Flagella
flagella
Mechano-adaptation in a large protein complex
Macromolecular protein complexes perform essential biological functions across life forms. A fundamental, though yet unsolved question in biology is how the function of such complexes is regulated by intracellular or extracellular signals. For instance, we have little understanding of how forces affect multi-protein machines whose function is often mechanical in nature. We address this question by studying the bacterial flagellar motor, a large complex that powers swimming motility in many bacteria. This rotary motor autonomously adapts to changes in mechanical load by adding or removing force-generating ‘stator’ units that power rotation. In the bacterium Escherichia coli, up to 11 units drive the motor at high load while all the units are released at low load. We manipulate motor load using electrorotation, a technique in which a rapidly rotating electric field applies an external torque on the motor. This allows us to change motor load at will and measure the resulting stator dynamics at single-unit resolution. We found that the force generated by the stator units controls their unbinding, forming a feedback loop that leads to autoregulation of the assembly. We complemented our experiments with theoretical models that provide insight into the underlying molecular interactions. Torque-dependent remodeling takes place within seconds, making it a highly responsive control mechanism, one that is mediated by the mechano-chemical tuning of protein interactions.
Microalgal motility through day/night cycles
We have characterised the motility of the swimming microalga Chlamydomonas reinhardtii as a function of day/night cycles, to which the microalgal growth is entrained. Intriguingly, we find that the microalgae swim almost twice as fast during the night than during the day. I will connect this result with the bioenergetics of flagellar propulsion, discussing consequences for the distributions of cells in lab-based and environmental water columns.
Coordinated motion of active filaments on spherical surfaces
Filaments (slender, microscopic elastic bodies) are prevalent in biological and industrial settings. In the biological case, the filaments are often active, in that they are driven internally by motor proteins, with the prime examples being cilia and flagella. For cilia in particular, which can appear in dense arrays, their resulting motions are coupled through the surrounding fluid, as well as through surfaces to which they are attached. In this talk, I present numerical simulations exploring the coordinated motion of active filaments and how it depends on the driving force, density of filaments, as well as the attached surface. In particular, we find that when the surface is spherical, its topology introduces local defects in coordinated motion which can then feedback and alter the global state. This is particularly true when the surface is not held fixed and is free to move in the surrounding fluid. These simulations take advantage of a computational framework we developed for fully 3D filament motion that combines unit quaternions, implicit geometric time integration, quasi-Newton methods, and fast, matrix-free methods for hydrodynamic interactions and it will also be presented.
Swimming and crawling of Euglena gracilis: a tale with many twists
Euglena gracilis is an interesting unicellular protist, also because it can adopt different motility strategies: swimming by flagellar propulsion, or crawling thanks to large amplitude shape changes of the whole body (a behavior known as “metaboly”, or “amoeboid motion”). Swimming trajectories are helical. The are powered by the beating of a single emerging flagellum, which spans non-planar waveforms in the shape of a twisted lasso. Finally the harmoniously coordinated shape changes that make metaboly possible, reminiscent of peristaltic waves, arise form the relative sliding of its pellicle strips, resulting in twisted helical bundles. We will report on the most recent findings on these interconnected topics, for which helical shapes provide a striking fil rouge.
Microorganism locomotion in viscoelastic fluids
Many microorganisms and cells function in complex (non-Newtonian) fluids, which are mixtures of different materials and exhibit both viscous and elastic stresses. For example, mammalian sperm swim through cervical mucus on their journey through the female reproductive tract, and they must penetrate the viscoelastic gel outside the ovum to fertilize. In micro-scale swimming the dynamics emerge from the coupled interactions between the complex rheology of the surrounding media and the passive and active body dynamics of the swimmer. We use computational models of swimmers in viscoelastic fluids to investigate and provide mechanistic explanations for emergent swimming behaviors. I will discuss how flexible filaments (such as flagella) can store energy from a viscoelastic fluid to gain stroke boosts due to fluid elasticity. I will also describe 3D simulations of model organisms such as C. Reinhardtii and mammalian sperm, where we use experimentally measured stroke data to separate naturally coupled stroke and fluid effects. We explore why strokes that are adapted to Newtonian fluid environments might not do well in viscoelastic environments.
Sperm have got the bends
The journey of development begins with sperm swimming through the female reproductive tract en-route to the egg. In order to successfully complete this journey sperm must beat a single flagellum, propelling themselves through a wide range of fluids, from liquified semen to viscous cervical mucus. It is well-known that the beating tail is driven by an array of 9 microtubule doublets surrounding a central pair, with interconnecting dynein motors generating shear forces and driving elastic wave propagation. Despite this knowledge, the exact mechanism by which coordination of these motors drives oscillating waves along the flagellum remains unknown; hypothesised mechanisms include curvature control, sliding control, and geometric clutch. In this talk we will discuss the mechanisms of flagellar bending, and present a simple model of active curvature that is able to produce many of the various sperm waveforms that are seen experimentally, including those in low and high viscosity fluids and after a cell has ‘hyperactivated’ (a chemical process thought to be key for fertilization). We will show comparisons between these simulated waveforms and sperm that have been experimentally tracked, and discuss methods for fitting simulated mechanistic parameters to these real cells.
