# Active depinning of bacterial droplets: The collective surfing of Bacillus subtilis

Marc Hennes, Julien Tailleur, Gaëlle Charron, Adrian Daerr

<figure><img src="image_1.png" /></figure>

## To cite this version:

Marc Hennes, Julien Tailleur, Gaëlle Charron, Adrian Daerr. Active depinning of bacterial droplets: The collective surfing of Bacillus subtilis. Congrès des 150 ans de la SFP, Jul 2023, Paris, France. . hal-04947043

HAL Id: hal-04947043

https://hal.science/hal-04947043v1

Submitted on 14 Feb 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>
### Context: Surface translocation of microbes

<figure><img src="image_3.png" /><figcaption>Swarming
B. subtilis</figcaption></figure>

* Rich phenomenology, mechanisms unclear

* Often sharp colony boundary: no leaving colony alone

* Mouvement strongly constrained by interfacial forces

<figure><img src="image_4.png" /><figcaption>Capillary forces enormous at bacterial scale</figcaption></figure>

<figure><img src="image_5.png" /></figure>

Sketch of capillary forces (vertical cut)

Bacteria confined between substrate and free surface.

Model substrate: Agar hydrogel

* Role of wetting and contact line depinning ?

### Wetting of bacterial droplets

* Wetting hysteresis pins even millimetric water droplets to vertical

<figure><img src="image_6.png" /></figure>

* Bacteria depin their drop

bacterial suspension 2 µl, 1 % vol.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

* Bacteria achieve depinning on very shallow slopes ...

↑ drop speed

<figure><img src="image_9.png" /></figure>

Bond number

$$Bo = \frac{\text{drop weight}}{\text{capillary forces}}$$

$$= \frac{\rho V g \sin \alpha}{V^{1/3} \gamma}$$

▲ ... initial droplet of bacterial suspension

★ ... water

* ... but this colony surfing occurs only after several hours.

### B. subtilis produces biosurfactant surfactin ▶ reduces surface tension

<figure><img src="image_10.png" /></figure>

Moving grid profilometry

<figure><img src="image_11.png" /></figure>

### Drop volume increases prior to depinning

<figure><img src="image_12.png" /></figure>

Initial volume $V_i = 2 \mu l$

* slow volume decrease for water

* the more bacteria (OD 1.5 > OD 0.27), the faster volume increases

* supernatant (SN = bacteria removed) behaves the same initially

* surfactant deficient strain 168 ⇒ no volume increase, as for water

* pure surfactant solution ⇒ volume increase

### Bacterial droplets harness 3 distinct effects to unpin

drop depinning condition:

Weight > Contact line pinning force

$$\rho V g \sin \alpha > \text{cst.} \cdot V^{(1/3)} \quad \gamma (\cos \theta_r - \cos \theta_a)$$

Volume ↗, Surface tension ↘ ⇒ $Bo = \rho V^{2/3} g \sin \alpha / \gamma$ increases,

Wetting hysteresis ↘ ⇒ critical Bond from 0.2 down to ~0.03

<figure><img src="image_13.png" /></figure>

### Control experiments

* Are bacteria really needed ?

1. Centrifuge & filter, keep supernatant (SN)

2. Put droplets of bacterial suspension and of SN side-by-side

<figure><img src="image_14.png" /></figure>

⇒ Contamination triggers drop sliding

* How many bacteria are needed ?

Fraction of colony surfing events (N = 5) for different bacterial concentrations (OD) in the deposited drop.

<figure><img src="image_15.png" /></figure>

* OD ≤ 0.02

no surfing (out of five exp.)

* OD ≥ 0.27

surfing always occurs

* Do propelling forces contribute to depinning ?

Pressure from active swimmers could help depin contact line

Compare with surfing probability for non-flagellated mutant

<figure><img src="image_16.png" /></figure>

⇒ Colony surfing independent of individual motility

© CNRS

105, 1073, pns. 1703997114