Development of selective medium for IMP-type carbapenemase-producing Enterobacteriaceae in stool specimens

BACKGROUND Identification of carbapenemase-producing Enterobacteriaceae (CPE) in faecal specimens is challenging. This fact is particularly critical because low-level carbapenem-resistant organisms such as IMP-producing CPE are most prevalent in Japan. We developed a modified selective medium more suitable for IMP-type CPE. METHODS Fifteen reference CPE strains producing different types of β-lactamases were used to evaluate the commercially available CHROMagar KPC and chromID CARBA as well as the newly prepared MC-ECC medium (CHROMagar ECC supplemented with meropenem, cloxacillin, and ZnSO4) and M-ECC medium (CHROMagar ECC supplemented with meropenem and ZnSO4). A total of 1035 clinical samples were then examined to detect CPE using chromID CARBA and M-ECC medium. RESULTS All tested strains producing NDM-, KPC-, and OXA-48-carbapenemases were successfully cultured in the media employed. Although most of the IMP-positive strains did not grow in CHROMagar KPC, chromID CARBA, or MC-ECC, all tested strains grew on M-ECC. When faecal samples were applied to the media, M-ECC medium allowed the best growth of IMP-type CPE with a significantly higher sensitivity (99.3%) than that of chromID CARBA (13.9%). CONCLUSIONS M-ECC medium was determined as the most favourable selective medium for the detection of IMP-type CPE as well as other types of CPE.