37 research outputs found
Particle diffusion in active fluids is non-monotonic in size
We experimentally investigate the effect of particle size on the motion of
passive polystyrene spheres in suspensions of Escherichia coli. Using particles
covering a range of sizes from 0.6 to 39 microns, we probe particle dynamics at
both short and long time scales. In all cases, the particles exhibit
super-diffusive ballistic behavior at short times before eventually
transitioning to diffusive behavior. Surprisingly, we find a regime in which
larger particles can diffuse faster than smaller particles: the particle
long-time effective diffusivity exhibits a peak in particle size, which is a
deviation from classical thermal diffusion. We also find that the active
contribution to particle diffusion is controlled by a dimensionless parameter,
the Peclet number. A minimal model qualitatively explains the existence of the
effective diffusivity peak and its dependence on bacterial concentration. Our
results have broad implications on characterizing active fluids using concepts
drawn from classical thermodynamics.Comment: 5 Figure
The propagation of active-passive interfaces in bacterial swarms
Propagating interfaces are ubiquitous in nature, underlying instabilities and
pattern formation in biology and material science. Physical principles
governing interface growth are well understood in passive settings; however,
our understanding of interfaces in active systems is still in its infancy.
Here, we study the evolution of an active-passive interface using a model
active matter system, bacterial swarms. We use ultra-violet light exposure to
create compact domains of passive bacteria within Serratia marcescens swarms,
thereby creating interfaces separating motile and immotile cells.
Post-exposure, the boundary re-shapes and erodes due to self-emergent
collective flows. We demonstrate that the active-passive boundary acts as a
diffuse interface with mechanical properties set by the flow. Intriguingly,
interfacial velocity couples to local swarm speed and interface curvature,
suggesting that an active analogue to classic Gibbs-Thomson-Stefan conditions
controls boundary propagation. Our results generalize interface theories to
mixing and segregation in active systems with collective flows
Dynamic nuclear structure emerges from chromatin crosslinks and motors
The cell nucleus houses the chromosomes, which are linked to a soft shell of
lamin filaments. Experiments indicate that correlated chromosome dynamics and
nuclear shape fluctuations arise from motor activity. To identify the physical
mechanisms, we develop a model of an active, crosslinked Rouse chain bound to a
polymeric shell. System-sized correlated motions occur but require both motor
activity {\it and} crosslinks. Contractile motors, in particular, enhance
chromosome dynamics by driving anomalous density fluctuations. Nuclear shape
fluctuations depend on motor strength, crosslinking, and chromosome-lamina
binding. Therefore, complex chromatin dynamics and nuclear shape emerge from a
minimal, active chromosome-lamina system.Comment: 18 pages, 21 figure
How cells wrap around virus-like particles using extracellular filamentous protein structures
Nanoparticles, such as viruses, can enter cells via endocytosis. During
endocytosis, the cell surface wraps around the nanoparticle to effectively eat
it. Prior focus has been on how nanoparticle size and shape impacts
endocytosis. However, inspired by the noted presence of extracellular vimentin
affecting viral and bacteria uptake, as well as the structure of coronaviruses,
we construct a computational model in which both the cell-like construct and
the virus-like construct contain filamentous protein structures protruding from
their surfaces. We then study the impact of these additional degrees of freedom
on viral wrapping. We find that cells with an optimal density of filamentous
extracellular components (ECCs) are more likely to be infected as they uptake
the virus faster and use relatively less cell surface area per individual
virus. At the optimal density, the cell surface folds around the virus, and
folds are faster and more efficient at wrapping the virus than crumple-like
wrapping. We also find that cell surface bending rigidity helps generate folds,
as bending rigidity enhances force transmission across the surface. However,
changing other mechanical parameters, such as the stretching stiffness of
filamentous ECCs or virus spikes, can drive crumple-like formation of the cell
surface. We conclude with the implications of our study on the evolutionary
pressures of virus-like particles, with a particular focus on the cellular
microenvironment that may include filamentous ECCs.Comment: 15 pages, 8 figure
Cell nuclei as cytoplasmic rheometers
Some researchers probe the mechanics of cells by perturbing them from the outside, such as using an atomic force microscope probe to record the amount of deformation of the cell in response to applying a prescribed force at a defined speed. Other researchers probe the mechanics of cells by perturbing them from the inside, an example of which is particle-tracking microrheology, in which the spontaneous motion of submicron, passive fluorescent beads ballistically injected earlier into the cell decodes the cell moduli. Both types of probes are typically composed of nonliving material. In this issue of Biophysical Journal, Moradi and Nazockdas cleverly propose to use the cell nucleus itself as a rheological probe for the mechanics of the cytoplasm (1). The cell nucleus is typically the largest and the stiffest organelle in eukaryotic cells. The surrounding cytoplasm contains other organelles and the cytoskeleton, which is comprised different kinds of semiflexible polymers, including actin, microtubules, and intermediate filaments. For cells that are confined by geometries on the scale of the size of the cell, the nucleus is minimally deformed and can therefore be approximated as a rigid sphere. It is in this limit that the authors ask the following questions. As a cell moves inside a microchannel, what does the motion of the cell nucleus, in response to deformations in the cell cortex, reveal about the rheology of the cytoplasm? Is it viscoelastic? Is it porous? Is it a poroelastic network? Is it something else? Answers to such questions will help us better understand cell function, such as how the cytoplasm reorganizes in response to changes in a cell physical environment
Bacteria colonies modify their shear and compressive mechanical properties in response to different growth substrates
Bacteria build multicellular communities termed biofilms, which are often
encased in a self-secreted extracellular matrix that gives the community
mechanical strength and protection against harsh chemicals. How bacteria
assemble distinct multicellular structures in response to different
environmental conditions remains incompletely understood. Here, we investigated
the connection between bacteria colony mechanics and the colony growth
substrate by measuring the oscillatory shear and compressive rheology of
bacteria colonies grown on agar substrates. We found that bacteria colonies
modify their own mechanical properties in response to shear and uniaxial
compression with the increasing agar concentration of their growth substrate.
