Emergence in Japan of an imipenem-susceptible, meropenem-resistant Klebsiella pneumoniae carrying blaIMP-6

We identified 5 Klebsiella pneumoniae isolates showing high resistance to β-lactams except imipenem and designated them ISMRK (imipenem-susceptible but meropenem-resistant Klebsiella). They carried the blaIMP-6 and blaCTX-M-2 on a self-transmissible plasmid. ISMRK may be falsely categorized as susceptible to carbapenems if imipenem is used to screen carbapenem resistance.