Research

To sink or Not to sink

Micoorganisms are usually denser than the medium they live in, and therefore have the tendency to sediment. When they’re motile, this might not look like a big problem, but when they are not… well… they need to find a way to stay afloat. How do they do it? We are lucky to have been involved in a very interesting project led by Joseph Christie-Oleza on the sinking behaviour of cyanobacteria. It turns out that pili help cyanos stay afloat! And, surprisingly, help fend off grazers as well. The results have just been published in Nature Communications. Congrats to all and in particular to Joseph!

Swarming, Traffic Jams and Biofilms

Transient traffic jam within a monolayer bacterial swarm. Notice the cells being pushed out of the monolayer!

Absolutely delighted that Iago’s paper on the transition from bacterial swarming to biofilms is now out on eLife.

When the expansion of a B. subtilis swarm is hindered (even just by a simple barrier!) cells at the front pile up through a physical process similar to a traffic jam (a transient one in the movie!). This in turn leads to the emergence of a localised biofilm. This is the first direct report we could find of a transition between swarming and biofilm! Great work by Iago and fab collaboration with Munehiro Asally!

Ah! …here’s eLife‘s press release!

Myosin tracking with iSCAT and a Galaxy-recognition Python library

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We recently had the great opportunity to collaborate with Darius Koester to study the behaviour of myosin II bundles and actin filaments. Our side of the work was spearheaded by Lewis Mosby, who adapted a Python library originally used to feature galaxies to recognise the myosin bundles, and analysed in great detail their motile behaviour. Part of this work is published in The Biophysical Journal  (preprint here). The detailed description of the myosin tracking can be found in a Special Issue of The Journal of Physics D, from the IOP (preprint here).

Confinement “diversifies” micro-swimmers

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From soil bacteria to sperm swimming in the fallopian tubes, microorganisms are often found to swim within confined environments. What is the effect of confinement on their flow fields? In a new paper, recently published in Physical Review Letters, we combine experiment and modelling to show that -contrary to expectations- the variety of microbial flow fields is greatly increased under confinement. This can in turn have have qualitative effects on both the biology (e.g., feeding currents) and the physics (e.g., collective behaviour) of microorganisms in confinement. This work was done in collaboration with Raphael Jeanneret and Mitya Pushkin.

 

In Phase or Out of Phase, that is the question

 

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How do cilia synchronise? Through hydrodynamics? Elasticity? Intracellular coupling? The mechanism seems to depend on whether these oscillators belong to same cell or not. In the latter case, we have shown that hydrodynamic interactions suffice; in the former, however, direct intracellular coupling between the flagella is necessary (see here, here, and here). How is this coupling acting? How can it promote opposite types of synchronisation? Our idea is that synchronisation states depend on the cell actively stiffening/relaxing the internal fibres joining the ciliary basal bodies. We explore this hypothesis in our new paper, recently accepted in J. Roy. Soc. Interface, looking at a  minimal model of “cilia coupled by intracellular connections”. (ArXiv preprint. Full version and Supplementary Informations including animations). A big Thank to U. Melbourne and its Department of Mathematics and Statistics for hosting Marco during the final developments of this work!!

Pilot Project from Network Plus

We’re delighted to share the news that we have received travel funds from the EPSRC Network Plus Emergence and Physics Far From Equilibrium to kickstart a collaboration with the groups of Dr. Giorgio Volpe (UCL, UK), Dr. Nuno Araújo (U. Lisbon, Portugal) and Dr. Idan Tuval (IMEDEA-UIB, Spain). The project, which will start later this year, focusses on understanding and controlling transport properties of binary suspensions where microscopic active particles interact with passive ones (cargoes).

Microparticle entrainment à la Taylor

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Close contact between microorganisms underpins fundamental interactions including infection, microbial grazing and fertilisation, but whether or not these interactions actually happen depends critically on the duration of contact. For swimming microorganisms, prolonged contact with an object should manifest as entrainment, and its extend hinge on the physics of escape from the entrained state. At present, neither the existence of entrainment nor the physical mechanisms determining its duration are well established. In our new paper, published in Physical Review Fluids, we combine experiments and theory to show that particle entrainment is indeed a generic feature of swimming microorganisms, and that its duration depends on an interplay between advection and diffusion. A Taylor-dispersion-type theory rationalises the dependence of the distribution of contact times on swimmers’ parameters, and predicts an optimal size for entrainment (~1μm), which we confirm experimentally.  [ArXiv Preprint]

Chlamydomonas Phototaxis: turn and what?

Phototaxis is one of the main categories of motility regulation by microorganisms. Arguably, it is particularly important for motile micro algae, due to their photosynthetic activity. One of the organisms where it has been studied the most is our beloved micro alga Chlamydomonas reinhardtii. Currently, we have a pretty good idea of the mechanism leading the cells to reorient towards/away from the light, but not much is known about what happens after they’ve reoriented…. In our recent paper we start looking into this, with surprising results.

Phase Defects

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Does a stronger interaction always make for a more stable system? Certainly not for synchronising oscillators, as we show in a paper just accepted in Physical Review Fluids. There we study the behaviour of a strip of colloidal rotors as the system is lifted from a no-slip surface. As the hydrodynamic coupling strengthens, the system develop recurring phase defects which worsen its synchronisation. Our simulations show that defects result from a competition between short-range and long-range coupling. The paper is currently accessible through the ArXiv.

Update: The paper has been published (open access) and is now available here.