Team:Imperial College London/Drylab/Autoinduction

Autoinduction

Starting the encapsulation process is related to diauxic growth of bacteria under the presence of two carbon sources. 3 models from literature have been discussed and implemented in this section and more information about each can be found in the extra tabs. After analysis of the models, we picked one of them as the best representation for our data. The sections below will provide a summary about each model, and at the end, discuss which one was picked and why. The diauxic growth phenomenon is tricky to understand, and the models we have chosen illustrate different approaches that groups have taken to explain what happens when bacteria switch to feeding from a primary carbon source to a secondary carbon source.

Kompala et al [1]

Goal of the models

• Provide understanding of the diauxic growth phenomenon
• Find out the time taken to consume glucose in the medium to characterize the CRP promoter.

Kompala et al cybernetic model

• This is a cybernetic model developed by Kompala et al [1].
• It is based on Michaelis-Menten assumptions for substrate consumption.
• The substrates in this case are our carbon sources: glucose, maltose, xylose, galactose...

Rationale:
In a mixture of carbon sources, glucose is energetically more favorable to consume(more on this is discussed in the autoinduction section).
This model includes the effects of enzyme activation and inhibition, as well as competition for metabolism of sugars [1].
Figure 1 from [2]

The red and green lines represent the concentrations of glucose (S1) and a secondary substrate (S2). As we can see, S1 is used up before S2. During this phase, the population (X- blue trace) grows exponentially and saturates when S1 runs out. Once it runs out, there is a switch phase, followed by metabolism of a secondary carbon source and entering a second exponential growth phase.

  About Kompala's cybernetic model

Zhao Lu et al model

• This model was taken from Zhao Lu et al [2] and it is based on lactose transport into the cell and the dynamics of the lac operon.
• Zhao Lu et al takes into account several factors that are not included in Kompala's model, such as:
  • The effects of catabolite repression and inducer exclusion [2]( see chemical induction for further explanation).
  • The "lag effect" seen when we switch from glucose to a secondary source. In the case of lactose, this delay is caused by the production of permeases and b-galactosidase.
  • Furthermore, other carbon sources we have used in our autoinduction experiments such as galactose, xylose, mannose... are also related to lactose (in structure) so we believe that the dynamics of the lac operon have an important part to play in the metabolism of secondary carbon sources.

  About Zhao Lu's model

References

[1]Kompala, D.S., D. Ramkrishna and G.T. Tsao, "Cybernetic modeling of microbial growth on multiple substrates," Biotechnology and Bioengineering 26 :1272-1281 (1984).
[2] Wolfram: http://demonstrations.wolfram.com/DiauxicGrowthOfBacteriaOnTwoSubstrates
[3] Zhao Lu et al. "Dynamics of Glucose-Lactose Diauxic Growth in E.coli". arXiv:0708.1993v1 [q-bio.OT] 15 Aug 2007