Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E42: Chemotaxis Meets PhysiologyInvited
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Sponsoring Units: GSNP Chair: Terence Hwa, Univ of California - San Diego Room: LACC 502B |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E42.00001: Theory of chemotactic ring propagation and the fitness advantage of cue-driven range expansion Invited Speaker: Jonas Cremer Many bacterial species are capable to sense and actively follow chemical gradients. For Escherichia coli, this chemotactic response belongs to the best-characterized subjects of molecular biology, but much less is known about its physiological function and fitness advantage. Previous studies have suggested chemotaxis as a foraging strategy under starvation conditions for which swimming is triggered as an emergency response to find new nutrient sources. However, this hypothesis has never been probed rigorously. Towards uncovering the physiological role of chemotaxis, we have systematically investigated the collective motion of cells along self-generated chemotactic gradients and its dependence on growth conditions*. Presenting experiments and a theoretical analysis, I discuss how the interplay of chemotactic sensing, proliferation, metabolite uptake, and swimming leads to the spreading and growth of an initially localized population. The collective migration dynamics of cells within ring-shaped fronts is described by a modified Keller-Segel model, emphasizing the crucial role of growth physiology and nutrient utilization. Coupled to the front propagation via pushed waves, proliferation of cells in the back drives overall population growth. As a consequence of the dynamics and the active regulation of motility genes, the speed of migration increases strongly with better growth conditions. By the integration of chemoattractant sensing into directed movement, the cue-driven form of range expansion described here is fast and easily outcompetes the canonical form of range-expansion via pulled waves and the Fisher-Kolmogorov dynamics. Overall, the results stand in strong contrast to the foraging hypothesis under starvation conditions but suggest the opposite: chemotaxis as an effective and foresighted strategy to optimize population growth and expansion when local conditions are good. |
Tuesday, March 6, 2018 8:36AM - 9:12AM |
E42.00002: Evolution at the front Invited Speaker: Seppe Kuehn Bacterial motility is intimately linked to growth through the allocation of cellular resources. This link creates physiological constraints which govern the spatiotemporal dynamics of growing and migrating bacterial populations. Here we present results which illuminate the evolutionary implications of these constraints. We select Escherichia coli for faster migration through a porous environment, a process which depends on both motility and growth. Using high-throughput single-cell tracking we find that a trade-off between swimming speed and growth rate constrains the evolution of faster migration. Evolving faster migration in rich medium results in slow growth and fast swimming, while evolution in minimal medium results in fast growth and slow swimming. Whole genome sequencing shows that in each condition parallel genomic evolution drives adaptation through different mutations. Through precise genetic engineering we show that the trade-off is mediated by antagonistic pleiotropy through mutations that likely disrupt negative regulation. A geometric model of the evolutionary process shows that the genetic capacity of an organism to vary traits can qualitatively depend on its environment, which in turn alters its evolutionary trajectory. Our results suggest that understanding how genetic architecture interacts with constraints on phenotypic variation may provide a route to predicting evolutionary dynamics more broadly. We present recent work which explores this possibility experimentally. |
Tuesday, March 6, 2018 9:12AM - 9:48AM |
E42.00003: Evolutionary stability of bacterial motility to spatially dependent selection Invited Speaker: Chenli Liu Bacterial chemotaxis is one of the well-studied systems. Attention has been focused mostly on the migrating cells. Much less is known about the adaptive value of chemotaxis to the population as a whole. Here we introduce a simple evolution protocol to study the effect of chemotaxis on the growth and colonization of E. coli behind the migrating front. The migration speeds of the selected populations depended distinctly on the selection distance: Cells selected at large distances migrated faster while those selected at small distances migrated slower. This surprising result, which relates the migration speed of a population to its fitness behind the front, is elucidated by performing quantitative spatial competition assays for co-migrating strains with different motilities: We reveal an intriguing parasitic interaction by which a slow strain can surf in the wake of a fast strain and preferentially propagate its own progeny, thereby dominating the population behind the front for an extended distance. Mathematical models are developed to relate the outcome of this competition process quantitatively to the evolution dynamics, predicting what migration speed is stable to selection at which distance. These predictions are quantitatively validated by repeating the evolution in conditions supporting different migration speeds of the ancestor. The precise relation between the migration speed of a strain and the position it is evolutionarily stable, despite the ease by which the migration speed can evolve, suggests that E. coli fine tune their motility to stably occupy ecological niches of defined spatial extent. |
Tuesday, March 6, 2018 9:48AM - 10:24AM |
E42.00004: Revealing bacterial free energy dynamics during loss of viability Invited Speaker: Teuta Pilizota Proton Motive Force (PMF) is an electrochemical gradient of protons across a biological membrane that consists of two components: membrane potential and the difference in pH. It plays a crucial role in the energetics of life, powering the production of ATP, transport of different molecules across the membrane, and as in the case of Escherichia coli, motility. As a coarse grain model of E. coli energetics, the cell can be approximated by a simple electrical circuit, with a battery and internal resistance representing catabolism. In external environments of pH equal to that of E. coli’s cytoplasm (~7.5) membrane potential drop in the circuit is equal to PMF. This representation allows us to ‘probe’ different components of the circuit by measuring PMF directly. For example, we can estimate changes in the resistance of the membrane under different stress conditions, and from it, infer the characteristics of the stress applied. To measure PMF in individual E. coli cells we use the bacterial flagellar motor and back focal plane interferometry while exposing the cells to several different stresses: photodamage, butanol, and indole. From the measurements, and using the circuit analogy we offer explanations for the nature of the membrane damage caused by each stress and discuss the model predictions for the overall cell energy maintenance. |
Tuesday, March 6, 2018 10:24AM - 11:00AM |
E42.00005: Physics of bacterial chemotaxis: From molecular mechansims to cellular behaviors Invited Speaker: Yuhai Tu More than 40 years ago, Berg and Brown discovered that E. coli cells perform a run-and-tumble style random walk biased towards higher concentrations of attractants. Around the same time, a phenomenological model of chemotaxis was proposed by Keller and Segel based on a drift-diffusion equation. Since then, much progress has been made in uncovering the molecular mechanism of this cellular navigation system. In this talk, we will discuss some of our recent work in developing an ab initio theory [1-2] to understand bacterial chemotaxis behaviors based on molecular mechanisms of the chemotaxis signaling pathway [3] and how theoretical predictions compare with quantitative microfluidics-based experiments [4,5]. |
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