Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D7: From Molecular Control to Spatiotemporal Patterns in Bacteria and Beyond |
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Sponsoring Units: GSNP Chair: Herbert Levine, University of California, San Diego Room: Ballroom C3 |
Monday, March 21, 2011 2:30PM - 3:06PM |
D7.00001: Bacterial strategies for chemotaxis response Invited Speaker: Bacteria respond to chemical cues by performing a biased random walk that enables them to migrate towards attractants and away from repellents. Bias is achieved by regulating the duration of the bacterial runs as a function of the environment, inferred from the history of chemoattractant detections experienced by the bacterium. This time-signal is processed using a time convolution function that can be assayed measuring the response of the bacterium to short pulses of chemoattractant. The convolution constitutes an elementary form of memory, which is encoded at the molecular level by the processes of (de-)methylation and (de-)phosphorylation of the underlying biochemical network. While the latter is being characterized in detail, the functional reasons shaping the bacterial chemotactic response are largely unknown. We show that the chemotactic response observed experimentally is the strategy that ensures the highest minimum (MaxiMin) uptake of chemoattractant, in any field thereof. The consequence is that adaptation of the chemotactic bacterial system appears to be evolutionary driven by the need to cope with space-time environmental fluctuations rather than the extension of the dynamic range of response. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D7.00002: Arrested phase separation in reproducing bacteria: a generic route to pattern formation? Invited Speaker: In this talk I will present a generic mechanism by which reproducing microorganisms can form stable patterns. This mechanism is based on the competition between two separate ingredients. First, a diffusivity that depends on the local population density can promote phase separation, generating alternating regions of high and low densities. Then, this is opposed by the logistic law for birth and death of microorganisms which allows only a single uniform density to be stable. The result of this contest is an arrested nonequilibrium phase separation in which dense droplets or rings become separated by less dense regions, with a characteristic steady-state length scale. I will illustrate this mechanism by considering a model of run-and-tumble bacteria, for which a density dependent diffusivity can stem from either a decrease of the swim speed or an increase of the tumbling rate at high density. No chemotaxis is assumed in this model, yet it predicts the formation of patterns strikingly similar to those believed to result from chemotactic behavior. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D7.00003: A tunable sequential and periodic pattern formed by coupling cell motility with density Invited Speaker: The ability of living organisms to form patterns is an untapped resource for synthetic biology. We aim to generate unique patterns by rewiring the genetic circuitry controlling cell motility. Specifically, E. coli cells are programmed to regulate their movement by sensing local cell density. Interesting patterns are formed by newly engineered cells. An engineered low-density mover strain spreads outwards and autonomously forms a sequential and periodic pattern. Moreover, we build a theoretical model that satisfactorily fits our current experimental data, and also predicts some parameters which may significantly affect the pattern formation. The study of this self-organized spatial distribution of cells may help us to probe the principles underlying the formation of natural biological patterns, and to prepare for future engineering of biological structures. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D7.00004: Self-organized biological patterns driven by growth and expansion Invited Speaker: The reaction-diffusion (RD) model, involving the diffusion of two types of morphogens (``activator'' and ``inhibitor'') whose interaction stimulates their own synthesis, is an established paradigm to explain the autonomous generation of space-filling patterns in biology. Starting from random initial perturbations, the RD model typically generates patterns via the development of finite-wavelength dynamical instabilities in confined geometries. In this talk, I will describe examples where elements of the RD model, together with the open, expanding geometries offered by growing biological systems, give rise to novel strategies to generate well-defined patterns in space and time. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D7.00005: Deadly competition between sibling bacterial colonies Invited Speaker: As a result of stress due to nutrient limitation or antibiotics, competing individual bacteria within a single colony may lyse sibling cells to release nutrients (cannibalism) or DNA (fratricide). However, we have recently shown that competition is not limited to individuals, but can occur at the colony level [A. Be'er et al., PNAS 106, 428 (2009); A. Be'er et al., PNAS 107, 6258 (2010).] In response to the presence of an encroaching sibling colony, \textit{Paenibacillus dendritiformis} bacteria secrete a lethal protein, lysing cells at the interface between the colonies. Analysis of the proteins secreted by these competing sibling colonies, combined with a mathematical model, shows how colonies maintain their growth by self-regulating the secretion of two proteins: subtilisin (a well-known growth promoter), and Slf (a previously unknown protein, which is lethal). The results also explain why a single colony is not inhibited by its own secretions. [Preview Abstract] |
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