Shifting process timings

This page explores a few practical concepts to shifting process timings.

Assuming we have captured the parameters for a given process to be the following:

• Initial cell concentration, N₀ = 5×10⁶ cells/mL
• Apparent lag phase, λ = 2.4 h
• Maximum specific growth rate, µ_max = 1.04 h^-1
• Maximum carrying capacity, Κ = 1×10¹⁰ cells/mL

The evolution of bacterial growth can be modelled using an exponential growth model, and consequently visualized as shown below.

Figure legend: A visual representation of the exponential growth model. The model assumes constant growth rate and therefore does not account for the acceleration and deceleration that occur in real bacterial growth. It is important to note that this model is not accurate for predicting cell concentrations immediately after the culture exits the lag phase or just before it enters the stationary phase. Nevertheless, this simplification is often acceptable when the goal is to establish an intuitive understanding of the process.

If a culture transfer is planned at 10⁹ cells/mL, the model predicts that this concentration is reached after approximately 7.5 hours — which may not fit conveniently within a normal working day.

Strategies for shifting process timings

There are two ways to change the timing of a process.

• Change of maximum specific growth rate, µ_max
• Change of initial cell concentration, N₀

Changing specific growth rate, µ

Raising the maximum specific growth rate is generally difficult because it is limited by the organism’s genetic and metabolic capacity. Improvements usually require strain development (via selection or engineering) or highly optimized growth conditions, both of which demand substantial effort, time, and understanding of the organism’s metabolism.

Lowering the growth rate is comparatively easy and reliable. It can be done simply by changing environmental parameters such as lowering temperature. Such adjustment is straightforward to implement and can be used intentionally to synchronize growth with other process steps.

Hint: When assessing specific growth rates, it is important to always use a direct cell counting technique such as BactoBox® because optical density measurements are heavily affected by changes in morphology, and cells change significantly in size throughout a growth curve.

Changing initial cell concentration, N₀

One of the simplest ways to shift process timing is by adjusting the initial concentration (N₀​).

Increasing the starting cell concentration shortens the time needed to reach a given concentration, although the relationship is not linear — once N₀ approaches the stationary-phase concentration, the culture no longer experiences full exponential growth.

Conversely, lowering N₀ can be an effective strategy for controlling process timing. By extending the exponential growth phase, it ensures that the culture remains active and reaches the target concentration at a convenient time, such as early the next morning.

Modelling process timings with changed N₀

If N₀​ is changed to 5×10⁷ or 10² cells/mL, the predicted growth curves change dramatically as illustrated in the figure below.

Figure legend: Growth curves simulated using a simple exponential model starting from initial cell concentrations, N₀, of 1×10², 5×10⁶, and 5×10⁷ cells/mL. The model predicts that these populations reach 1×10⁹ cells/mL after 16 hours, 7.5 hours, and 5.5 hours, respectively.

It is clear how one can achieve a process which reaches the target concentration for e.g. a culture transfer during the next working day if the initial concentration is very low. Similarly, the process can also be sped up by increasing the initial cell concentration, although there are often practical limitations to this approach.