The transthyretin-like protein (TLP) from Salmonella enterica subspecies I is a periplasmic protein with high level structural similarity to a protein found in mammals and fish. In humans, the protein homologue, transthyretin, binds and carries retinol and thyroxine, and a series of other, unrelated aromatic compounds. Here we show that the amino acid sequence of the TLP from different species, subspecies and serovars of the Salmonella genus is highly conserved and demonstrate that the TLP gene is constitutively expressed in S. Typhimurium and that copper and other divalent metal ions severely inhibit enzyme activity of the TLP, a cyclic amidohydrolase that hydrolyses 5-hydroxyisourate (5-HIU). In order to determine the in vivo role of the S. Typhimurium TLP, we constructed a strain of mouse-virulent S. Typhimurium SL1344 bearing a mutation in the TLP gene (SL1344 ΔyedX). We assessed the virulence of this strain via oral inoculation of mice and chickens. Whilst SL1344 ΔyedX induced a systemic infection in both organisms, the bacterial load detected in the faeces of infected chickens was significantly reduced when compared to the load of S. Typhimurium SL1344. These data demonstrate that the TLP gene is required for survival of S. Typhimurium in a high uric acid environment such as chicken faeces, and that metabolic traits of Salmonellae in natural and contrived hosts may be fundamentally different. Our data also highlight the importance of using appropriate animal models for the study of bacterial pathogenesis especially where host-specific virulence factors or traits are the subject of the study.

Mechanical response of nano-based composites is generally influenced by interaction of filler and matrix at interface. Increasing filler-loading within the composite may cause spatial limitation toward best dispersion of filler, and since synthesizing a totally agglomerated-free nanocomposite is difficult, filler and matrix interaction needs to be perfectly modeled. A micromechanical model is developed in this study based on the common Halpin-Tsai theory to predict the elastic stiffness of vinyl ester/exfoliated graphite platelet nanocomposites. The model considers near-rational ideal (uniformly dispersed) mixed with clustered filler-network to simulate filler-distribution conditions. A filler-dispersion level based on the filler concentration has been proposed mathematically in this study. Predictions of the proposed model considering filler morphology were compared with the predictions of the Halpin-Tsai model and the experimentally obtained results as well. The proposed model shows better accuracy in terms of stiffness over predictions of the HalpinTsai model and appears in a very good agreement with the experimental results obtained for vinyl ester nanocomposites.

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A micromechanical model using properties of both polymer and filler is proposed to address creep response in case of nanocomposites consisting of graphite nanoparticles embedded in a polymer matrix. Empirical creep models like Burgers and Findley models have been frequently used for creep simulation of polymeric-based nanocomposites. An elastic stiffness model has been established first to simulate stiffness property of nanocomposites with emphasis on filler distribution conditions. Creep stiffness is then predicted from the outstanding stiffness model based on the linear elastic-viscoelastic principle. In the model development, a parameter termed as additional constraint stiffness factor has been defined in the model to account for the constraining effect played by nanoparticles on the polymer chains. Magnitude of constraint factor indicates performance of nanoparticles in preventing polymer chains motion under creep loading of the nanocomposite. Creep predictions of the model with and without effect of the additional constraint stiffness factor are compared with experimental results obtained for vinyl ester reinforced nanocomposites. Predictions of the model disregarding the additional constraint stiffness factor overestimate the experimental results and do not provide good agreement. For each of the studied nanocomposites, the present model was able to provide creep response according to different filler percentage and different temperature along with the same stiffness function and similar constraint factors, in each case.

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