EDTA-modified carbapenem inactivation method (eCIM) for detecting IMP Metallo-β-lactamase-producing Pseudomonas aeruginosa: an assessment of increasing EDTA concentrations

BackgroundPrompt identification of carbapenemase-harboring organisms is valuable in informing therapeutic and infection-control measures. The modified carbapenem inactivation method (mCIM) and EDTA-modified carbapenem inactivation method (eCIM) are inexpensive and easy to interpret phenotypic tests endorsed by the Clinical and Laboratory Standards Institute (CLSI) for the detection of carbapenemase-harboring Enterobacterales. Only mCIM is endorsed by CLSI for detecting carbapenemase-harboring Pseudomonas aeruginosa. eCIM's ability to delineate serine and metallo-beta -lactamases (MBL) could be advantageous in areas prevalent with carbapenemase-harboring P. aeruginosa. A recent assessment of mCIM/eCIM on MBL-harboring P. aeruginosa demonstrated high eCIM sensitivity for NDMs and VIMs but not for IMP-producers. Therefore, this study aimed to determine whether increasing EDTA concentrations would enhance eCIM sensitivity for a collection of IMP-harboring P. aeruginosa isolates.Twenty-six IMP-harboring P. aeruginosa isolates were utilized. For test validation, additional P. aeruginosa isolates harboring NDM (n=3), VIM (n=3), KPC (n=8), wild-type (n=1), and Enterobacterales isolates harboring IMP (n=6) and NDM (n=1) were assessed. The mCIM test was conducted as outlined by CLSI. Simultaneously, the eCIM test was performed with the standard 5mM EDTA concentration and doubling EDTA concentrations: 10mM, 20mM, and 40mM.ResultsConcentration-dependent improvement was observed among the IMP-harboring P. aeruginosa with eCIM sensitivities at 0, 31, 85, and 100% respectively. Remaining Enterobacterales and P. aeruginosa responded concordantly with their genotype at the standard 5mM eCIM concentration, with doubling EDTA concentrations providing no greater sensitivity.ConclusionCombination of mCIM and an eCIM with a 40mM EDTA concentration appropriately capture IMP-harboring P. aeruginosa without sacrificing test utility for other carbapenemase-harboring isolates.