A Novel, Integron-Regulated, Class C β-Lactamase

AmpC-type β-lactamases severely impair treatment of many bacterial infections, due to their broad spectrum (they hydrolyze virtually all β-lactams, except fourth-generation cephalosporins and carbapenems) and the increasing incidence of plasmid-mediated versions. The original chromosomal AmpCs are often tightly regulated, and their expression is induced in response to exposure to β-lactams. Regulation of mobile ampC expression is in many cases less controlled, giving rise to constitutively resistant strains with increased potential for development or acquisition of additional resistances. We present here the identification of two integron-encoded ampC genes, bla_IDC-1 and bla_IDC-2 (integron-derived cephalosporinase), with less than 85% amino acid sequence identity to any previously annotated AmpC. While their resistance pattern identifies them as class C β-lactamases, their low isoelectric point (pI) values make differentiation from other β-lactamases by isoelectric focusing impossible. To the best of our knowledge, this is the first evidence of an ampC gene cassette within a class 1 integron, providing a mobile context with profound potential for transfer and spread into clinics. It also allows bacteria to adapt expression levels, and thus reduce fitness costs, e.g., by cassette-reshuffling. Analyses of public metagenomes, including sewage metagenomes, show that the discovered ampCs are primarily found in Asian countries.

A fluorescein-labeled AmpC β-lactamase allows rapid characterization of β-lactamase inhibitors by real-time fluorescence monitoring of the β-lactamase-inhibitor interactions

Rapid emergence of class C β-lactamases has urged an immediate need for developing class C β-lactamase specific inhibitors for effective clinical treatment. To facilitate the development of effective class C β-lactamase inhibitors, we propose a new approach for a rapid analysis of the interaction of AmpC β-lactamase and its inhibitors using our recently developed V211Cf fluorescent β-lactamase biosensor during drug screening. Since the fluorescein of V211Cf can report the local environment change in the active site of AmpC β-lactamase, fluorescence responses of V211Cf toward its substrates/inhibitors can provide real-time traces of the dynamic change of the interaction of the β-lactamase with its substrates/inhibitors. In this study, we found that V211Cf displayed distinct fluorescence signal patterns toward different kinds of inhibitors (including clavulanic acid, sulbactam, tazobactam and 2-thiopheneboronic acid) due to the differences in their interactions with β-lactamase. V211Cf not only enables a high throughput screening for inhibitors but can also provide a rapid preliminary indication on the inhibitor's potency and stability to β-lactamase's hydrolytic action as well as how the inhibitors interact with the target enzyme, thereby speeding up the drug discovery and development cycle of class C β-lactamase inhibitors.