When we first designed our project, it soon came to our mind that we had to ensure the containment of our bacteria from the environment. The best way to do that, apart from the physical containment performed by the porous membrane [LIEN HYPERTEXTE], was to implement our bacterial interface with a kill-switch.

A kill-switch is, literally, something that tells a bacterium whether to live or die, depending on specific environmental conditions. These switches are of many kinds and have been increasingly used in recent years, with the rapid development of synthetic biology [1]. They mainly work by introducing in the bacteria gene constructs that block important genes or express toxins when exposed to a specific environmental condition. We realized an educational video destined to explain to the public audience the principle of a kill-switch. This video can be found here (LIEN HYPERTEXTE).

Our interface is eventually destined to “cohabit” in a human body. We want to make sure that the bacteria we create cannot develop outside the place we put it in. Thus, our kill-switch makes sure that when the environmental conditions of the inner human body are not met, bacteria die.

Reviewing the literature led us to read a very interesting article presenting the cryodeath kill-switch, invented by Mr. Finn Stirling and Colleagues in Pamela Silver's laboratory in 2017, which is triggered by temperatures under 37°C [2]. For obvious reasons, this kill switch sounded like a perfect fit for our application. Contacting Finn comforted us in that choice, and we had the chance to exchange a lot, not only concerning the kill switch but on our project in general. After reviewing the original sequence, we modified it so that it would become a biobrick, and we removed what was unnecessary to us, resulting in BBa_K2616002. (lien vers Parts)

The kill-switch functions by the mean of the toxin/antitoxin couple CcdB/CcdA. The toxin targets and inhibits the GyrA subunit of DNA gyrase, an essential bacterial enzyme that catalyzes the super-coiling of double-stranded closed circular DNA [3].

In the kill-switch, the transcription of CcdB and CcdA are promoted by two different promoters. CcdA is promoted by the promoter Plac. It is one of the best known inducible promoters in synthetic biology and bacteria engineering in general. Mr. Finn Stirling made it constitutive in his design by removing the LacI binding sites and increasing the strength of the RNAP binding sites. This ensures a constant low expression of antitoxin [4]. On the contrary, the toxin CcdB is promoted by the promoter PcspA. This promoter was originally discovered before the Cold Shock Protein A in many bacteria, as a way to cope with environmental stress [5]. This promoter ensures a very low transcription number at 37°C, but it increases drastically below 22°C. According to the results of Hoynes, et al., PcspA is induced about 10-fold at 27°C [6].

Overall, at 37°C, the quantity of antitoxin CcdA is high enough to cope with the leaky low level of toxin produced. However, if bacteria happen to be in an environment at a lower temperature, the toxin promoter is not repressed anymore, the quantity of toxin becomes too important, and the bacteria is not able to grow (see figure 1). This allows us to make sure that our genetically modified organisms will not spread out in an open environment.

To test the efficiency of our kill-switch, we decided to culture BL21 E. coli transformed with it at several temperatures (15°C, 20°C, 25°C and 37°C). The growth was followed by measuring the optical density at 600 nm every 30 minutes for 6 hours, followed by two additional points at 18 hours and at 72 hours. Each experiment was done in a triplicate and the standard deviations were calculated for every point. On figure 2, we show that the bacteria transformed with the kill-switch showed no measurable growth at 15°C and at 20°C during the 72 hours of the experiment, whereas the control population grew normally.

At 25°C, the kill-switch population grew more slowly than the control for the first 18 hours, but the growth eventually started to reach normal values at 72 hours.

Finally, at 37°C there was no difference in the growth of the kill-switch population compared to the control bacteria.

To conclude, the cryodeath kill-switch is a highly efficient way to prevent bacterial growth below 22°C. In the original design by Mr. Finn Stirling, the kill-switch was introduced in the genome of E. coli bacteria using lambda red recombineering. We successfully managed to modify this design, clone it into a plasmid and making it a novel composite biobrick, while keeping its efficiency.

• “Kill switches limit modified microbes,” Nature, vol. 528, no. 7581, pp. 166–167, Dec. 2015.
  
  
• F. Stirling et al., “Rational Design of Evolutionarily Stable Microbial Kill Switches,” Mol. Cell, vol. 68, no. 4, p. 686–697.e3, Nov. 2017.
  
  
• R. J. Reece and A. Maxwell, “DNA Gyrase: Structure and Function,” Crit. Rev. Biochem. Mol. Biol., vol. 26, no. 3–4, pp. 335–375, Jan. 1991.
  
  
• T. P. Malan and W. R. McClure, “Dual promoter control of the Escherichia coli lactose operon.,” Cell, vol. 39, no. 1, pp. 173–80, Nov. 1984.
  
  
• R. Keto-Timonen, N. Hietala, E. Palonen, A. Hakakorpi, M. Lindström, and H. Korkeala, “Cold Shock Proteins: A Minireview with Special Emphasis on Csp-family of Enteropathogenic Yersinia,” Front. Microbiol., vol. 7, Jul. 2016.
  
  
• A. Hoynes-O’Connor, T. Shopera, K. Hinman, J. P. Creamer, and T. S. Moon, “Enabling complex genetic circuits to respond to extrinsic environmental signals,” Biotechnol. Bioeng., vol. 114, no. 7, pp. 1626–1631, Jul. 2017.