Threshold effects for two pathogens spreading on a network

Diseases spread through host populations over the networks of contacts between individuals, and a number of results about this process have been derived in recent years by exploiting connections between epidemic processes and bond percolation on networks. Here we investigate the case of two pathogens in a single population, which has been the subject of recent interest among epidemiologists. We demonstrate that two pathogens competing for the same hosts can both spread through a population only for intermediate values of the bond occupation probability that lie above the classic epidemic threshold and below a second higher value, which we call the coexistence threshold, corresponding to a distinct topological phase transition in networked systems.Comment: 5 pages, 2 figure

Dissection of Nodule Development by Supplementation of \u3cem\u3eRhizobium leguminosarum\u3c/em\u3e biovar \u3cem\u3ephaseoli\u3c/em\u3e Purine Auxotrophs with 4-Aminoimidazole-5-Carboxamide Riboside

Purine auxotrophs of Rhizobium leguminosarum biovar phaseoli CFN42 elicit uninfected pseudonodules on bean (Phaseolus vulgaris L.). Addition of 4-aminoimidazole-5-carboxamide (AICA) riboside to the root medium during incubation of the plant with these mutants leads to enhanced nodule development, although nitrogenase activity is not detected. Nodules elicited in this manner had infection threads and anatomical features characteristic of normal nodules, such as peripheral vasculature rather than the central vasculature of the pseudonodules that were elicited without AICA riboside supplementation. Although 105 to 106 bacteria could be recovered from these nodules after full development, bacteria were not observed in the interior nodule cells. Instead, large cells with extensive internal membranes were present. Approximately 5% of the normal amount of leghemoglobin and 10% of the normal amount of uricase were detected in these nodules. To promote the development of true nodules rather than pseudonodules, AICA riboside was required no later than the second day through no more than the sixth day following inoculation. After this period, removal of AICA riboside from the root medium did not prevent the formation of true nodules. This observation suggests that there is a critical stage of infection, reached before nodule emergence, at which development becomes committed to forming a true nodule rather than a pseudonodule

Uniqueness of Ground States for Short-Range Spin Glasses in the Half-Plane

We consider the Edwards-Anderson Ising spin glass model on the half-plane $Z \times Z^+$ with zero external field and a wide range of choices, including mean zero Gaussian, for the common distribution of the collection J of i.i.d. nearest neighbor couplings. The infinite-volume joint distribution $K(J,\alpha)$ of couplings J and ground state pairs $\alpha$ with periodic (respectively, free) boundary conditions in the horizontal (respectively, vertical) coordinate is shown to exist without need for subsequence limits. Our main result is that for almost every J, the conditional distribution $K(\alpha|J)$ is supported on a single ground state pair.Comment: 20 pages, 3 figure

From iron oxides to infections

I was fortunate to have met Terry Beveridge early during my time in graduate school, when my advisor, Francois Morel (a geochemist), recognized the importance of my having a microbiologist on my PhD advising committee. At the time, I was studying microbial precipitation of arsenic trisulfide (As_2S_3) by Desulfotomaculum auripigmentum and needed the help of a geomicrobiologist to take electron micrographs of my samples (Newman et al., 1997). What better person to ask than Terry Beveridge? Not knowing much about what I was doing, I naively went off to Guelph to spend a week working in Terry’s laboratory during the fall of 1994. Little did I know that this would be the start of one of the most inspirational scientific and personal relationships of my career. From that week forward, I turned to Terry whenever I had a question about microbiology, and looked forward to his responses: Terry had a way of making me feel like I had hit upon something profound (when in fact my questions were pretty trivial), and his answers not only were thoughtful and informed, but had the effect of getting me to think about other problems that were much more interesting

Evolution of Spatially Inhomogeneous Eco-Systems: An Unified Model Based Approach

Recently we have extended our the "unified" model of evolutionary ecology to incorporate the {\it spatial inhomogeneities} of the eco-system and the {\it migration} of individual organisms from one patch to another within the same eco-system. In this paper an extension of our recent model is investigated so as to describe the {\it migration} and {\it speciation} in a more realistic way.Comment: Latex, 10 pages, 8 figure

