Phenotypic detection of AmpC β-lactamases, extended-spectrum- β-lactamases and metallo-β-lactamases in Enterobacteriaceae using a resazurin microtitre assay with inhibitor-based methods.

Dissemination of antibiotic resistance in Enterobacteriaceae mediated by AmpC, ESBL and MBL β-lactamases is clinically significant. A simple, relatively quick method for the detection of these resistance phenotypes would greatly improve chemotherapeutic recommendation. This technology would provide valuable input in our surveillance of resistance on a global stage, particularly if the methodology could be applicable to resource poor settings. A resazurin microtitre plate (RMP) assay incorporating cloxacillin, clavulanic acid, and EDTA for the rapid phenotypic identification of AmpC, Extended-spectrum-β-lactmase (ESBL), metallo-β-lactamase (MBL) and the co-existence of β-lactamases has been developed. A total of 47 molecularly characterised Enterobacteriaceae clinical isolates producing AmpCs, ESBLs, co-producers of ESBL and AmpC, MBLs, and co-producers of ESBL and MBL were phenotypically examined using the RMP assay. The ceftazidime (CAZ)-based and cefotaxime (CTX)-based RMP assay successfully detected all 16 AmpC, 14 ESBL, 9 MBL producers, 6 ESBL-AmpC co-producers, and 2 ESBL-MBL co-producers without false positive results. The CAZ-based assay was more reliable in detecting AmpC alone, while the CTX-based assay performed better in identifying co-producers of ESBL and AmpC. There was no difference in detection of ESBL and MBL producers. The findings of the present study suggest that use of the RMP assay with particular β-lactamase inhibitors explicitly detects three different β-lactamases, as well as co-existence of β-lactamases within 6 h after initial isolation of the pathogen. This assay is applicable to carry out in any laboratory, is cost-effective and easy to interpret. It could be implemented in screening patients, controlling infection and for surveillance purposes

Insights into the mechanism of calcium activation of adseverin

Actin remodeling is a key step in many cellular processes. Adseverin and gelsolin are calcium dependent actin remodeling proteins in gelsolin superfamily. Gelsolin is involved in cell motility whereas adseverin participates in cell secretion. Gelsolin has a C-terminal extension that forms an a-helix, which covers an F-actin-binding site in the absence of calcium. Equilibrium dialysis experiments suggest that adseverin has one rate-limiting step during activation with respect to actin severing whereas gelsolin has two rate-limiting steps. The second rate-limiting step of gelsolin has been attributed to the unlatching of the C-terminal helix. However, the details of the calcium activation of adseverin and gelsolin remain unclear. Results presented in this study have demonstrated that in solution adseverindisplays different conformations fromgelsolin. However, the calcium induced conformational changes of these proteins are similar. The N-terminal half of adseverin displays an inactive conformation in the absence of calcium that is activated by calcium binding at A3. Calcium binding at the calcium-binding site straightens the long helix that disruptsthe key interactions at the A1:A3 interface (R97-E314 and F64-M310), which is unnecessary for gelsolin. Calcium activation at the C-terminal half of adseverin involves domains rearrangement induced by cooperative calcium binding at A4 and AS but A6. However calcium binding at A6 is key to the activation of the full-length molecule. Moreover actin filament depolymerization assay suggests that the C-terminal half may present actin filament severing activity in the present of calcium. Results in the thesis have provided insights into the details of calcium activation of adseverin providing mechanistic explanations to such questions as, "Why does adseverin require less calcium than gelsolin in inducing severing activity?" "Why is actin filament severing activity by the N-terminus of adseverin calcium dependent whereas it is calcium independent in gelsolin?" Moreover the results have helped explain the common mechanism of calcium activation of the C- terminal half of adseverin and gelsolin.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

The Structural Basis of the Control of Actin Dynamics by the Gelsolin Superfamily Proteins

