Enhanced diffusion by reciprocal swimming

Purcell's scallop theorem states that swimmers deforming their shapes in a time-reversible manner ("reciprocal" motion) cannot swim. Using numerical simulations and theoretical calculations we show here that in a fluctuating environment, reciprocal swimmers undergo, on time scales larger than that of their rotational diffusion, diffusive dynamics with enhanced diffusivities, possibly by orders of magnitude, above normal translational diffusion. Reciprocal actuation does therefore lead to a significant advantage over non-motile behavior for small organisms such as marine bacteria

Hydrodynamic attraction of swimming microorganisms by surfaces

Cells swimming in confined environments are attracted by surfaces. We measure the steady-state distribution of smooth-swimming bacteria (Escherichia coli) between two glass plates. In agreement with earlier studies, we find a strong increase of the cell concentration at the boundaries. We demonstrate theoretically that hydrodynamic interactions of the swimming cells with solid surfaces lead to their re-orientation in the direction parallel to the surfaces, as well as their attraction by the closest wall. A model is derived for the steady-state distribution of swimming cells, which compares favorably with our measurements. We exploit our data to estimate the flagellar propulsive force in swimming E. coli

A two-sphere model for bacteria swimming near solid surfaces

We present a simple model for bacteria like \emph{Escherichia coli} swimming near solid surfaces. It consists of two spheres of different radii connected by a dragless rod. The effect of the flagella is taken into account by imposing a force on the tail sphere and opposite torques exerted by the rod over the spheres. The hydrodynamic forces and torques on the spheres are computed by considering separately the interaction of a single sphere with the surface and with the flow produced by the other sphere. Numerically, we solve the linear system which contains the geometrical constraints and the force-free and torque-free conditions. The dynamics of this swimmer near a solid boundary is very rich, showing three different behaviors depending on the initial conditions: (1) swimming in circles in contact with the wall, (2) swimming in circles at a finite distance from the wall, and (3) swimming away from it. Furthermore, the order of magnitude of the radius of curvature for the circular motion is in the range $8-50\,\mu$m, close to values observed experimentally.Comment: 10 pages, 4 figure

Floppy swimming: Viscous locomotion of actuated elastica

Actuating periodically an elastic filament in a viscous liquid generally breaks the constraints of Purcell's scallop theorem, resulting in the generation of a net propulsive force. This observation suggests a method to design simple swimming devices - which we call "elastic swimmers" - where the actuation mechanism is embedded in a solid body and the resulting swimmer is free to move. In this paper, we study theoretically the kinematics of elastic swimming. After discussing the basic physical picture of the phenomenon and the expected scaling relationships, we derive analytically the elastic swimming velocities in the limit of small actuation amplitude. The emphasis is on the coupling between the two unknowns of the problems - namely the shape of the elastic filament and the swimming kinematics - which have to be solved simultaneously. We then compute the performance of the resulting swimming device, and its dependance on geometry. The optimal actuation frequency and body shapes are derived and a discussion of filament shapes and internal torques is presented. Swimming using multiple elastic filaments is discussed, and simple strategies are presented which result in straight swimming trajectories. Finally, we compare the performance of elastic swimming with that of swimming microorganisms.Comment: 23 pages, 6 figure

Novel ultrastructures of Treponema primitia and their implications for motility

Members of the bacterial phylum Spirochaetes are generally helical cells propelled by periplasmic flagella. The spirochete Treponema primitia is interesting because of its mutualistic role in the termite gut, where it is believed to cooperate with protozoa that break down cellulose and produce H2 as a by-product. Here we report the ultrastructure of T. primitia as obtained by electron cryotomography of intact, frozen-hydrated cells. Several previously unrecognized external structures were revealed, including bowl-like objects decorating the outer membrane, arcades of hook-shaped proteins winding along the exterior and tufts of fibrils extending from the cell tips. Inside the periplasm, cone-like structures were found at each pole. Instead of the single peptidoglycan layer typical of other Gram-negative bacteria, two distinct periplasmic layers were observed. These layers formed a central open space that contained two flagella situated adjacent to each other. In some areas, the inner membrane formed flattened invaginations that protruded into the cytoplasm. High-speed light microscopic images of swimming T. primitia cells showed that cell bodies remained rigid and moved in a helical rather than planar motion. Together, these findings support the 'rolling cylinder' model for T. primitia motility that posits rotation of the protoplasmic cylinder within the outer sheath

A Randomized Trial of Manual Phone Calls Versus Automated Text Messages for Peripheral Nerve Block Follow-Ups.

Mobile phone applications (apps) have been used for patient follow-up in the postoperative period, specifically to assess for complications and patient satisfaction. Few studies have evaluated their use in regional anesthesia. The objective of this study was to compare follow-up response rates using manual phone calls versus an automated patient outreach (APO) app for peripheral nerve block patients. We hypothesized that the response rate would be higher in the APO group. A mobile app, JeffAnesthesia, was developed, which sends notifications to patients to answer survey questions in the app. We randomly assigned patients who received peripheral nerve blocks for postoperative pain to either a manual phone call or an APO app group, with follow-up in each category occurring between postoperative days (POD) 14-21 and 90-100. In total, 60 patients were assigned to the phone call group and 60 patients to the APO app group. Between POD 14-21, 9 (15%) patients were reached in the manual phone call arm, and 16 (26.7%) patients were reached in the APO arm (p = 0.117). At POD 90-100, follow-up was successful with 5 (8.2%) in the manual phone call group vs. 3 (5.0%) patients in the APO app group (p = 0.300). Overall response rate was poor, with comparable response rates between groups. The APO method may reduce time spent by anesthesia staff on follow-up calls, but our data do not suggest this method improves response rates significantly. Further studies are needed to better understand the reasons for the poor response rate and strategies for improvement