Infection of Soybean and Pea Nodules by \u3cem\u3eRhizobium\u3c/em\u3e spp. Purine Auxotrophs in the Presence of 5-aminoimidazole-4-Carboxamide Riboside

Purine auxotrophs of various Rhizobium species are symbiotically defective, usually unable to initiate or complete the infection process. Earlier studies demonstrated that, in the Rhizobium etli-bean symbiosis, infection by purine auxotrophs is partially restored by supplementation of the plant medium with 5-amino-imidazole-4-carboxamide (AICA) riboside, the unphosphorylated form of the purine biosynthetic intermediate AICAR. The addition of purine to the root environment does not have this effect. In this study, purine auxotrophs of Rhizobium fredii HH303 and Rhizobium leguminosarum 128C56 (bv. viciae) were examined. Nutritional and genetic characterization indicated that each mutant was blocked in purine biosynthesis prior to the production of AICAR. R. fredii HH303 and R. leguminosarum 128C56 appeared to be deficient in AICA riboside transport and/or conversion into AICAR, and the auxotrophs derived from them grew very poorly with AICA riboside as a purine source. All of the auxotrophs elicited poorly developed, uninfected nodules on their appropriate hosts. On peas, addition of AICA riboside or purine to the root environment led to enhanced nodulation; however, infection threads were observed only in the presence of AICA riboside. On soybeans, only AICA riboside was effective in enhancing nodulation and promoting infection. Although AICA riboside supplementation of the auxotrophs led to infection thread development on both hosts, the numbers of bacteria recovered from the nodules were still 2 or more orders of magnitude lower than in fully developed nodules populated by wild-type bacteria. The ability to AICA riboside to promote infection by purine auxotrophs, despite serving as a very poor purine source for these strains, supports the hypothesis that AICAR plays a role in infection other than merely promoting bacterial growth

Realistic spin glasses below eight dimensions: a highly disordered view

By connecting realistic spin glass models at low temperature to the highly disordered model at zero temperature, we argue that ordinary Edwards-Anderson spin glasses below eight dimensions have at most a single pair of physically relevant pure states at nonzero low temperature. Less likely scenarios that evade this conclusion are also discussed.Comment: 18 pages (RevTeX; 1 figure; to appear in Physical Review E

Photomixotrophic growth of Rhodobacter capsulatus SB1003 on ferrous iron

This study investigates the role iron oxidation plays in the purple non-sulfur bacterium Rhodobacter capsulatus SB1003. This organism is unable to grow photoautotrophically on unchelated ferrous iron [Fe(II)] despite its ability to oxidize chelated Fe(II). This apparent paradox was partly resolved by the discovery that SB1003 can grow photoheterotrophically on the photochemical breakdown products of certain ferric iron–ligand complexes, yet whether it could concomitantly benefit from the oxidation of Fe(II) to fix CO_2 was unknown. Here, we examine carbon fixation by stable isotope labeling of the inorganic carbon pool in cultures growing phototrophically on acetate with and without Fe(II). We show that R. capsulatus SB1003, an organism formally thought incapable of phototrophic growth on Fe(II), can actually harness the reducing power of this substrate and grow photomixotrophically, deriving carbon both from organic sources and from fixation of inorganic carbon. This suggests the possibility of a wider occurrence of photoferrotrophy than previously assumed

A Variational Principle for the Asymptotic Speed of Fronts of the Density Dependent Diffusion--Reaction Equation

We show that the minimal speed for the existence of monotonic fronts of the equation $u_t = (u^m)_{xx} + f(u)$ with $f(0) = f(1) = 0$, $m >1$ and $f>0$ in $(0,1)$ derives from a variational principle. The variational principle allows to calculate, in principle, the exact speed for arbitrary $f$. The case $m=1$ when $f'(0)=0$ is included as an extension of the results.Comment: Latex, postcript figure availabl