Evaluation of the modi ed sodium mercaptoacetic acid double disk synergy test for detecting the metallo-β-lactamase producing Enterobacteriaceae

Background: Sodium mercaptoacetic acid double disk synergy test (SMA-DDSTs) is frequently used in Japan to detect metallo-β-lactamase (MBL) producer easily. In this study, we evaluated SMA-DDST at the points of the antimicrobial disk, and the distance between SMA and an antimicrobial disk. Methods: Forty-one clinical isolates ceftazidime (CAZ)-resistant Enterobacteriaceae and 12 reference strains were tested. Those isolates included 17, 19, 4 and 13 of only IMP-, IMP with other classes of β-lactamases-, New Delhi MBL with other classes of β-lactamasesand non-MBL-producers, respectively. SMA-DDSTs were performed with CAZ, imipenem (IPM) and meropenem (MEPM)-containing disks with the distance between SMA and antimicrobial disk at 11 mm or 16 mm (centerto-center). Results: The sensitivities were 27/40 (68%), 17/40 (43%), 35/40 (88%), 29/40 (73%), 25/40 (63%), and 39/40 (98%) for 16 mm CAZ-SMA-DDST, 16 mm IPMSMA-DDST, 16 mm MEPM-SMA-DDST, 11 mm CAZ-SMA-DDST, 11 mm IPMSMA-DDST, and 11 mm MEPM-SMA-DDST, respectively. CAZ and MEPM disk showed the highest sensitivity for only MBL-producing isolates and MBL with coproducing other classes of β-lactamases-producing isolates, respectively. Among any of antimicrobials used in this study, 11 mm SMA-DDSTs showed higher sensitivity than those 16 mm. The specificity was 100% for all SMA-DDSTs.