The effect of dilution rate on secreted proteins
There are three ways that the concentrations of a protein produced by bacterial cells can behave. Which category the protein falls into depends on the way in which the production of a protein is related to the growth of bacterial cells. There are three categories.
- Growth linked This is where the production of a protein is directly linked to the growth of a cell. Examples of this could be cell wall proteins where the production of the protein is a requirement for growth so is obviously growth linked. Most proteins are growth linked
- Non Growth Linked and Pseudo Non Growth Linked. This is where the bacterial cells produce a set amount of protein per cell regardless of growth rate. Examples of non growth linked proteins this are synthesis of a few specific enzymes and some antibiotics. Many growth linked proteins can behave in a non growth linked fashion in some situations inside a chemostat. For example if the growth linked protein requires a specific metabolite for it’s production and that is at a sub optimal concentration then it will be produced at a maximum rate limited by the media and not the activation at it’s promoter or the energy charge of the cell and therefore not be linked to growth.
- Partially Growth linked. Many proteins are growth linked for some growth rates and non-growth linked for others. This category is essentially for all proteins whose expression profile does not fit into the above two categories. Many examples of this are quorum sensing systems, virulence factors, conjugation genes or genes related to competency. It is incredibly difficult to make a predictive model of these systems as there is no way other than by experiment to test what will happen when you vary the growth rate.
These will behave in exactly the same way as OD. Ie they will reach a steady state where the concentration inside the chemostat is not dependent on the flow rate through the chemostat. The mathematical proof is available (on page 158 of reference 1).
The concentration of these in the media will fall as dilution rate in the chemostat increases. This is due to washout of the protein and a constant production rate so you usually get an exponential decrease in the concentration of the protein.
These will show a combination of the above behaviours with short growth linked phases and short non growth linked phases.
1. S. John Pritt Principles of microbial and cell cultivation (1975) Blackwell scientific publications.