A single-component multidrug transporter of the major facilitator superfamily is part of a network that protects Escherichia coli from bile salt stress

Resistance to high concentrations of bile salts in the human intestinal tract is vital for the survival of enteric bacteria such as Escherichia coli. Although the tripartite AcrAB–TolC efflux system plays a significant role in this resistance, it is purported that other efflux pumps must also be involved. We provide evidence from a comprehensive suite of experiments performed at two different pH values (7.2 and 6.0) that reflect pH conditions that E. coli may encounter in human gut that MdtM, a single-component multidrug resistance transporter of the major facilitator superfamily, functions in bile salt resistance in E. coli by catalysing secondary active transport of bile salts out of the cell cytoplasm. Furthermore, assays performed on a chromosomal ΔacrB mutant transformed with multicopy plasmid encoding MdtM suggested a functional synergism between the single-component MdtM transporter and the tripartite AcrAB–TolC system that results in a multiplicative effect on resistance. Substrate binding experiments performed on purified MdtM demonstrated that the transporter binds to cholate and deoxycholate with micromolar affinity, and transport assays performed on inverted vesicles confirmed the capacity of MdtM to catalyse electrogenic bile salt/H+ antiport

Why do top predators engage in superpredation? FRom an empirical scenario to a theoretical framework

Lethal interactions can shape ecosystem structure, and consequently understanding their causes is ecologically relevant. To improve both empirical and theoretical knowledge on superpredation (i.e. predation on high-order predators), we studied an eagle owl population, including its main prey and mesopredators, and then we crossed these results with existing theories to provide a reasoning framework. We ftted our feld data into four main causes explaining lethal interactions: food stress, opportunistic superpredation, removal of a competitor, and removal of a potential threat. Empirically, superpredation seemed to be mostly determined by the combination of the food-stress and opportunistic-superpredation hypotheses, which highlights the complexity of the factors triggering superpredation. Therefore, besides being a response to lower food availability, superpredation may also represent an effective mechanism to remove potential predators and/or competitors, either intentionally or not. Our theoretical framework focused on the decision-making process in superpredation, considering four inter-related stages: encountering; attacking; and capturing a mesopredator; as well as consuming a mesopredator once killed. Superpredation almost certainly results from a complex process of decision-making, accounting for costs and benefts assessed moment-to-moment and for each mesopredator individual. It is time to build bridges between theoretical and empirical studies to further understand the mechanisms driving complex interactions among top predators and mesopredators

Exposure to nitric oxide protects against oxidative damage but increases the labile iron pool in sorghum embryonic axes

Sodium nitroprusside (SNP) and diethylenetriamine NONOate (DETA NONOate), were used as the source of exogenous NO to study the effect of NO upon germination of sorghum (Sorghum bicolor (L.) Moench) seeds through its possible interaction with iron. Modulation of cellular Fe status could be an important factor for the establishment of oxidative stress and the regulation of plant physiology. Fresh and dry weights of the embryonic axes were significantly increased in the presence of 0.1 mM SNP, as compared to control. Spin trapping EPR was used to assess the NO content in axes from control seeds after 24 h of imbibition (2.4±0.2 nmol NO g−1 FW) and seeds exposed to 0.01, 0.1, and 1 mM SNP (3.1±0.3, 4.6±0.2, and 6.0±0.9 nmol NO g−1 FW, respectively) and 1 mM DETA NONOate (6.2±0.6 nmol NO g−1 FW). Incubation of seeds with 1 mM SNP protected against oxidative damage to lipids and maintained membrane integrity. The content of the deferoxamine–Fe (III) complex significantly increased in homogenates of axes excised from seeds incubated in the presence of 1 mM SNP or 1 mM DETA NONOate as compared to the control (19±2 nmol Fe g−1 FW, 15.2±0.5 nmol Fe g−1 FW, and 8±1 nmol Fe g−1 FW, respectively), whereas total Fe content in the axes was not affected by the NO donor exposure. Data presented here provide experimental evidence to support the hypothesis that increased availability of NO drives not only protective effects to biomacromolecules, but to increasing the Fe availability for promoting cellular development as well

Towards Rational Deployment of Multiple Heuristics in A* (Extended Abstract)

In this paper we discuss and experiment with Lazy A*, a variant of A* where heuristics are evaluated lazily and with Rational Lazy A*, which decides whether to compute the more expensive heuristics at all, based on a myopic value of information estimate. Full version appears in IJCAI-2013

Partial-Expansion A* with Selective Node Generation

A* is often described as being 'optimal,' in that it expands the minimum number of unique nodes. But, A* may generate many extra nodes which are never expanded. This is a performance loss, especially when the branching factor is large. Partial Expansion A* (PEA*) addresses this problem when expanding a node, n, by generating all the children of n but only storing children with the same f-cost as n. We introduce an enhanced version of PEA* (EPEA*). Given a priori domain knowledge, EPEA* only generates the children with the same f-cost as the parent. State-of-the-art results were obtained for a number of domains. Drawbacks of EPEA* are also discussed. A full version of this paper appears in the proceedings of AAAI-201