Many bacterial species use flagella for self-propulsion in aqueous media. In the soil, which is a complex and structured environment, water is found in microscopic channels where viscosity and water potential depend on the composition of the soil solution and the degree of soil water saturation. Therefore, the motility of soil bacteria might have special requirements. An important soil bacterial genus is Bradyrhizobium, with species that possess one flagellar system and others with two different flagellar systems. Among the latter is B. diazoefficiens, which may express its subpolar and lateral flagella simultaneously in liquid medium, although its swimming behaviour was not described yet. These two flagellar systems were observed here as functionally integrated in a swimming performance that emerged as an epistatic interaction between those appendages. In addition, each flagellum seemed engaged in a particular task that might be required for swimming oriented toward chemoattractants near the soil inner surfaces at viscosities that may occur after the loss of soil gravitational water. Because the possession of two flagellar systems is not general in Bradyrhizobium or in related genera that coexist in the same environment, there may be an adaptive tradeoff between energetic costs and ecological benefits among these different species.

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The method of Riemannian geometry has been successful in the context of equilibrium thermodynamics. In this work, we extend this approach to non-equilibrium processes. As a geometric-differential frame of non-equilibrium systems, we consider in our study the geometric properties of a manifold associated with simple but typical non-equilibrium models. We consider a Uhlenbeck-Ornstein process and the formal structure of the probability density function solution of the Fokker-Planck equation. We propose a general geometric strategy for the construction of macroscopic potentials in non-equilibrium problems. This macroscopic potential is a function of the transport coefficient and is associated with system instabilities.

Metabolic control of glutamine and glutamate synthesis from ammonia and oxoglutarate in Escherichia coli is tight and complex. In this work, the role of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) regulation in this control was studied. Both enzymes form a linear pathway, which can also have a cyclic topology if glutamate-oxoglutarate amino transferase (GOGAT) activity is included. We modelled the metabolic pathways in the linear or cyclic topologies using a coupled nonlinear differential equations system. To simulate GS regulation by covalent modification, we introduced a relationship that took into account the levels of oxoglutarate and glutamine as signal inputs, as well as the ultrasensitive response of enzyme adenylylation. Thus, by including this relationship or not, we were able to model the system with or without GS regulation. In addition, GS and GDH activities were changed manually. The response of the model in different stationary states, or under the influence of N-input exhaustion or oscillation, was analyzed in both pathway topologies. Our results indicate a metabolic control coefficient for GDH ranging from 0.94 in the linear pathway with GS regulation to 0.24 in the cyclic pathway without regulation, employing a default GDH concentration of 8 microM. Thus, in these conditions, GDH seemed to have a high degree of control in the linear pathway while having limited influence in the cyclic one. When GS was regulated, system responses to N-input perturbations were more sensitive, especially in the cyclic pathway. Furthermore, we found that effects of regulation against perturbations depended on the relative values of the glutamine and glutamate output first-order kinetic constants, which we named k(6) and k(7), respectively. Effects of regulation grew exponentially with a factor around 2, with linear increases of (k(7) - k(6)). These trends were sustained but with lower differences at higher GS concentration. Hence, GS regulation seemed important for metabolic stability in a changing environment, depending on the cell's metabolic status.

We consider a multicomponent ensemble of charged fermions which are constrained to move on the plane. By just retaining particle-particle ladder diagrams in Goldstone's expression for the energy shift and approximating the kernel of the resultant integral equation, we obtain for this system an analytical Yasuhara-like formula for the contact pair correlations that includes screening effects. An undesirable pseudo-screening due to the kernel approximation is overcome by adding a corrective term into the integral equation. In particular, we use the case in which the system has only two species of opposite charge to model a quantum well as a two-dimensional electron-hole plasma. The electron-hole correlation at contact so calculated is taken as the enhancement factor in the photoluminescence from the quantum well and it is checked against available experimental data for the density dependence of the electron-hole plasma lifetime.

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