Colistin-heteroresistance in carbapenemase-producing Enterobacter species causing hospital-acquired infections among Egyptian patients

Objectives: Colistin is the last resort for treating carbapenemase-producing Enterobacter. Colistin-heteroresistance is a new concern as it may cause treatment failure. Our study aimed to detect colistin-heteroresistance among carbapenemase-producing Enterobacter species causing hospital-acquired infections in patients at Mansoura University Hospitals (MUHs). Methods: Sensitivity of recovered Enterobacter species to imipenem, meropenem and colistin was estimated by the broth dilution method. Carbapenemase production was detected with the Carba NP test and confirmed with multiplex PCR. Population analysis profile (PAP) was performed to assess colistin-heteroresistance. Enterobacter isolates with colistin MIC ≤ 2 μg/mL had subpopulations growing at colistin concentration > 2 μg/mL were considered heteroresistant. Isolates with subpopulations growing at colistin concentrations two times higher than MIC but ≤ 2 μg/mLwere considered heterogeneous. Results: Of 115 Enterobacter isolates collected during the period of the study, 61 (53%) were cabapenem-resistant. Of these, 49 isolates (42.6%) were carbapenemase-producers, including Enterobacter cloacae complex (37; 75.5%) and Enterobacter aerogenes (12; 24.5%). The most prevalent carbapenemase gene was blaNDM (20 isolates; 40.8%). Seven isolates were colistin-resistant (7/115; 6.1%). Seventeen isolates (34.7% of carbapenemase-producers) were colistin-heteroresistant and two isolates had heterogeneous profiles. Most of these isolates were E. cloacae complex (12/17) and from bloodstream infection (10/17). The frequency of heteroresistant subpopulations ranged from 1 × 10−5 to 5.5 × 10−4. Conclusion: Carbapenem-resistant Enterobacter is a common resistant pathogen in the hospital setting. Colistin-heteroresistance among carbapenemase-producing Enterobacter is a growing serious medical problem as colistin is considered the last hope for treating infections caused by these multidrug-resistant pathogens

A new beam-column model for seismic analysis of RC frames – Part II: Model Verification

In this investigation, the performance of the simplified Flexibility-Based Fiber Model (FBFM), proposed in Part I of this study, is evaluated. The proposed model relies on calculating the inelastic lengths at the ends of the Reinforced Concrete (RC) beam-column member in every load increment and using preset flexibility distribution functions along the inelastic lengths to integrate the overall element response. The model eliminates the need for monitoring the responses of many segments distributed along the member length which results in a significant reduction in computations. The model performance is evaluated in this study on a one element level of a beam-column element and on a structure level of a 3-story frame. The selected structures are subjected to static pushover, static cyclic and earthquake loading conditions. The results of the proposed model are compared with the outcomes of the conventional FBFMs. The comparison is achieved using global performance parameters such as the maximum drift ratios and local performance parameters such as the maximum strains in steel and concrete at the plastic hinge regions. The analysis conducted indicates that the proposed model is capable of describing with satisfactory accuracy and computational efficiency the response of RC frame structures

A new beam-column model for seismic analysis of RC frames – Part I: Model derivation

In this study, a reliable and computationally efficient beam-column model is proposed for seismic analysis of Reinforced Concrete (RC) frames. The model is a simplified version of the Flexibility-Based Fiber Models (FBFMs), which rely on dividing the element length into small segments and dividing the cross section of each segment into concrete and steel fibers. In the proposed model, only the two end sections are subdivided into fibers and uniaxial material models that consider the various behavioral characteristics of steel and concrete under cyclic loading conditions are assigned for the cross section fibers. The proposed model is simpler than the FBFMs as it does not require monitoring the responses of many segments along the element length, which results in a significant reduction in computations. The inelastic lengths at the ends of the proposed model are divided into two inelastic zones; cracking and yielding. The inelastic lengths vary according to the loading history and are calculated in every load increment. The overall response of the RC member is estimated using preset flexibility distribution functions along the element length. A flexibility factor η is utilized to facilitate selecting the proper flexibility distribution shape. The proposed model is implemented into the computer program DRAIN-2DX

Behavior of stub girder floor system with partial shear connection

A stub-girder floor system is a composite system constructed from a continuous steel beam and a reinforced concrete slab separated by a series of short, typically wide, flange sections called stubs. The finite element method has been used in the analysis of this composite system where it is capable to represent the constituent parts, adopt adequate elements and use appropriate solution techniques. As the behavior of stub-girders presents significant nonlinear effects, it is fundamental that the interaction of all different components should be properly modeled as well as the interface behavior. The present work focuses on the modeling of stub-girders with full and partial shear connection in two and three dimensions. The proposed model contains all the main structural parameters and their associated nonlinearities (concrete slab, steel beam, stubs, and shear connectors). In this model, the shear connectors are modeled as springs to consider the geometry of studs in addition to the nonlinearity due to the interaction between the shear connector and the concrete slab. Tests and numerical results available in the literature are used to validate the models. Based on the proposed finite element model, an extensive parametric study of stub-girders is performed, considering the material properties, relative dimensions and shear connector characteristics, where valuable recommendations and conclusions are achieved

Evaluating the vertical vibration response of footbridges using a response spectrum approach

In this paper, the vertical vibration response of footbridges subjected to dynamic loads induced by walking humans is assessed via a response spectrum approach. The dynamic walking load in the vertical direction is applied on the bridges using two different loading schemes: (1) a stationary load at the mid-span and (2) a moving load across the bridge. The response spectrum analysis is carried out using a Generalized Single Degree of Freedom procedure which has been verified by comparing its predictions with the results of a Multi-Degrees of Freedom modeling. The results obtained indicated that the response spectrum approach is capable of accurately predicting the footbridge vibration response. The results obtained also indicated that the main parameters that affect the induced accelerations in footbridges due to the human walking loads in the vertical direction are the footbridge mass and damping ratio