Bacterial swarms thicken up

New paper out on B. subtilis swarming dynamics! The paper, published in Frontiers of Soft Matter, is a collaboration with Iago and Munehiro and builds on our previous paper on eLife. Here we dwell into the characterisation of the transition from a monolayer to a multilayer swarm. Turns out that this looks like a first order phase transition and can take either a “nucleation and growth” route or a more sudden “spinodal” decomposition route…

The curious case of confined active-passive systems

We’re excited to share our latest publication in Nature Communications, where we investigate the dynamics of mixed active-passive systems. In these systems, the passive particles are buffeted around by the active components like swimming microorganisms or synthetic active particles. These suspensions are fascinating both at a fundamental level (how can we describe/prescribe the average behaviour of the passive particles?) and -possibly- for future technological applications (directed transport at the microscopic scale). Here we show that confining an active-passive system leads to a non-uniform distribution of the passive species in a predictable way. We then use confinement to induce the mixed system to spontaneously un-mix and separate out the passive components! This is Steve’s first paper, in collaboration with Raphaël Jeanneret (LPENS Paris, France) and Idan Tuval (IMEDEA UIB-CSIC, Spain).

The Narrow-Escape of a Microorganism

A microalga escapes one pool only to find itself in another.

Escaping through narrow apertures involves rare events and therefore is usually quite hard. It is also a classical problem for both Brownian and ballistic particles. Interestingly, microorganisms can find themselves having to find and go through a narrow aperture. Their case is peculiar as it bridges the Brownian and ballistic cases. Our new paper, just out in Physical Review Research, looks at this problem with a mix of experiments and simulations. As is often the case, we find an unexpected twist in the story…. A great collaboration with Antoine Allard, Mathieu Souzy, Jean François Louf, Matteo Contino and Idan Tuval.

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


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


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




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!!