequencing and analysis. Among these, 203 co-harbored blaCTX-M-2 group, with 90% of them demonstrating high conjugation frequency and broad-spectrum resistance to β-lactams. Of the remaining 17 isolates, nine lacked blaCTX-M, while eight co-harbored blaCTX-M-1 group (n = 2) or blaCTX-M-9 group (n = 6). Eleven isolates carried nontransferable plasmids with genetic structures distinct from those of blaIMP-6 and blaCTX-M-2 co-encoding plasmids, including eight non-incompatibility N plasmids. Fifteen isolates carried only blaIMP-6-encoding plasmids; two carried plasmids with blaIMP-6 and blaCTX-M (blaCTX-M-27 or blaCTX-M-65). This novel study revealed that blaIMP-6 can exist without blaCTX-M-2 on diverse, often nontransferable plasmids, suggesting distinct, lower dissemination pathways compared to those of epidemic blaCTX-M-2 co-carrying plasmids and highlighting previously overlooked plasmids that necessitate close monitoring.