Paradoxical carbapenemase activity detected by modified carbapenemase inactivation (mCIM) method in <i>Citrobacter sedlakii</i>

Screening for carbapenemase-producing Enterobacterales is a necessity for hospitals serving high-risk patient populations. Detection methods often incorporate the modified carbapenemase inactivation method (mCIM) test to verify carbapenemase production in suspect isolates. Here, we demonstrate the case of a <i>Citrobacter sedlakii</i> isolate recovered from carbapenemase-surveillance culture that was strongly mCIM positive for carbapenemase production, but phenotypically nonresistant to carbapenems. The modified carbapenemase inactivation method with EDTA (eCIM) test, which suggests the presence of a metallo-β-lactamase, was positive as well. Liquid chromatography mass spectrometry additionally confirmed the loss of meropenem during the mCIM test. Whole-genome sequencing detected only the presence of the chromosomal class A serine β-lactamase SED-1 but did not identify any known carbapenemase or additional β-lactamases. Further characterization suggested that the large inoculum utilized in mCIM tests can give the impression of carbapenemase production in non-resistant isolates. Additionally, the large inoculum, combined with weak carbapenemase activity, can mimic the phenotype expected for metallo-β-lactamase in the eCIM test. These results suggest that utilization of the mCIM and eCIM tests without appropriate susceptibility testing can lead to erroneous detection of carbapenemase production.IMPORTANCEPhenotypic tests including the mCIM are often used for detection of carbapenemase production in Enterobacterales. The presence of a carbapenemase gene is confirmed by PCR assays and, while rarely seen, a positive mCIM assay and negative PCR can alert the laboratory to the presence of rare carbapenemases (such as GES and IMI). We report a false-positive mCIM test, which we demonstrate is due to the heavy inoculum of the organism used in the assay. Whole-genome sequencing showed that a class A β-lactamase (<i>bla_SED-1</i>) was present, and mass spectrometry demonstrated degradation of meropenem during the mCIM assay. The inoculum effect should be considered when interpreting mCIM and eCIM assays where a carbapenemase gene is not detected.