Fluid flows created by swimming bacteria drive self-organization in confined suspensions

Concentrated suspensions of swimming microorganisms and other forms of active matter are known to display complex, self-organized spatiotemporal patterns on scales that are large compared with those of the individual motile units. Despite intensive experimental and theoretical study, it has remained unclear the extent to which the hydrodynamic flows generated by swimming cells, rather than purely steric interactions between them, drive the self-organization. Here we use the recent discovery of a spiral-vortex state in confined suspensions of Bacillus subtilis to study this issue in detail. Those experiments showed that if the radius of confinement in a thin cylindrical chamber is below a critical value, the suspension will spontaneously form a steady single-vortex state encircled by a counter-rotating cell boundary layer, with spiral cell orientation within the vortex. Left unclear, however, was the flagellar orientation, and hence the cell swimming direction, within the spiral vortex. Here, using a fast simulation method that captures oriented cell–cell and cell–fluid interactions in a minimal model of discrete particle systems, we predict the striking, counterintuitive result that in the presence of collectively generated fluid motion, the cells within the spiral vortex actually swim upstream against those flows. This prediction is then confirmed by the experiments reported here, which include measurements of flagella bundle orientation and cell tracking in the self-organized state. These results highlight the complex interplay between cell orientation and hydrodynamic flows in concentrated suspensions of microorganisms.In the wide variety of systems termed “active matter” (1, 2), one finds the spontaneous appearance of coherent dynamic structures on scales that are large compared with the individual motile units. Examples range from polar gels (3, 4), bacterial suspensions (5–10), and microtubule bundles (11) to cytoplasmic streaming (12, 13). At high concentrations, suspensions of rod-like bacteria are known to arrange at the cellular scale with parallel alignment as in nematic liquid crystals (5, 14), but with local order that is polar, driven by motility (15, 16). At meso- and macroscopic scales, coherent structures such as swirls, jets, and vortices at scales 10 μm to 1 mm have been experimentally observed (5–10). Many studies have focused on how complex cell interactions can give rise to macroscopic organization and ordering, and the role of self-generated fluid flows in the dynamics of dense suspensions is still under debate (8, 10, 17–21). This controversy is due in part to the inherent complexity of the systems under investigation and the difficulty in making faithful mathematical models.Microswimmers such as Escherichia coli, Bacillus subtilis, and Chlamydomonas rheinhardtii produce dipolar fluid flows through the combined action of their flagella and cell body on the fluid. In the far field, they are well described as “pusher” or “puller” stresslets (22–24), corresponding to the case of flagella behind or in front of the cell body. These fluid flows affect passive tracers (25, 26), as well as swimmers: their motion is subject to convection and shear reorientation induced by neighboring organisms, which can lead to complex collective organization. Macroscopic fluid flows emerge from the collective motion of a colony of motile bacteria, and the suspension can exhibit a quasi-turbulent dynamics (5). Microorganisms like B. subtilis live in porous environments, such as soil, where contact with surfaces is inevitable as mesoscale obstacles and confinement are the norm. Recent experiments give insight into the interactions of single microorganisms with surfaces (24, 27–29), yet suspension dynamics in confinement has only begun to be investigated (30), and the role of the collectively generated fluid flows in the macroscopic organization has yet to be fully understood.Recently, Wioland et al. (30) showed that a dense suspension of B. subtilis, confined into a flattened drop, can self-organize into a spiral vortex, in which a boundary layer of cells at the drop edge moves in the opposite direction to the bulk circulation. This spatiotemporal organization is driven by the presence of the circular boundary and the interactions of bacteria with it. At the interface, the packed cells move at an angle to the tangential that is dictated by the drop curvature, swimmer size, and shape. This macroscopic nonequilibrium pattern and double circulation were not anticipated by theory and have not been seen in any simulations of discrete particle systems due to the computational difficulty of capturing both confinement and complex interactions between elongated swimmers. Although previous simulations have demonstrated the importance of hydrodynamics in populations of spherical squirmers (31) and rod-shaped swimmers (32), they do not consider boundary effects and the elongated shape of the swimmers in the steric interactions. On the other hand, continuum models of motile suspensions that include fluid dynamics and have been successful in explaining large-scale patterns (32, 33), have either ignored confinement or interactions with surfaces, or, if addressing confinement (34), have imposed boundary conditions that generally do not resolve the orientations of the bacteria at the interface. Thus, the conditions at boundaries and microscopic interactions between cells warrant careful consideration in the modeling of these suspensions so that the macroscopic dynamics and organization are correctly captured.Here, we elucidate the origin and nature of the spontaneous emergence of the spiral vortex and cellular organization in a confined motile suspension. A computational model is described for bacterial suspensions in which the direct and hydrodynamic interactions between the swimmers and the confining circular interface can be tuned. The cells are represented as oriented circles or ellipses subject to cell–cell and cell–fluid interaction, whereas the fluid flow is the total of the pusher dipolar fluid flows produced by each swimmer’s locomotion. It is shown that, although some circulation under conditions of confinement may arise with direct interactions only, hydrodynamics are necessary and crucial to reproduce and explain the double circulation that is observed experimentally. Simulations (Fig. 1 A–C) are able to reproduce the emergence of the spiral vortex from an isotropic state (Fig. 1 D–F) and give insights into the origin of the microscopic organization of the bacteria in the drop. The computational results show the remarkable feature that cells in the bulk of the drop swim against the stronger colony-generated fluid flow and thus have a net backward motion. We confirm this observation by measuring the orientation of the cells and of their flagella through suitable fluorescent labeling methods.Open in a separate windowFig. 1.Snapshots of the bacterial suspension self-organization from simulations (A–C) and experiments (D–F). (A–C) An initially isotropic suspension of microswimmers inside a circle with diameter 12ℓ (ℓ = individual swimmer length). Black dots indicate the swimming direction. The swimmer-generated fluid flow is shown superimposed in each plot (blue arrows). (D–F) A dense suspension of B. subtilis in a drop, 70 μm in diameter. (Upper) Bright field. (Lower) Images processed by edge-detection filtering. Initial disordered state is obtained by shining a blue laser that causes cells to tumble. In both simulations and experiments, the suspension organization initiates at the boundary, as seen in B and E. See also Movie S1.     

