Wave reflection in a reaction-diffusion system: breathing patterns and attenuation of the echo.

Formation and interaction of the one-dimensional excitation waves in a reaction-diffusion system with the piecewise linear reaction functions of the Tonnelier-Gerstner type are studied. We show that there exists a parameter region where the established regime of wave propagation depends on initial conditions. Wave phenomena with a complex behavior are found: (i) the reflection of waves at a growing distance (the remote reflection) upon their collision with each other or with no-flux boundaries and (ii) the periodic transformation of waves with the jumping from one regime of wave propagation to another (the periodic trigger wave).

Biological factors underlying regularity and chaos in aquatic ecosystems: simple models of complex dynamics.

This work is focused on the processes underlying the dynamics of spatially inhomogeneous plankton communities. We demonstrate that reaction-diffusion mathematical models are an appropriate tool for searching and understanding basic mechanisms of complex spatio-temporal plankton dynamics and fractal properties ofplanktivorous fish school walks

The mechanism of fractal-like structure formation by bacterial populations.

Three types of population growth and development of chemotaxic motile bacteria Escherichia coli on semi-solid nutrient media are investigated: a) stable development - circular symmetrical waves; b) bursts; c) fractal-like self-organization. Experimental investigation of the burst formation is presented. The microscopic analysis of growing, fractal-like structures is carried out, and a mechanism for such structure formation is suggested. It is supposed that fractal-like bacterial structures growth is based on the principle of successively forming multiple micro-bursts. A mathematical model has been suggested to reproduce the experimental results. The structures obtained by numerical modeling of population growth in the parameter space 'substrate concentration - bacterial movement rate' reproduce the corresponding experimental structures in the space 'nutrient concentration in the media - the density of the media'.

Structural organization and phase behavior of DNA-calcium-dipalmitoylphosphatidylcholine complex.

Freeze-fracture study of ultrastructure of DNA--calcium--dipalmitoylphosphatidylcholine (DPPC) complex was carried out at different temperatures. For high-speed cryofixation from controllable initial temperatures, a special thermostatic chamber was designed. The fracture surface of the complex was found to be considerably different from the initial DPPC liposomes: 1) the period of ripple phase was 25 nm in contrast to 15 nm for control samples; 2) the ripple phase was observed at temperatures ranging from 6 degreesC to lipid melting temperature; 3) at temperature above the lipid melting unordered worm-like folds were formed on the fracture surface. Their length was correlated with the length of DNA fragments used in the experiment. We suppose that DNA molecules adsorbed on the membrane surface were segregated to clusters, resulting in formation of a new phase with specific structure and properties.

Liposomes in gene therapy. Structural polymorphism of lipids and effectiveness of gene delivery.

The effective gene delivery to target cells is a basic challenge of gene therapy. In this review attention is focused on liposomal vehicles. They represent not only an alternative, but also an extension to the other methods of transfection where both biological and synthetic materials are utilized. The structural and phase transformations of lipids are generally inherent to nature and can be applied in practice. The liposomes could serve as a specific matrix for materials composed of proteins, polysaccharides, and various synthetic polymers. The investigation of structural polymorphism of lipids and DNA--lipid complexes is important for designing of effective carriers of genes and drugs.

DNA initiates polymorphic structural transitions in lecithin.

The inverted micellar phase, obtained by treating lecithin and Ca(2+)-DNA complex with chloroform, was used as an intermediate step in the preparation of DNA-Ca(2+)-lecithin complex. DSC analysis demonstrated the involvement of a large fraction of lipid in the interaction with DNA. Freeze-fracture electron microscopy revealed (i) rod-like structures on the hydrophobic fracture surface of membranes and (ii) regular bundles of fibrils with a repeat distance of about 6 nm, which were located free in solution. Similar regular bundles of fibrils were also revealed by staining the samples with uranyl acetate. According to the suggested model, the observed structures are hexagonally packed inverted lipid tubes, with DNA located in their central cores. The possible biological relevance of the capability of Ca(2+)-DNA to initiate polymorphic phase transitions of lecithin is discussed.

Lysis of Escherichia coli cells induced by bacteriophage T4.

Structural changes in the envelope of Escherichia coli cells accompanying their lysis from without by bacteriophage T4 have been studied. The hypothesis concerning the role of collapse of membrane potential and formation of periplasmic vesicles in the process of lysis from without has been advanced.

Instability of waves formed by motile bacteria.

Many motile chemotactic bacteria (for instance, Escherichia coli) inoculated at some point in a semisolid nutrient medium can form circular expanding population waves. The formation of these motile structures is due to chemotaxis. The circular waves originate from an expanding bacterial lawn (a parent population). The regular shape of these waves results from the isotropic distribution of freely diffusible nutrient molecules which are also attractants. In this paper we show that the regular shape of the bacterial population waves can be spontaneously disturbed. As this takes place arc-shaped population waves ('bursts') are formed. It was found that initially the mean length of the cells forming the bursts was greater than that of the parent cell population. But then it decreased resulting in a value characteristic of the parent population.