Frustrated Self-Assembly of Non-Euclidean Crystals of Nanoparticles
Self-organized complex structures in nature, e.g., viral capsids, hierarchical biopolymers, and bacterial flagella, offer efficiency, adaptability, robustness, and multi-functionality. Can we program the self-assembly of three-dimensional (3D) complex structures using simple building blocks, and reach similar or higher level of sophistication in engineered materials? Here we present an analytic theory for the self-assembly of polyhedral nanoparticles (NPs) based on their crystal structures in non-Euclidean space. We show that the unavoidable geometrical frustration of these particle shapes, combined with competing attractive and repulsive interparticle interactions, lead to controllable self-assembly of structures of complex order. Applying this theory to tetrahedral NPs, we find high-yield and enantiopure self-assembly of helicoidal ribbons, exhibiting qualitative agreement with experimental observations. We expect that this theory will offer a general framework for the self-assembly of simple polyhedral building blocks into rich complex morphologies with new material capabilities such as tunable optical activity, essential for multiple emerging technologies.
Exploring the evolution of motile curved bacteria using a regularized Stokeslet Boundary Element Method and Pareto optimality theory
Bacteria exhibit a bewildering diversity of morphologies, but despite their impact on nearly all aspects of life, they are frequently classified into a few general categories, usually just “spheres” and “rods.” Curved-rod bacteria are one simple variation observed in many environments, particularly the ocean. However, why so many species have evolved this shape is unknown. We used a regularized Stokeslet Boundary Element Method to model the motility of flagellated, curved bacteria. We show that curvature can increase swimming efficiency, revealing a widely applicable selective advantage. Furthermore, we show that the distribution of cell lengths and curvatures observed across bacteria in nature is predicted by evolutionary trade-offs between three tasks influenced by shape: efficient swimming, the ability to detect chemical gradients, and reduced cost of cell construction. We therefore reveal shape as an important component of microbial fitness.
Motility-dependent pathogenicity of a spirochetal bacterium
Motility is a crucial virulence factor for many species of bacteria, but it is not fully understood how bacterial motility is practically involved in pathogenicity. This time I will give a talk on the association of motility with pathogenicity in the zoonotic spirochete bacterium Leptospira. Recently, we measured swimming force of individual leptospires using optical tweezers and found that they can generate ~30 times of the swimming force of E. coli. We also observed that leptospires increase the reversal frequency of swimming at the gel-liquid interface, resembling host dermis exposed to contaminated water (Abe et al., 2020, Sci Rep). These could be involved in percutaneous infection of the spirochete. We have shown that Leptospira not only swims in liquid but also moves over solid surfaces (Tahara et al., 2018, Sci Adv). We quantified the surface motility called “crawling” on cultured kidney tissues from various mammals, showing that pathogenic leptospires crawl over the tissue surfaces more persistently that non-pathogenic ones (Xu et al., 2020, Front Microbiol). I will discuss the spirochete motility related to pathogenicity from the biophysical viewpoint.
Synthetic swimmers: microorganism swimming without microorganisms
The effect of non Newtonian liquid rheology on the swimming performance of microorganisms is still poorly understood, despite numerous recent studies. In our effort to clarify some aspects of this problem, we have developed a series of magnetic synthetic swimmers that self-propel immersed in a fluid by emulating the swimming strategy of flagellated microorganisms. With these devices, it is possible to control some aspects of the motion with the objective to isolate specific effects. In this talk, recent results on the effects of shear-thinning viscosity and viscoelasticity on the motion of helical swimmers will presented and discussed. Also, a number of other new uses of the synthetic swimmers will be presented including swimming across gradients, swimming in sand, interactions and rheometry.
Swimming in the third domain: archaeal extremophiles
Archaea have evolved to survive in some of the most extreme environments on earth. Life in extreme, nutrient-poor conditions gives the opportunity to probe fundamental energy limitations on movement and response to stimuli, two essential markers of living systems. Here we use three-dimensional holographic microscopy and computer simulations to show that halophilic archaea achieve chemotaxis with power requirements one hundred-fold lower than common eubacterial model systems. Their swimming direction is stabilised by their flagella (archaella), enhancing directional persistence in a manner similar to that displayed by eubacteria, albeit with a different motility apparatus. Our experiments and simulations reveal that the cells are capable of slow but deterministic chemotaxis up a chemical gradient, in a biased random walk at the thermodynamic limit.
Spinners, not swimmers: how sperm flagella fooled us for 350 years - now in 3D!
In the 17th century, Antonie van Leeuwenhoek used one of the earliest microscopes to see how sperm swim. He described the sperm as a “living animalcule” with a “tail, which, when swimming, lashes with a snakelike movement, like eels in water”. Strikingly, this perception of how sperm moves has not changed since. Indeed, anyone today with a modern microscope would make the same observation: sperm swim forward by wiggling their tail symmetrically side-to-side. Our new research using 3D microscopy shows that we have all been victims of a sperm deception, an illusion. Only now we can see that for 350 years we have been wrong about how sperm actually swims.