Implementation and impact of a multidisciplinary coagulation factor stewardship program at an academic medical center

Journal of Thrombosis and Thrombolysis -     

Does pure robotic partial nephrectomy provide similar perioperative outcomes when compared to the combined laparoscopic–robotic approach?

Laparoscopic and robotic partial nephrectomy have become the preferred option for surgical management of incidentally discovered small renal tumors. Currently there is no consensus on which aspects of the procedure should be performed laparoscopically versus robotically. We believe that combining a laparoscopic exposure and hilar dissection followed by tumor extirpation and renorrhaphy with robotic assistance provides improved perioperative outcomes compared to a pure robotic approach alone. We performed a comparison of perioperative outcomes between combined laparoscopic–robotic partial nephrectomy—or hybrid procedure—and pure robotic partial nephrectomy (RPN). A multi-center retrospective analysis of patients undergoing RPN and hybrid PN using the da Vinci S system^® was performed. Patient data were reviewed for demographic and perioperative variables. Statistical analysis was performed using the Welch t test and linear regression, and nonparametric tests with similar significance results. Thirty-one patients underwent RPN while 77 patients underwent hybrid PN between 2007 and 2011. Preoperative variables were comparable in both groups with the exception of lesion size and nephrometry score which were significantly higher in patients undergoing hybrid PN. Length of surgery, estimated blood loss and morphine used were significantly less in the hybrid group, while warm ischemia time was significantly longer. The difference in WIT was accounted for in this data by adjusting for nephrometry score. In our multi-center series, the hybrid approach was associated with a shorter operative time, reduced blood loss and lower narcotic usage. We believe this approach is a valid alternative to RPN.     

Schistosomiasis as a rare cause of recurrent acute appendicitis – A case report

INTRODUCTION

We are presenting a case of schistosomiasis in a 41 year old lady who presented with right iliac fossa pain for 3 years. The pain worsened and the frequency increased in the last 3 months prior to referral. The ultrasound was unremarkable. Her bowel habits were normal and there was no vomiting. There was no blood in the stool or in the urine.

PRESENTATION OF CASE

The abdomen was soft except on deep palpation. There was slight tenderness in the right lower quadrant. A repeat ultrasound was unremarkable. The full blood count was within the normal range. A diagnosis of recurrent acute appendicitis was made and an interval appendicectomy was performed.

DISCUSSION

Histopathology results revealed schistosomiasis of the appendix. There was no acute inflammation but there was fibrous obliteration of the distal lumen of the appendix and reactive lymphoid hyperplasia.

CONCLUSION

This is the first case in a country with relatively clean drinking water. There are no irrigation schemes but there are seasonal rivers and streams. The patient admits to swimming in these streams during childhood. Clinical features of schistosomiasis were not elicited.     

Six Weeks of Voluntary Exercise don’t Protect C57BL/6 Mice Against Neurotoxicity of MPTP and MPP+

Exercise improves the central nervous system (CNS) functions and is widely recommended for neurological patients with, e.g., Alzheimer’s and Parkinson’s disease (PD). However, exercise-induced neuroprotection is an open discussion. Here, the intranasal administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 65 mg/kg) caused death of dopaminergic neurons in the substantia nigra pars compacta and depletion of dopamine in the striatum of C57BL/6 mice. 1-Methyl-4-phenylpyridinium, the active metabolite of MPTP, also inhibited complex-I activity of mitochondria isolated from the CNS of mice. However, 6 weeks of exercise on voluntary running wheels did not protect against nigrostriatal neurodegeneration or mitochondrial inhibition, suggesting that benefits of exercise for PD may not be associated with neuroprotection. The literature presents other candidates, such as neurotrophins or increased antioxidant defenses.