Biochemical Characterization of VIM-39, a VIM-1-Like Metallo-β-Lactamase Variant from a Multidrug-Resistant Klebsiella pneumoniae Isolate from Greece

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

The Molecular Mechanisms Underlying Hidden Phenotypic Variation among Metallo-β-Lactamases

Genetic variation among orthologous genes has been largely formed through neutral genetic drift while maintaining the functional role of these genes. However, because the evolution of gene occurs in the context of each host organism, their sequence changes are also associated with adaptation to a specific environment. Thus, genetic variation can create critical phenotypic variation, particularly when genes are transferred to a new host by horizontal gene transfer. Unveiling "hidden phenotypic variation" is particularly important for genes that confer resistance to antibiotics. However, our understanding of the molecular mechanisms that underlie phenotypic variation remains limited. Here we sought to determine the extent of phenotypic variation in the B1 metallo-β-lactamase (MBL) family and its molecular basis by systematically characterizing eight MBL orthologs, including NDM-1 and VIM-2 and IMP-1. We found that these MBLs confer diverse levels of resistance. The phenotypic variation cannot be explained by variation in catalytic efficiency alone; rather, it is the combination of the catalytic efficiency and abundance of functional periplasmic enzyme that best predicts the observed variation in resistance. The level of functional periplasmic expression varied dramatically between MBL orthologs. This was the result of changes at multiple levels of each ortholog's: (1) quantity of mRNA, (2) amount of MBL expressed, and (3) efficacy of functional enzyme translocation to the periplasm. Overall, it is the interaction between each gene and the host's underlying cellular processes (transcription, translation, and translocation) that determines MBL genetic incompatibility through horizontal gene transfer. These host-specific processes may constrain the effective spread and deployment of MBLs to certain host species and could explain the current observed distribution bias.

The rapid spread of carbapenem-resistant Enterobacteriaceae

Carbapenems, our one-time silver bullet for multidrug resistant bacterial infections, are now threatened by widespread dissemination of carbapenem-resistant Enterobacteriaceae (CRE). Successful expansion of Enterobacteriaceae clonal groups and frequent horizontal gene transfer of carbapenemase expressing plasmids are causing increasing carbapenem resistance. Recent advances in genetic and phenotypic detection facilitate global surveillance of CRE diversity and prevalence. In particular, whole genome sequencing enabled efficient tracking, annotation, and study of genetic elements colocalized with carbapenemase genes on chromosomes and on plasmids. Improved characterization helps detail the co-occurrence of other antibiotic resistance genes in CRE isolates and helps identify pan-drug resistance mechanisms. The novel β-lactamase inhibitor, avibactam, combined with ceftazidime or aztreonam, is a promising CRE treatment compared to current colistin or tigecycline regimens. To halt increasing CRE-associated morbidity and mortality, we must continue quality, cooperative monitoring and urgently investigate novel treatments.