Genomic Analysis of an IMP-1-Producing Atlantibacter hermannii Bloodstream Isolate Carrying a Transferable blaIMP-1-Harbouring IncHI2/pST1 Plasmid Related to Historical Enterobacterales Plasmids in Japan

Atlantibacter hermannii is an uncommon member of the Enterobacterales rarely associated with human infection, and genomic data on antimicrobial resistance in this species remain limited. Carbapenemase-producing A. hermannii isolates have rarely been described. We characterized an IMP-1-producing A. hermannii bloodstream isolate recovered in Japan and assessed the structure and transferability of its blaIMP-1-harbouring IncHI2 plasmid. Minimum inhibitory concentrations were determined by broth microdilution. Complete genome sequencing was performed using Oxford Nanopore technology, followed by comparative plasmid analysis and conjugation experiments. The isolate showed reduced susceptibility to carbapenems and resistance to extended-spectrum cephalosporins, whereas aztreonam, aztreonam–avibactam, and cefiderocol retained in vitro activity. Genome sequencing revealed a circular chromosome and two plasmids, including a 289-kb IncHI2 plasmid belonging to plasmid sequence type 1 (pST1) and harbouring blaIMP-1 within a class 1 integron. Comparative analysis showed structural conservation with IncHI2/pST1 plasmids identified in Japanese Enterobacterales isolates in 2007 and 2018. Conjugation experiments confirmed transfer to Escherichia coli, with markedly higher transfer frequencies at 30–35 °C than at 37 °C. These findings provide genomic and functional evidence for the continued detection of closely related blaIMP-1-harbouring IncHI2/pST1 plasmids among different Enterobacterales species in Japan over an extended period.