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.
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.
Non-equilibrium molecular assembly in reshaping and cutting cells
A key challenge in modern soft matter is to identify the principles that govern the organisation and functionality in non-equilibrium systems. Current research efforts largely focus on non-equilibrium processes that occur either at the single-molecule scale (e.g. protein and DNA conformations under driving forces), or at the scale of whole tissues, organisms, and active colloidal and microscopic objects. However, the range of the scales in-between — from molecules to large-scaled molecular assemblies that consume energy and perform work — remains under-explored. This is, nevertheless, the scale that is crucial for the function of a living cell, where molecular self-assembly driven far from equilibrium produces mechanical work needed for cell reshaping, transport, motility, division, and healing. Today I will discuss physical modelling of active elastic filaments, called ESCRT-III filaments, that dynamically assemble and disassemble on cell membranes. This dynamic assembly changes the filaments’ shape and mechanical properties and leads to the remodelling and cutting of cells. I will present a range of experimental comparisons of our simulation results: from ESCRT-III-driven trafficking in eukaryotes to division of evolutionary simple archaeal cells.
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.