Team:Paris Bettencourt/Modeling/Assisted diffusion


Team IGEM Paris 2011

Assisted diffusion

Introduction to the model

Inspired by the experiments of Dubey and Ben-Yehuda we asked ourselves several questions. What kind of process could do this molecular transfer? How can we characterize it? It could be an active process, a passive diffusion or something else. Several arguments can be opposed to the active process hypothesis:

  • During the process, an exchange of molecules of different natures takes place. These molecules have nothing to do with the natural components of a cell (GFP, calcein, etc.). Thus there is no specificity of transport, and there should be no specific mechanism of active transport.
  • Unlike the mammalian cells, the bacterial tubes seem to have no "railroad" to guide the transported molecules.

The question is :

Can we develop a theoretical model of "active" transfer that can justify what was observed in the orignal article?

We need to know if such a model can be designed starting from physical laws and if this model can explain quantitatively the transfer through the nanotubes. Due to its purely physical nature, our model can also shed some light as to the nanotube formation.

We managed to come up with an idea for such a process, and in this page we propose a new model that can possibly explain the speed of the molecule exchange through the nanotubes.

As a matter of fact, the difference in membrane tension between two bacteria could lead to a pressure difference. That could induce a small cytoplasmic transfer to reach internal pressure equilibrium.


Click on the circles on the above picture to discover our assisted diffusion model in details

Model description in few words :

We do not know what is the mechanism behind nanotube formation. We can suppose they are made of lipid membrane within a cell-wall like matrix, as suggested by the original electron microscopy experiment. When the membrane of the two cells fuse, there might be a difference of tension between the two phospholipid bilayers. This phenomenon might lead to a movement of lipids from one membrane to the other. The newly arrived phospholipids change the membrane tension of the bacterium. As a consequence, they change the internal Laplace pressure ΔPLap of the bacterium.

where τ is membrane tension, R is a radius of bacterium.

A simple analogy of clothesline can help to understand what is happening. You need to pull more your clothesline to put more clothes on it. When you pull the rope by two ends you create a tension. The bigger the tension, the more weight (pressure) you can put on the rope.

All of this will lead to establishing the pressure difference at the tube extremities and we will get a Poiseuille's flow. Constituents diluted in water will move from one cell to another unidirectionally and faster than by simple diffusion. This is a fast process that we have named the "assisted diffusion".

The assisted diffusion model in 3 bullet points:

  • Characteristic time of the process is about 100 ns
  • The effect strongly depends on the initial phospholipid distribution on the membrane of two connected bacteria
  • The phenomenon predicts the flux of only 0.1 % of the cytoplasme, not enough to explain the GFP experiment of the original paper

We have done a back of the envelope calculation to see whether an order of magnitude is acceptable.


  1. Intercellular Nanotubes Mediate Bacterial Communication, Dubey and Ben-Yehuda, Cell, 2011, available here
  2. BioNumbers here