Dynamical effects induced by long range activation in a nonequilibrium reaction-diffusion system.

We show both experimentally and numerically that the time scales separation introduced by long range activation can induce oscillations and excitability in nonequilibrium reaction-diffusion systems that would otherwise only exhibit bistability. Namely, we show that in the chlorite-tetrathionate reaction, where the autocatalytic species H+ diffuses faster than the substrates, the spatial bistability domain in the nonequilibrium phase diagram is extended with oscillatory and excitability domains. A simple model and a more realistic model qualitatively account for the observed dynamical behavior. The latter model provides quantitative agreement with the experiments.

Spatial bistability and waves in a reaction with acid autocatalysis.

The phenomenon of spatial bistability has recently been proposed for a comprehensive understanding of a number of chemical patterns observed in open spatial reactors consisting of thin films of gel diffusively fed from one side. We study experimentally and numerically this phenomenon in the tetrathionate-chlorite reaction characterized by an acid superautocatalysis. We focus on the similarities and differences with previous studies on the chlorine dioxide-iodide reaction. In addition, we show that this reaction, which is only bistable in a continuous stirred tank reactor, can exhibit oscillatory and traveling waves when diffusion comes into play. Our computations suggest that the nonstationary behaviour originates from differential diffusive transport.

Chemical morphogenesis: turing patterns in an experimental chemical system.

Patterns resulting from the sole interplay between reaction and diffusion are probably involved in certain stages of morphogenesis in biological systems, as initially proposed by Alan Turing. Self-organization phenomena of this type can only develop in nonlinear systems (i.e. involving positive and negative feedback loops) maintained far from equilibrium. We present Turing patterns experimentally observed in a chemical system. An oscillating chemical reaction, the CIMA reaction, is operated in an open spatial reactor designed in order to obtain a pure reaction-diffusion system. The two types of Turing patterns observed, hexagonal arrays of spots and parallel stripes, are characterized by an intrinsic wavelength. We identify the origin of the necessary diffusivity between activator and inhibitor. We also describe a pattern growth mechanism by spot splitting that recalls cell division.

Adsorption of plasma proteins onto anticoagulant polystyrene derivatives: a fluorescence study.

Quenching of fluorescence was used to monitor adsorption of thrombin (T), antithrombin (AT) and their inactive complex (T-AT) onto three anticoagulant biomaterials made of polystyrene beads bearing the functional groups of heparin. An adsorption capacity of 0.12 mumol of T per mg of polymer allowed the formation of a monolayer of protein at the polymer surface. An affinity constant of 3 x 10(7) l.mol-1 between thrombin and polymer was estimated, whatever the polymer used. The affinity of T-AT was similar although weaker. Desorption of proteins from the polymeric interface by means of polycations (polybrene and polylysine) showed that the inactive complex T-AT is more quantitatively and easily released than thrombin.

[Oscillations in population densities of the bacterial prey-predator couple Escherichia coli-Bdellovibrio bacteriovorus: experimental study and theoretical model]. / Oscillations des densités de population du couple bactérien proie-prédateur escherichia coli-Bdellovibrio bacteriovorus: étude expérimentale et modèle théorique.

Oscillations in population densities in the bacterial predator-prey Bdellovibrio bacteriovorus-Escherichia coli system were investigated both experimentally and theoretically. Experimental conditions for observing (damped) oscillations were first determined in a closed system and then used in an open system, i.e. in a chemostat, when an adequate flux of nutritive medium was added. The experimentally observed oscillations were always unstable with poorly reproducible amplitude and period. A theoretical model was used in order to explain this behaviour. It was first presented and satisfactorily tested for the same bacterial couple operating in a closed system, and allowed an experimental determination of its kinetic parameters. When adapted to open system conditions, it yielded computer-simulations which showed oscillations of the population densities in good agreement with those experimentally observed. It also showed that stable oscillations were not possible, the only "focus" in the predator-prey-density plane being an unstable one with no surrounding limit cycle.

Experimental study of synchronization phenomena under periodic light irradiation of a nonlinear chemical system.

A photosensitive chemical oscillating reaction, i.e., the Briggs-Rauscher (B.R.) reaction, exhibiting a wealth of nonlinear behavior, when performed in a continuous-flow stirred-tank reactor, and subjected to periodic light irradiation, is studied as an experimental example of entrainment phenomena observable in biological systems. The adaptation patterns under periodic light irradiation are elucidated by means of the response of the system to continuous and single-pulse light irradiation. It is shown that self-oscillating states, excitable steady states and bistable systems can exhibit the same types of synchronization patterns when submitted to periodic external forces with appropriate amplitude and time scale conditions.

[Periodic changes of populations in a prey-predator system: Escherichia coli-Bdellovibrio bacteriovorus]. / Variations périodiques de populations dans un système proieprédateur: Escherichia coli-Bdellovibrio bacteriovorus

The prey-predator system Escherichia coli-Bdellovibrio bacterivorus was investigated in a very poor medium. Optical density recording and numerations of the bacterial species showed synchronous oscillations of the concentration of both microbial populations.

Kinetics of a bacterial culture growth: validity of the affinity rule in biological systems.

The kinetic study of a process is usually performed by measuring a convenient intensive property, P, as a function of time. The "affinity rule" states that, when a given process takes place under different external constraints (e.g., different temperatures, pressures, pH values, etc.), the various P versus time curves are related by an affinity transformation parallel to the time axis: in other words the P versus log time curves are parallel and can be superimposed by translation. The validity of the rule has been extensively tested in chemical and physiochemical processes, but there is no evidence as yet that it extends to biological systems. The present paper shows that the rule is indeed valid for the kinetics of growth of an Escherichia coli culture at various temperatures and pH values. More extended experiments are necessary to prove or disprove the general validity of the rule in biological systems, but its practical interest is evident: whenever it is valid it will be possible, from a very small number of measurements, to predict the complete behavior of the system in a number of various external conditions