Field of Science

Why doesn't all the GTA get taken up?

I've been modelling the production and uptake of GTA particles in a culture, hoping to understand the cause of the surprising GTA-accumulation curve I described in the previous post.  But this has led me to a more fundamental surprise.
Only a very small fraction of the cells in a GTA+ culture produce GTA particles and lyse, and all the other cells are able to bind GTA particles and take up their DNA.  So why doesn't all the new GTA quickly get taken up by all the surviving cells?
Here are the basic principles I've been assuming, based on what's in the literature:  GTA production:  Cells in exponential growth don't produce GTA.  The GTA genes are turned on as the culture density gets high and growth slows.  Once the culture reaches its stationary-phase density GTA production stops.  GTA uptake:  Cells in exponential growth express the capsule genes at a low level and bind GTA particles with moderate efficiency.  The capsule genes are turned up when culture density reaches a quorum-sensing threshold, and ability to bind GTA particles gradually increases.  Stationary phase cells bind GTA particles efficiently.  GTA decay: BTA particles are moderately unstable, so they fall apart with some unknown probability.

Let's put some numbers to this:

  1. Assume that 1% of cells produce GTA over the course of the permissive stage.
  2. Assume that each producer cell produces 100 particles and then dies.
  3. Assume that each non-producer cell can take up 1 GTA particle.

Result:  All the GTA particles are taken up.  The concentration of GTA particles in the medium falls to zero.

In reality, assumption 1 is likely to be an overestimate, and assumption 3 an underestimate.  I'm going to do some experiments to see if I can clarify what's going on.

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