These findings highlight that mechanical interactions between bacteria and
their microenvironment are an important element in bacteria colony development,
which can aid in developing strategies to disrupt or reduce biofilm growth.Comment: biophysics, soft matter, biofilm rheology, biofilm mechanic
Dynamic remodeling of fiber networks with stiff inclusions under compressive loading
The ability of tissues to sustain and withstand mechanical stress is critical to tissue development and healthy tissue maintenance. The mechanical properties of tissues are typically considered to be dominated by the fibrous extracellular matrix (ECM) component of tissues. Fiber network mechanics can capture certain mechanical features of tissues, such as shear strain stiffening, but is insufficient in describing the compressive response of certain tissues and blood clots that are rich in extracellular matrix. To understand the mechanical response of tissues, we employ a contemporary mechanical model, a fibrous network of fibrin embedded with inert bead inclusions that preserve the volume-conserving constraints of cells in tissues. Combining bulk mechanical rheology and a custom imaging device, we show that the presence of inclusions alters the local dynamic remodeling of the networks undergoing uniaxial compressive strains and demonstrate non-affine correlated motion within a fiber-bead network, predicted to stretch fibers in the network and lead to the ability of the network to stiffen under compression, a key feature of real tissues. These findings have important implications for understanding how local structural properties of cells and ECM fibers impact the bulk mechanical response of real tissues
Materials science and mechanosensitivity of living matter
Living systems are composed of molecules that are synthesized by cells that use energy sources within their surroundings to create fascinating materials that have mechanical properties optimized for their biological function. Their functionality is a ubiquitous aspect of our lives. We use wood to construct furniture, bacterial colonies to modify the texture of dairy products and other foods, intestines as violin strings, bladders in bagpipes, and so on. The mechanical properties of these biological materials differ from those of other simpler synthetic elastomers, glasses, and crystals. Reproducing their mechanical properties synthetically or from first principles is still often unattainable. The challenge is that biomaterials often exist far from equilibrium, either in a kinetically arrested state or in an energy consuming active state that is not yet possible to reproduce de novo. Also, the design principles that form biological materials often result in nonlinear responses of stress to strain, or force to displacement, and theoretical models to explain these nonlinear effects are in relatively early stages of development compared to the predictive models for rubberlike elastomers or metals. In this Review, we summarize some of the most common and striking mechanical features of biological materials and make comparisons among animal, plant, fungal, and bacterial systems. We also summarize some of the mechanisms by which living systems develop forces that shape biological matter and examine newly discovered mechanisms by which cells sense and respond to the forces they generate themselves, which are resisted by their environment, or that are exerted upon them by their environment. Within this framework, we discuss examples of how physical methods are being applied to cell biology and bioengineering
Quenching active swarms: effects of light exposure on collective motility in swarming Serratia marcescens
Swarming colonies of the light-responsive bacteria Serratia marcescens grown on agar exhibit robust fluctuating large-scale flows that include arrayed vortices, jets and sinuous streamers. We study the immobilization and quenching of these collective flows when the moving swarm is exposed to intense wide-spectrum light with a substantial ultraviolet component. We map the emergent response of the swarm to light in terms of two parameters-light intensity and duration of exposure-and identify the conditions under which collective motility is impacted. For small exposure times and/or low intensities, we find collective motility to be negligibly affected. Increasing exposure times and/or intensity to higher values suppresses collective motility but only temporarily. Terminating exposure allows bacteria to recover and eventually reestablish collective flows similar to that seen in unexposed swarms. For long exposure times or at high intensities, exposed bacteria become paralysed and form aligned, jammed regions where macroscopic speeds reduce to zero. The effective size of the quenched region increases with time and saturates to approximately the extent of the illuminated region. Post-exposure, active bacteria dislodge immotile bacteria; initial dissolution rates are strongly dependent on duration of exposure. Based on our experimental observations, we propose a minimal Brownian dynamics model to examine the escape of exposed bacteria from the region of exposure. Our results complement studies on planktonic bacteria, inform models of patterning in gradated illumination and provide a starting point for the study of specific wavelengths on swarming bacteria