Metallo-β-lactamases (MBLs) are a class of Zn(II)-containing enzymes that hydrolyse the β-lactam bond in β-lactam-containing antibiotics, resulting in products that no longer exhibit antibacterial activity. Currently, there are 29 known MBLs (not including variants), and these enzymes have been classified on the basis of primary amino acid sequence and functionality into three or four classes (see www.mbled.uni-stuttgart.de for an updated MBL database)1. MBLs in classes B1 and B3 are known for their ability to hydrolyse all β-lactam antibiotics, except monobactams, including penicillins, carbapenems and cephalosporins. The class B2 enzymes exhibit a narrower substrate preference and preferentially hydrolyse carbapenems2. The most clinically important MBLs belong to class B1, which contains VIM (Verona integrinencoded MBLs), IMP (imipenemase) and NDM (New Delhi MBL), and all of these enzymes have multiple clinical variants (VIM-1—VIM-46, IMP-1—IMP-51, and NDM-1—NDM-16) (www.lahey.org/Studies/). The presence of multiple variants poses a challenge to identify clinical inhibitors because the variants often exhibit different substrate specificities and are affected differently by known non-clinical inhibitors3.

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