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Pattern formation in active matter and biology: bacterial mixtures and vegetation

Grant number: 21/10139-2
Support type:Scholarships in Brazil - Post-Doctorate
Effective date (Start): January 01, 2022
Effective date (End): November 30, 2022
Field of knowledge:Physical Sciences and Mathematics - Physics
Principal researcher:Nathan Jacob Berkovits
Grantee:Pablo Souza de Castro Melo
Home Institution: Instituto de Física Teórica (IFT). Universidade Estadual Paulista (UNESP). Campus de São Paulo. São Paulo , SP, Brazil
Associated research grant:16/01343-7 - ICTP South American Institute for Fundamental Research: a regional center for theoretical physics, AP.TEM


The emergence of complex biological functions depends on the spontaneous formation of spatiotemporal patterns between agents such as cells, plants, and animals. In the first part of this project, we will investigate the collective behavior of binary mixtures of bacterial cells and how they undergo cooperative or competitive pattern formation. We will focus on phenomena similar to so-called "motility-induced phase separation" found in collections of active (i.e., motile) particles like bacterial fluids, where persistent motion takes the role of "attractive forces" in generating agglomeration. It is known that, compared with systems of identical particles, the phenomenology of passive (i.e., nonmotile) mixtures is much richer. The study ofbacterial mixtures is therefore of great relevance as different bacterial species andstrains frequently coexist in Nature. This project aims to discover new features thatappear only for mixtures, e.g., the slow approach to stationary segregation incrowded environments, for which there are no results. Our research questions will beanswered via a combination of theoretical methods, simulations, and collaborationwith experimentalists.Initially, we will consider reciprocal mixtures where both types of bacteria have thesame motility properties, i.e., the same self-propulsion speeds and reorientation rates.As found in Biology, we will consider the scenario where the motility properties ofeach type are affected reciprocally by the presence of the other type. These"quorum-sensing" interactions can be achieved and tuned in the lab by geneticallymodifying each type's biochemical signalling; in doing so, their motility propertieswill depend on the concentration of highly-diffusive molecules produced exclusivelyby the other type. With this mechanism, it was shown that co-localization (or anti-localization) of the bacterial types emerges. Nonetheless, several questions remain open. One example is the environmental coupling that arises for bacterial mixtures compartmentalized in spatial niches. Another question is whether bacterial types, say, A and B, can regulate their motilities such that A moves at higherself-propulsion speed in the presence of B while B moves at lower self-propulsionspeed in the presence of A, effectively leading to a phenomenon preliminarily dubbed"type chasing". Finally, we will consider mixtures of bacteria with different motilities.Answering these questions will provide important steps towards avoiding thepathogenic formation of bacterial agglomerates found in medical contamination.Secondly, we will investigate the minimal ecological requirements for the formationof vegetation patterns. Particularly, we will study the structural effects of rainfallspatial gradients (as at the border of deserts) and their seasonal temporal variability.These features are expected to generate the coexistence of distinct patterns. A similarbehavior occurs during the phase separation of thermodynamic fluids, where abruptenvironmental changes are known to generate secondary domains on a matrix ofevolving primary structures. Later in the dynamics, all domains merge together as thesystem approaches thermodynamic equilibrium. For vegetation patterns, however,thermodynamic equilibrium is absent, meaning that primary and secondary"domains" coexist indefinitely. The implications of such primary-secondary domaincoupling will be analyzed. Our results will be compared with image analysis fromavailable satellite data.This project is designed to bring outstanding advances to the lively fields of activematter and theoretical ecology. It involves a synergy of interests as well as theoreticaland experimental expertises. The proposal is physically relevant, viable, and presentsgreat potential for applications. (AU)

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