SMB-1, a Novel Subclass B3 Metallo-β-Lactamase, Associated with ISCR1and a Class 1 Integron, from a Carbapenem-Resistant Serratia marcescens Clinical Isolate

Abstract

ABSTRACTA carbapenem-resistantSerratia marcescensstrain, 10mdr148, was identified in a Japanese hospital in 2010. The carbapenem resistance of this strain was attributed to the production of a novel metallo-β-lactamase (MBL), named SMB-1 (Serratiametallo-β-lactamase). SMB-1 possessed a zinc binding motif, H(Q)XHXDH (residues 116 to 121), H196, and H263 and was categorized as a member of subclass B3 MBL. SMB-1 has 75% amino acid identity with the most closely related MBL, AMO1, of uncultured bacterium, recently identified through the metagenomic analysis of apple orchard soil. The introduction ofblaSMB-1intoEscherichia coliconferred resistance to a variety of β-lactam antibiotics, penicillins, cephalosporins, and carbapenems, but not aztreonam, a resistance pattern consistent with those of other MBLs. SMB-1 demonstrated highkcatvalues of >500 s−1for carbapenems, resulting in the highest hydrolyzing efficiency (kcat/Km) among the agents tested. The hydrolyzing activity of SMB-1 was well inhibited by chelating agents. TheblaSMB-1gene was located on the chromosome ofS. marcescensstrain 10mdr148 and at the 3′ end of the ISCR1element in complex with a typical class 1 integron carryingaac(6′)-IbandcatB3gene cassettes. Downstream ofblaSMB-1, the second copy of the 3′conserved segment and ISCR1were found. To our knowledge, this is the first subclass B3 MBL gene associated with an ISCR1element identified in anEnterobacteriaceaeclinical isolate. A variety of antibiotic resistance genes embedded with ISCR1have been widely spread amongEnterobacteriaceaeclinical isolates, thus the further dissemination ofblaSMB-1mediated by ISCR1transposition activity may become a future concern.