Pseudospectra and stability radii of analytic matrix functions with application to time-delay systems

AbstractDefinitions for pseudospectra and stability radii of an analytic matrix function are given, where the structure of the function is exploited. Various perturbation measures are considered and computationally tractable formulae are derived. The results are applied to a class of retarded delay differential equations. Special properties of the pseudospectra of such equations are determined and illustrated

Role of lysozyme inhibitors in the virulence of avian pathogenic Escherichia coli

Lysozymes are key effectors of the animal innate immunity system that kill bacteria by hydrolyzing peptidoglycan, their major cell wall constituent. Recently, specific inhibitors of the three major lysozyme families occuring in the animal kingdom (c-, g- and i-type) have been discovered in Gram-negative bacteria, and it has been proposed that these may help bacteria to evade lysozyme mediated lysis during interaction with an animal host. Escherichia coli produces two inhibitors that are specific for c-type lysozyme (Ivy, Inhibitor of vertebrate lysozyme; MliC, membrane bound lysozyme inhibitor of c-type lysozyme), and one specific for g-type lysozyme (PliG, periplasmic lysozyme inhibitor of g-type lysozyme). Here, we investigated the role of these lysozyme inhibitors in virulence of Avian Pathogenic E. coli (APEC) using a serum resistance test and a subcutaneous chicken infection model. Knock-out of mliC caused a strong reduction in serum resistance and in in vivo virulence that could be fully restored by genetic complementation, whereas ivy and pliG could be knocked out without effect on serum resistance and virulence. This is the first in vivo evidence for the involvement of lysozyme inhibitors in bacterial virulence. Remarkably, the virulence of a ivy mliC double knock-out strain was restored to almost wild-type level, and this strain also had a substantial residual periplasmic lysozyme inhibitory activity that was higher than that of the single knock-out strains. This suggests the existence of an additional periplasmic lysozyme inhibitor in this strain, and indicates a regulatory interaction in the expression of the different inhibitors

RpoS-independent evolution reveals the importance of attenuated cAMPCRP regulation in high hydrostatic pressure resistance acquisition in E. coli

High hydrostatic pressure (HHP) processing is an attractive non-thermal alternative to food pasteurization. Nevertheless, the large inter- and intra-species variations in HHP resistance among foodborne pathogens and the ease by which they can acquire extreme resistance are an issue of increasing concern. Since RpoS activity has been considered as a central determinant in the HHP resistance of E. coli and its pathovars, this study probed for the potential of an E. coli MG1655 ΔrpoS mutant to acquire HHP resistance by directed evolution. Despite the higher initial HHP sensitivity of the ΔrpoS mutant compared to the wild-type strain, evolved lineages of the former readily managed to restore or even succeed wild-type levels of resistance. A number of these ΔrpoS derivatives were affected in cAMP/CRP regulation, and this could be causally related to their HHP resistance. Subsequent inspection revealed that some of previously isolated HHP-resistant mutants derived from the wild-type strain also incurred a causal decrease in cAMP/CRP regulation. cAMP/CRP attenuated HHP-resistant mutants also exhibited higher resistance to fosfomycin, a preferred treatment for STEC infections. As such, this study reveals attenuation of cAMP/CRP regulation as a relevant and RpoS-independent evolutionary route towards HHP resistance in E. coli that coincides with fosfomycin resistance

Does virulence assessment of Vibrio anguillarum using sea bass (Dicentrarchus labrax) larvae correspond with genotypic and phenotypic characterization?

Background: Vibriosis is one of the most ubiquitous fish diseases caused by bacteria belonging to the genus Vibrio such as Vibrio (Listonella) anguillarum. Despite a lot of research efforts, the virulence factors and mechanism of V. anguillarum are still insufficiently known, in part because of the lack of standardized virulence assays. Methodology/Principal Findings: We investigated and compared the virulence of 15 V. anguillarum strains obtained from different hosts or non-host niches using a standardized gnotobiotic bioassay with European sea bass (Dicentrarchus labrax L.) larvae as model hosts. In addition, to assess potential relationships between virulence and genotypic and phenotypic characteristics, the strains were characterized by random amplified polymorphic DNA (RAPD) and repetitive extragenic palindromic PCR (rep-PCR) analyses, as well as by phenotypic analyses using Biolog's Phenotype MicroArray (TM) technology and some virulence factor assays. Conclusions/Significance: Virulence testing revealed ten virulent and five avirulent strains. While some relation could be established between serotype, genotype and phenotype, no relation was found between virulence and genotypic or phenotypic characteristics, illustrating the complexity of V. anguillarum virulence. Moreover, the standardized gnotobiotic system used in this study has proven its strength as a model to assess and compare the virulence of different V. anguillarum strains in vivo. In this way, the bioassay contributes to the study of mechanisms underlying virulence in V. anguillarum