Risk of inaccurate species identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and of false carbapenem resistance by automated susceptibility analysis of Enterobacter spp.

Enterobacter species were isolated from the sputum of an 84-year-old female patient with fever, chills, cellulitis in the right elbow, and pyogenic arthritis in the left shoulder. The bacteria were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and whole-genome sequencing (WGS). This strain was defined as a carbapenem-resistant Enterobacterales (CRE) by the MicroScan NG MIC 44 panel without applying the revised guidelines of the Korea Disease Control and Prevention Agency properly. MALDI-TOF MS is commonly used to determine species, but the accurate identification of Enterobacter subspecies remains challenging. An Etest or disk diffusion test is recommended to confirm ertapenem resistance when an isolate exhibits resistance solely to ertapenem among carbapenems.