Mechanisms and roles of the LuxS system, methyl recycling, and DNA methylation on the physiology of Campylobacter jejuni

Campylobacter jejuni is one of the leading causes of human bacterial gastroenteritis, and campylobacteriosis in sheep. The genetic diversity of this organism, the potential for multiple sources to transmit C. jejuni to humans, and the possession of a variety of virulence factors and antimicrobial resistance mechanisms make C. jejuni a serious health problem worldwide. The autoinducer-2 (AI-2)/LuxS system has been the focus of several studies for its potential applications to attenuate C. jejuni virulence. A study from our group found that the LuxS enzyme plays a critical role in virulence and fitness of C. jejuni IA3902 and 11168 strains. Mutagenesis of the luxS gene negatively impacted C. jejuni colonization of the gastrointestinal tract of several host species. However, the physiologic basis for this colonization defect is unclear. In addition to AI-2 production, LuxS is also a key enzyme involved in the activated methyl cycle (AMC). The AMC is an important source for the formation of S-adenosylmethionine, a methyl donor crucial to biological processes like DNA methylation. DNA methylation has also been linked with a diverse number of important physiological and pathogenic mechanisms in many bacteria, but is poorly understood in C. jejuni. The collective work from this thesis attempts to address some of the knowledge gaps on the role of LuxS, methyl recycling, and DNA methylation in C. jejuni physiology. Collectively, results from our study showed that luxS mutation interrupted the AMC resulting in significant changes to intracellular concentrations of several key metabolites. However, the colonization-associated factors tested on our luxS mutants in this thesis do not show evidence of being the primary mechanisms responsible for the luxS mutant\u27s decreased colonization ability. We proceeded to analyze the role of LuxS on DNA methylation and found that the luxS mutation had no appreciable effect on the methylome profile of the mutant. We also compared the methylome profiles of three important C. jejuni strains and found significant strain variability in the methylomes, which suggest a potential role for DNA methylation in Campylobacter pathobiology. The methylome studies also revealed a novel putative methyltransferase which we later confirmed and definitively assigned to a specific methylation motif. While mutagenesis of the methyltransferase gene resulted in a loss of methylation of its cognate motif we were unable to show an effect on the growth or motility phenotypes tested in our study. In summary, the luxS mutation demonstrated physiological effects on the AMC, but the colonization mechanisms affected by the mutation are still unknown. However, DNA methylation studies revealed strain-specific methylation profiles, including a unique methyltransferase, which may serve a biological and/or pathogenic purpose specific to the strain

Breaking a chaos-noise-based secure communication scheme

This paper studies the security of a secure communication scheme based on two discrete-time intermittently-chaotic systems synchronized via a common random driving signal. Some security defects of the scheme are revealed: 1) the key space can be remarkably reduced; 2) the decryption is insensitive to the mismatch of the secret key; 3) the key-generation process is insecure against known/chosen-plaintext attacks. The first two defects mean that the scheme is not secure enough against brute-force attacks, and the third one means that an attacker can easily break the cryptosystem by approximately estimating the secret key once he has a chance to access a fragment of the generated keystream. Yet it remains to be clarified if intermittent chaos could be used for designing secure chaotic cryptosystems.Comment: RevTeX4, 11 pages, 15 figure

Bilayer Hubbard model: Analysis based on the fermionic sign problem

The bilayer Hubbard model describes the antiferromagnet to spin singlet transition and, potentially, aspects of the physics of unconventional superconductors. Despite these important applications, significant aspects of its `phase diagram' in the interplane hopping $t_\perp$ versus on-site interaction $U$ parameter space, at half filling, are largely in disagreement. Here we provide an analysis making use of the average sign of weights over the course of the importance sampling in quantum Monte Carlo simulations to resolve several central open questions. Specifically, this metric of the weights clarifies the finite-sized metallic regimes at small $U$. Furthermore, at strong interactions, it points to the existence of a crossover from a correlated to uncorrelated band insulator not yet explored in a variety of existing, unbiased numerical methods. Our work demonstrates the versatility of using properties of the weights in quantum Monte Carlo simulations to reveal important physical characteristics of the models under study.Comment: 6+7 pages, 3+9 figures, update the figure

Simulations of cold electroweak baryogenesis: dependence on the source of CP-violation

We compute the baryon asymmetry created in a tachyonic electroweak symmetry breaking transition, focusing on the dependence on the source of effective CP-violation. Earlier simulations of Cold Electroweak Baryogenesis have almost exclusively considered a very specific CP-violating term explicitly biasing Chern-Simons number. We compare four different dimension six, scalar-gauge CP-violating terms, involving both the Higgs field and another dynamical scalar coupled to SU(2) or U(1) gauge fields. We find that for sensible values of parameters, all implementations can generate a baryon asymmetry consistent with observations, showing that baryogenesis is a generic outcome of a fast tachyonic electroweak transition