Formation of Ordered Patterns in Growing Colonies of Microorganisms
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Radiophysics and Quantum Electronics, Vol. 63, No. 1, June, 2020 (Russian Original Vol. 63, No. 1, January, 2020)
FORMATION OF ORDERED PATTERNS IN GROWING COLONIES OF MICROORGANISMS L. Sh. Tsimring ∗
UDC 577.359+579.262
In this work, we discuss the mechanisms of the ordered-pattern formation in the colonies of microorganisms, which are related to the instabilities of their collective motion. The nematic-order formation during the E.coli-bacterium growth inside an open microflow chamber is considered and intricate flower-like patterns, which emerge during the growth of the binary mixture of two types of bacteria (motile A. baylyi and non-motile E. coli) on the agar surface inside a Petri dish, are described. The experiments and the models, which explain the observed phenomena, are described in both cases.
1.
INTRODUCTION
In the late 1970s, under the guidance of L. A. Ostrovsky, the author started to study the instabilities of the hydrodynamic flows in application to the oceanographic problems. Our first joint work “Radiation instability of the shear flows in a stratified fluid” was published in 1981 [1]. Along with the subsequent work [2], the above-mentioned article laid the foundation for our cycle of works on the study of the properties of waves with negative energy. Their main results are reflected in review [3]. As is shown in [3], the waves with negative energy, which emerge only in the moving media, lead to the appearance of instabilities in the presence of dissipation. Later, the author dealt with the flows and their stability during the study of granulated media [4]. The situation in this field was complicated by the fact that the commonly accepted equations for describing the hydrodynamics of the granular media did not exist (they are not available even at present). Flow dynamics also proved to be important for the studies in biological physics, especially, description of nonlinear dynamics of the intracellular genetic chains. It plays an important role in the collective processes, which occur in the populations of the simplest unicellular organisms, i.e., bacteria. As it happens, in addition to collective biochemical processes (e.g., synchronous oscillations of the number of some proteins in the cells), motility, reproduction, and mechanical interaction of bacteria lead to their collective motion, which has the character of rather special flows on large scales. In some cases, these flows can become unstable and lead to the appearance of nontrivial patterns. It should be noted that studying these patterns is not only of academic interest. It appears that collective behavior gives a serious selective advantage to bacteria and helps them compete in their struggle for survival [5, 6]. Many bacteria types form biofilms on various surfaces to effectively fight stresses in an unfavorable environment [6–8]. In biofilms, bacteria tightly adjoin each other and interact not only by way of biochemical signaling or competing for limited feeding resources, but also via direct physical contacts. These contacts lead to the forma
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