Comparison of Test Methods for Detecting Metallo-β-Lactamase-Producing Gram-Negative Bacteria

The recent increase in gram-negative bacteria coproducing multiple classes of β-lactamases has made it difficult to accurately identify metallo-β-lactamase (MBL) producers. In the present study, six methods for detecting MBL producers were compared using 56 gram-negative bacterial isolates that produce various β-lactamases. Sodium mercaptoacetic acid (SMA) and EDTA were used as inhibitors for a double-disk synergy test (DDST), and antimicrobial agents, including ceftazidime (CAZ), imipenem, and meropenem (MEPM), along with the distance between the disks, were compared. The Etest(®), dry-plate DPD1(®), Cica-Beta(®), and modified Hodge test were also compared. Among the six methods compared, DDST using the SMA disk showed the highest sensitivity (Se) and specificity (Sp). In DDST, the clearest appearance of growth inhibition was observed when the distance between the disks was maintained at approximately 5 mm. A combination of CAZ and SMA successfully detected only MBL-producing isolates (Se, 87.5%; Sp, 100%), and MEPM exhibited the best performance in combination with SMA in the detection of MBL producers coproducing other classes of β-lactamases such as CTX-M- and CMY-type enzymes (Se, 79.1%; Sp, 100%). DDST using SMA and CAZ and/or MEPM is a simple, specific, and cost-effective method to screen MBL producers in routine clinical laboratory testing.