Infections caused by carbapenem-resistant Enterobacteriaceae (CRE) are a significant cause of morbidity and mortality worldwide. CRE are defined by the Centers for Disease Control and Prevention (CDC) as those Enterobacteriaceae that are resistant in vitro to any carbapenem antimicrobial. This means a minimum inhibitory concentration (MIC) of ≥ 2 mg/ml for ertapenem, or an MIC ≥ 4 mg/ml for doripenem, meropenem, or imipenem. In addition, those Enterobacteriaceae that are documented to produce a carbapenemase are also considered CRE, regardless of carbapenem MIC. For Enterobacteriaceae species that have intrinsic imipenem resistance – such as Morganella morganii, Proteus spp. and Providencia spp – resistance to carbapenems other than imipenem is required.2 This phenotypic definition includes both carbapenemase-producing Enterobacteriaceae (CPE or CP-CRE) as well as non-carbapenemase-producing CRE. A retrospective study evaluating the impact of carbapenemase production on outcomes after monomicrobial CRE bloodstream infections (BSI) found that patients infected with CP-CRE were at increased risk of dying within 14 d as compared with patients with non-carbapenemase-producing CRE (adjusted odds ratio, 4.92; 95% confidence interval 1.01–24.81). The authors speculated that increased virulence of CP-CRE rather than patient factors were the cause of this observation
