Edwardsiella Comparative Phylogenomics Reveal the New Intra/Inter-Species Taxonomic Relationships, Virulence Evolution and Niche Adaptation Mechanisms

Edwardsiella bacteria are leading fish pathogens causing huge losses to aquaculture industries worldwide. E. tarda is a broad-host range pathogen that infects more than 20 species of fish and other animals including humans while E. ictaluri is host-adapted to channel catfish causing enteric septicemia of catfish (ESC). Thus, these two species consist of a useful comparative system for studying the intricacies of pathogen evolution. Here we present for the first time the phylogenomic comparisons of 8 genomes of E. tarda and E. ictaluri isolates. Genome-based phylogenetic analysis revealed that E. tarda could be separate into two kinds of genotypes (genotype I, EdwGI and genotype II, EdwGII) based on the sequence similarity. E. tarda strains of EdwGI were clustered together with the E. ictaluri lineage and showed low sequence conservation to E. tarda strains of EdwGII. Multilocus sequence analysis (MLSA) of 48 distinct Edwardsiella strains also supports the new taxonomic relationship of the lineages. We identified the type III and VI secretion systems (T3SS and T6SS) as well as iron scavenging related genes that fulfilled the criteria of a key evolutionary factor likely facilitating the virulence evolution and adaptation to a broad range of hosts in EdwGI E. tarda. The surface structure-related genes may underlie the adaptive evolution of E. ictaluri in the host specification processes. Virulence and competition assays of the null mutants of the representative genes experimentally confirmed their contributive roles in the evolution/niche adaptive processes. We also reconstructed the hypothetical evolutionary pathway to highlight the virulence evolution and niche adaptation mechanisms of Edwardsiella. This study may facilitate the development of diagnostics, vaccines, and therapeutics for this under-studied pathogen

Boundary element method based computation of stress intensity factor by modified crack closure integral

The local smoothing scheme in conjunction with the modified crack closure integral technique has been adopted in the boundary element method to improve the accuracy of computed stress intensity factors. Simple relations have been derived for the case of linear, quadratic and quarter point elements around the crack tip. Case studies are presented to demonstrate improvement in the accuracy. While the displacement method gives a difference with the standard handbook solution up to 26%, the suggested method helps to reduce it to within 2%

Effect of modelling of traction and thermal singularities on accuracy of SIFS computation through modified crack closure integral in BEM

The effect of simultaneous modelling of both strain and traction singularities, and temperature derivative and heat flux singularities at the crack rip, on the accuracy of computation of stress intensity factors (SIFs) based on the modified crack closure integral in boundary element method is presented. Simple relations are given for SIF calculations. Results on mode I, mode II and mixed mode, crack subjected to mechanical and/or thermal loading are studied to illustrate the difference between partial and total modelling of the singularities. The dependence of accuracy of the SIFs on the crack tip element size is examined. The effect of order of Gaussian quadrature on the accuracy is also reported. (C) 1999 .

Further considerations in modified crack closure integral based computation of stress intensity factor in BEM

This paper deals with modified crack closure integral based computation of stress intensity factors in a boundary element method for problems with mechanical loading remote from the crack edges. The modified crack closure integral technique has been coupled with the local smoothing scheme to obtain simple relations for energy release rates for linear;quadratic and quarter point elements around the crack tip. Stress intensity factors calculated through the proposed formulation and the displacement method for a number of examples are compared, wherever possible, with data available in the literature. The results based on the proposed scheme are more accurate than those obtained by the displacement method. (C) 1998

A review of sif evaluation and modelling of singularities in bem

Various boundary element method (BEM) based approaches to solve crack problems are discussed. The displacement method, J-integral method and the modified crack closure integral (MCCI) method for the evaluation of the stress intensity factors (SIFs) are reviewed. Effects of partial and total modelling of singularities on the accuracy of the results have been presented. Elements capable of partial and total modelling of the wellknown square root singularities, variable order singularities, neighbouring variable order singularities, etc., are also reviewed. Case studies are included to illustrate the effectiveness of the various methods of calculation of the SIFs and the performance of the special elements

Modified crack closure integral based computation of stress intensity factors for 2-D thermoelastic problems through boundary element method

A modified crack closure integral (MCCI) based computation of stress intensity factors (SIFs) for thermal loading through boundary element method (BEM) is presented. Simple relations are given for the determination of stress intensity factors (SIFs) using the BEM results for linear, quadratic and quarter point elements employed around the crack tip. Examples of crack under mode I, mode II and mixed mode thermal and/or mechanical loading are examined. The computed SIFs are compared wherever possible with solutions available in the literature. The agreement is good. The effect of crack tip element size on the accuracy of results is reported. (C) 1999 Elsevier Science S.A.

BEM based evaluation of SIFs using modified crack closure integral technique under remote and/or crack edge loading

This paper deals with the boundary element method based evaluation of stress intensity factors for mode I and mixed mode problems under remote and/or crack edge loading. The modified crack closure integral technique has been used to enhance the accuracy of the computed stress intensity factors. Simple, ready to use, relations for the strain energy release rate have been obtained corresponding to linear, quadratic and quarter point elements surrounding the crack tip. The boundary element method is employed by breaking a domain into two subregions for the mixed mode problems. Case studies involving remote or crack edge loading for both mode I and mixed mode problems are presented to demonstrate the accuracy of the new scheme. The stress intensity factors based on the proposed scheme show better agreement with the standard solutions available in the literature than those obtained directly through the displacement method. (C) 1998 .