Spectroscopy of Blue Stragglers and Turnoff Stars in M67 (NGC 2682)

We have analyzed high-resolution spectra of relatively cool blue stragglers and main sequence turnoff stars in the old open cluster M67 (NGC 2682). We attempt to identify blue stragglers whose spectra are least contaminated by binary effects (contamination by a binary companion or absorption by circumstellar material). These ``best'' stragglers have metallicities ([Fe/H] = -0.05) and abundance ratios of the blue stragglers are not significantly different from those of the turnoff stars. Based on arguments from hydrodynamical models of stellar collisions, we assert that the current upper limits for the lithium abundances of all blue stragglers observed in M67 (by us and others) are consistent with no mixing during the formation process, assuming pre-main sequence and main sequence depletion patterns observed for M67 main sequence stars. We discuss composition signatures that could more definitively distinguish between blue straggler formation mechanisms in open cluster stars. We confirm the spectroscopic detection of a binary companion to the straggler S 1082. From our spectra, we measure a projected rotational speed of 90+/-20 km/sec for the secondary, and find that its radial velocity varies with a peak-to-peak amplitude of ~ 25 km/sec. Because the radial velocities do not vary with a period corresponding to the partial eclipses in the system, we believe this system is currently undergoing mass transfer. In addition we present evidence that S 984 is a true blue straggler (and not an unresolved pair). If this can be proven, our detection of lithium may indicate a collisional origin.Comment: 20 pages, 4 figures, to appear in October 2000 A

Predictors of non-pharmacological intervention in patients with paroxysmal atrial fibrillation:Value of neuroticism

Background: Non-pharmacological intervention is gaining increasing popularity in the treatment of patients with paroxysmal atrial fibrillation. We sought to investigate which factors play a role in the choice for non-pharmacological intervention with a particular focus on neuroticism. Methods: The study group comprised 73 patients with paroxysmal atrial fibrillation (mean age 55 13 years, 50 males). On average, patients had a 3-year-history of one symptomatic paroxysm per week lasting 2 It. The degree of neuroticism was assessed using the short scale Eysenck Personality Questionnaire. Results: During a mean follow-up period of 7.0 +/- 0.6 years, 20 patients (27%) underwent a non-pharmacological intervention for atria] fibrillation including His bundle ablation (n=1), maze operation (n=4), DDDR-pacemaker (n=10), pulmonary vein ablation (n=5). Multivariate regression analysis showed that age <55 years (odds ratio 5.3, 95% CI 1.1-24.5), frequency of paroxysms of atrial fibrillation > 1 per week (odds ratio 5.9, 95% CI 1.2-28.5) and total number of anti-arrhythmic drugs (class I and III) used > 2 (odds ratio 3.4, 95% CI 1.6-6.9) were predictive of non-pharmacological intervention (all p <0.05). In contrast, the degree of neuroticism was similar in patients who underwent non-pharmacological intervention as opposed to patients who did not undergo non-pharmacological intervention (4.5 +/- 3.3 vs. 4.0 +/- 2.9, p = NS). Conclusions: On the basis of this small study, neuroticism would not appear to play an important role in the decision to perform a nonpharmacological intervention. Instead, the data indicate that younger patients with pharmacologically refractory atrial fibrillation more often undergo non-pharmacological intervention. (c) 2005 Elsevier Ireland Ltd. All rights reserved

The cytosolic entry of diphtheria toxin catalytic domain requires a host cell cytosolic translocation factor complex

In vitro delivery of the diphtheria toxin catalytic (C) domain from the lumen of purified early endosomes to the external milieu requires the addition of both ATP and a cytosolic translocation factor (CTF) complex. Using the translocation of C-domain ADP-ribosyltransferase activity across the endosomal membrane as an assay, the CTF complex activity was 650–800-fold purified from human T cell and yeast extracts, respectively. The chaperonin heat shock protein (Hsp) 90 and thioredoxin reductase were identified by mass spectrometry sequencing in CTF complexes purified from both human T cell and yeast. Further analysis of the role played by these two proteins with specific inhibitors, both in the in vitro translocation assay and in intact cell toxicity assays, has demonstrated their essential role in the productive delivery of the C-domain from the lumen of early endosomes to the external milieu. These results confirm and extend earlier observations of diphtheria toxin C-domain unfolding and refolding that must occur before and after vesicle membrane translocation. In addition, results presented here demonstrate that thioredoxin reductase activity plays an essential role in the cytosolic release of the C-domain. Because analogous CTF complexes have been partially purified from mammalian and yeast cell extracts, results presented here suggest a common and fundamental mechanism for C-domain translocation across early endosomal membranes

Active particles in periodic lattices

Both natural and artificial small-scale swimmers may often self-propel in environments subject to complex geometrical constraints. While most past theoretical work on low-Reynolds number locomotion addressed idealised geometrical situations, not much is known on the motion of swimmers in heterogeneous environments. As a first theoretical model, we investigate numerically the behaviour of a single spherical micro-swimmer located in an infinite, periodic body-centred cubic lattice consisting of rigid inert spheres of the same size as the swimmer. Running a large number of simulations we uncover the phase diagram of possible trajectories as a function of the strength of the swimming actuation and the packing density of the lattice. We then use hydrodynamic theory to rationalise our computational results and show in particular how the far-field nature of the swimmer (pusher vs. puller) governs even the behaviour at high volume fractions