Rearrangement of the actin cytoskeleton occurs in a variety of cellular processes and structures and involves a wide spectrum of proteins. Among these, the gelsolin superfamily proteins (GSPs) control actin organization by severing filaments, capping filament ends and bundling filaments. Structural changes within the GSPs are key in controling their functions. This thesis is aimed in understanding the activation mechanisms of the C-terminal halves of GSPs through investigating the atomic structures of gelsolin, adseverin and villin. X-ray crystallography was used to determine the structures of C-terminal fragments of these 3 proteins. The results demonstrate that: 1) The structure of the activated form of the C-terminal half of gelsolin displays an open conformation, with the actin-binding site on gelsolin domain 4 (G4) fully exposed and all three type-II calcium binding sites (CBS) occupied. Neither actin nor the type-I calcium, which is normally sandwiched between actin and G4, is required to achieve this conformation. 2) Calcium ions at both type-I and type-II CBSs of gelsolin were exchangable within the crystals. Extraction of calcium ions from the CBSs triggered local conformation changes which we speculate are the initial steps toward restoration of the arrangement of domains found in the calcium-free inactive form of gelsolin in solution. 3) The long helix of G6 in the calcium-bound structure is similar to the helix of calcium-free isolated villin domain 6 (V6). 4) The conformation of the C-terminal half of adseverin in the active state is similar to that of gelsolin. These results suggest that the C-terminal halves of GSPs are activated before forming a complex with actin. The activation involves straightening the helix of domain 6 which is a key component in the global conformation changes of C-terminal halves of these proteins. The results also suggest that a calcium ion may bind to the type-I CBS on domain 4 of the active conformation of GSPs concurrently with forming the complex with actin, hence, stabilizing the GSP:actin complex.

Calcium ion exchange in crystalline gelsolin

Gelsolin is a calcium and pH-sensitive modulator of actin filament length. Here, we use X-ray crystallography to examine the extraction and exchange of calcium ions from their binding sites in different crystalline forms of the activated N and C-terminal halves of gelsolin, G1-G3 and G4-G6, respectively. We demonstrate that the combination of calcium and low pH activating conditions do not induce conformational changes in G4-G6 beyond those elicited by calcium alone. EGTA is able to remove calcium ions bound to the type I and type II metal ion-binding sites in G4-G6. Constrained by crystal contacts and stabilized by interdomain interaction surfaces, the gross structure of calcium-depleted G4-G6 remains that of the activated form. However, high-resolution details of changes in the ion-binding sites may represent the initial steps toward restoration of the arrangement of domains found in the calcium-free inactive form of gelsolin in solution. Furthermore, bathing crystals with the trivalent calcium ion mimic, Tb3+, results in anomalous scattering data that permit unequivocal localization of terbium ions in each of the proposed type I and type II ion-binding sites of both halves of gelsolin. In contrast to predictions based on solution studies, we find that no calcium ion is immune to exchange

Activation in isolation : exposure of the actin-binding site in the C-terminal half of gelsolin does not require actin

Gelsolin requires activation to carry out its severing and capping activities on F-actin. Here, we present the structure of the isolated C-terminal half of gelsolin (G4-G6) at 2.0 A resolution in the presence of Ca(2+) ions. This structure completes a triptych of the states of activation of G4-G6 that illuminates its role in the function of gelsolin. Activated G4-G6 displays an open conformation, with the actin-binding site on G4 fully exposed and all three type-2 Ca(2+) sites occupied. Neither actin nor the type-l Ca(2+), which normally is sandwiched between actin and G4, is required to achieve this conformation

Helix Straightening as an Activation Mechanism in the Gelsolin Superfamily of Actin Regulatory Proteins*

Villin and gelsolin consist of six homologous domains of the gelsolin/cofilin fold (V1–V6 and G1–G6, respectively). Villin differs from gelsolin in possessing at its C terminus an unrelated seventh domain, the villin headpiece. Here, we present the crystal structure of villin domain V6 in an environment in which intact villin would be inactive, in the absence of bound Ca2+ or phosphorylation. The structure of V6 more closely resembles that of the activated form of G6, which contains one bound Ca2+, rather than that of the calcium ion-free form of G6 within intact inactive gelsolin. Strikingly apparent is that the long helix in V6 is straight, as found in the activated form of G6, as opposed to the kinked version in inactive gelsolin. Molecular dynamics calculations suggest that the preferable conformation for this helix in the isolated G6 domain is also straight in the absence of Ca2+ and other gelsolin domains. However, the G6 helix bends in intact calcium ion-free gelsolin to allow interaction with G2 and G4. We suggest that a similar situation exists in villin. Within the intact protein, a bent V6 helix, when triggered by Ca2+, straightens and helps push apart adjacent domains to expose actin-binding sites within the protein. The sixth domain in this superfamily of proteins serves as a keystone that locks together a compact ensemble of domains in an inactive state. Perturbing the keystone initiates reorganization of the structure to reveal previously buried actin-binding sites