Because ocean water is typically resource-poor, bacteria may gain significant growth advantages if they can exploit the ephemeral nutrient patches originating from numerous, small sources. Although this interaction has been proposed to enhance biogeochemical transformation rates in the ocean, it remains questionable whether bacteria are able to efficiently use patches before physical mechanisms dissipate them. Here we show that the rapid chemotactic response of the marine bacterium Pseudoalteromonas haloplanktis substantially enhances its ability to exploit nutrient patches before they dissipate. We investigated two types of patches important in the ocean: nutrient pulses and nutrient plumes, generated for example from lysed algae and sinking organic particles, respectively. We used microfluidic devices to create patches with environmentally realistic dimensions and dynamics. The accumulation of P. haloplanktis in response to a nutrient pulse led to formation of bacterial hot spots within tens of seconds, resulting in a 10-fold higher nutrient exposure for the fastest 20% of the population compared with nonmotile cells. Moreover, the chemotactic response of P. haloplanktis was >10 times faster than the classic chemotaxis model Escherichia coli, leading to twice the nutrient exposure. We demonstrate that such rapid response allows P. haloplanktis to colonize nutrient plumes for realistic particle sinking speeds, with up to a 4-fold nutrient exposure compared with nonmotile cells. These results suggest that chemotactic swimming strategies of marine bacteria in patchy nutrient seascapes exert strong influence on carbon turnover rates by triggering the formation of microscale hot spots of bacterial productivity.chemotaxis ͉ marine snow ͉ microfluidics ͉ patchiness

Actin polymerization provides a powerful propulsion force for numerous types of cell motility. Although tremendous progress has been made in identifying the biochemical components necessary for actin-based motility, the precise biophysical mechanisms of force generation remain unclear. To probe the polymerization forces quantitatively, we introduce an experimental system in which lipid vesicles coated with the Listeria monocytogenes virulence factor ActA are propelled by actin polymerization. The polymerization forces cause significant deformations of the vesicle. We have used these deformations to obtain a spatially resolved measure of the forces exerted on the membrane using a model based on the competition between osmotic pressure and membrane stretching. Our results indicate that actin exerts retractile or propulsive forces depending on the local membrane curvature and that the membrane is strongly bound to the actin gel. These results are consistent with the observed dynamics. After a slow elongation of the vesicle from a spherical shape, the strong bonds between the actin gel and the membrane rupture if the retractile forces exceed a critical value, leading to a rapid release of the vesicle's trailing edge.

Experimental systems that provide temporal and spatial control of chemical gradients are required for probing into the complex mechanisms of eukaryotic cell chemotaxis. However, no current technique can simultaneously generate stable chemical gradients and allow fast gradient changes. We developed a microfluidic system with microstructured membranes for exposing neutrophils to fast and precise changes between stable, linear gradients of the known chemoattractant Interleukin-8 (IL-8). We observed that rapidly lowering the average concentration of IL-8 within a gradient, while preserving the direction of the gradient, resulted in temporary neutrophil depolarization. Fast reversal of the gradient direction while increasing or decreasing the average concentration also resulted in temporary depolarization. Neutrophils adapted and maintained their directional motility, only when the average gradient concentration was increased and the direction of the gradient preserved. Based on these observations we propose a two-component temporal sensing mechanism that uses variations of chemokine concentration averaged over the entire cell surface and localized at the leading edge, respectively, and directs neutrophil responses to changes in their chemical microenvironment.

We report for the first time that primary human neutrophils can undergo persistent, directionally biased movement away from a chemokine in vitro and in vivo, termed chemorepulsion or fugetaxis. Robust neutrophil chemorepulsion in microfluidic gradients of interleukin-8 (IL-8; CXC chemokine ligand 8) was dependent on the absolute concentration of chemokine, CXC chemokine receptor 2 (CXCR2), and was associated with polarization of cytoskeletal elements and signaling molecules involved in chemotaxis and leading edge formation. Like chemoattraction, chemorepulsion was pertussis toxin-sensitive and dependent on phosphoinositide-3 kinase, RhoGTPases, and associated proteins. Perturbation of neutrophil intracytoplasmic cyclic adenosine monophosphate concentrations and the activity of protein kinase C isoforms modulated directional bias and persistence of motility and could convert a chemorepellent to a chemoattractant response. Neutrophil chemorepulsion to an IL-8 ortholog was also demonstrated and quantified in a rat model of inflammation. The finding that neutrophils undergo chemorepulsion in response to continuous chemokine gradients expands the paradigm by which neutrophil migration is understood and may reveal a novel approach to our understanding of the homeostatic regulation of inflammation.

Anyone who has built a sandcastle recognizes that the addition of liquid to granular materials increases their stability. However, measurements of this increased stability often conflict with theory and with each other [1,2,3,4,5,6,7]. A friction-based Mohr-Coulomb model has been developed [3,8]. However, it distinguishes between granular friction and inter-particle friction, and uses the former without providing a physical mechanism. Albert, et al.[2] analyzed the geometric stability of grains on a pile's surface. The frictionless model for dry particles is in excellent agreement with experiment. But, their model for wet grains overestimates stability and predicts no dependence on system size. Using the frictionless model and performing stability analysis within the pile, we reproduce the dependence of the stability angle on system size, particle size, and surface tension observed in our experiments. Additionally, we account for past discrepancies in experimental reports by showing that sidewalls can significantly increase the stability of granular material.The experimental apparatus consists of a clear plexiglass drum which is rotated about an horizontal axis. We primarily use a drum with a diameter D of 28.5 cm and a width W which can be varied from 0 cm to 14.5 cm. A drum with D = 12.5 cm and W = 11.5 cm is also used to vary system size. The rotation rate ω is varied from 9.0×10 −4 rpm to 5.6×10 −1 rpm. Soda-lime glass spheres which have a density ρ of 2.4 g cm −3 , and with radius r = 0.25 mm, 0.3 mm, 0.5 mm, and 1.5 mm and size dispersity within 0.1 mm were used. The drum was 40% filled with grains premixed with a small amount of liquid. We report the amount of liquid added in terms of the volume fraction, V f , which is defined as the volume of the liquid divided by the volume occupied by the grains alone. The effect of surface tension of the liquid Γ was tested by using silicone oil and water which have Γ = 20 ± 1 dyne cm −1 and Γ = 70 ± 1 dyne cm −1 , respectively. Silicone oil with viscosity ν ranging from 5 cS to 1000 cS is used to study its impact on the measured inclination angles.The drum is back lit so that light could pass through a thin layer of grains that tends to accumulate on the sides of the drum, but is unable to pass through the bulk of the pile. Images were acquired with a mega-pixel resolution digital camera at a rate of three frames per second 100 50 time (s) (b) 32 30 28 26 Surface angle (deg.) 100 50 0 time (s) (a) FIG. 1: Avalanching regimes Glass spheres (radius r 0.5 mm) mixed with silicone oil in a rotating drum (width W 14.5 cm). (a) Stick-slip flow is observed for a rotation rate ω of 0.028 rpm and viscosity ν = 5 cS. (b) Continuous flow is observed for rotation rate of 0.28 rpm.and used to determine the surface slope with automated code. At this rate, the slope of the pile changed no more than 0.2 degrees between frames. This error in measurement is not significant given that the slope of the pile at the moment of avalanche was distributed over about two degrees in a given run....