Difference between revisions of "Team:Toulouse/Results"

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<center>   
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<center>   
 
     <div id="breadcrumb" class="clear" style="float: center;" >
 
     <div id="breadcrumb" class="clear" style="float: center;" >
 
  <ul>
 
  <ul>
 
         <li><a href="#attract">○ Attract</a></li>
 
         <li><a href="#attract">○ Attract</a></li>
 
<li><a href="#eradicate">○ Eradicate</a></li>
 
<li><a href="#eradicate">○ Eradicate</a></li>
 +
<li><a href="#regulate">○ Regulate</a></li>
 
<li><a href="#TPX">○ ApiColi containment and culture</a></li>
 
<li><a href="#TPX">○ ApiColi containment and culture</a></li>
       
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       </ul>
 
       </ul>
 
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<li><a href="#carbon">○ Choice of carbon source</a></li>
 
<li><a href="#carbon">○ Choice of carbon source</a></li>
 
<li><a href="#tox">○ Testing acids toxicity</a></li>
 
<li><a href="#tox">○ Testing acids toxicity</a></li>
 
 
</ul>
 
</ul>
 
</div>
 
</div>
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</center>
 
 
 
 
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  <ul>
 
  <ul>
 
         <li><a href="#varrotest2">- Test on varroas</a></li>
 
         <li><a href="#varrotest2">- Test on varroas</a></li>
<li><a href="#formaproduct">- Test of formate production</a></li>
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<li><a href="#cloningformate">- Cloning butyrate genes</a></li>
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<li><a href="#formaproduct">- Test of formate production</a></li>
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       </ul>
 
       </ul>
 
     </div>
 
     </div>
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<div class="subtitle" id="cloningformate">
 +
  <h3>Cloning formate genes</h3>
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  </div>
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<br>
 +
<b>Aim: to create the genetic tool to implement a formate pathway in <i>E. coli</i></b>
 +
</font>
 +
</div>
 +
  <div class="group">
 +
  <p class="text">
 +
  We designed and ordered synthetic genes to express the full pathway minus the <i>ccr</i> gene. The latter was already
 +
  placed behind <i>lacI</i> to be ready for circadian circle regulation. Thus, to build a biobrick with the full pathway, we cloned the <i>ccr</i> gene with all the genes necessary for butyrate production, as peresented below:
 +
  </p>
 +
  </div>
 +
 
 +
  <center>
 +
  <div style="one-half;padding:10px;">
 +
<img src="https://static.igem.org/mediawiki/2015/3/3b/TLSE_Parts_image5.PNG" style="width:60%;" />
 +
</div>
  
 +
<p class="legend">
 +
Figure 2: assembly of the synthetic pathway leading to formate production (3090 Kb).
 +
The first arrow represents the promoter, the others represent genes. The green circles are for the RBS and the red circle is for the terminator.
 +
</p>
 +
  </center>
 +
 
 +
 
 +
  <div class="group">
 +
  <p class="text">
 +
  This construction <a target="_blank" href="http://parts.igem.org/Part:BBa_K1587007">(BBa_K1587007)</a> was inserted in the plasmid pSB1C3 to be provided to the iGEM in the biobrick format.
 +
  </p>
 +
  </div>
 +
 +
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<b>Conclusion: The synthetic formate operon is ready to be used in <i>E. coli</i></b>
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</font>
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</div>
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<br> <br><br>
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<center><div class="title" id="regulate">
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  <h3>Regulate</h3>
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  </div>
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  </center>
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<center> 
 +
<div id="breadcrumb" class="clear" style="float: center;" >
 +
<ul>
 +
<li><a href="#logic">- NOT logic gate</a></li>
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<li><a href="#sensor">- Light sensor</a></li>
 +
</ul>
 +
</div>
 +
</center>
 +
 +
<div class="subtitle" id="logic">
 +
  <h3>NOT logic gate</h3>
 +
</div>
 +
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<br>
 +
<b>Aim: NOT logic gate construction</b>
 +
</font>
 +
</div>
 +
 +
<div class="group center">
 +
<p class="text">
 +
We want to express the butyrate synthetic pathway by day and the formate synthetic pathway by night. As a first step to develop our regulation module, we replaced the butyrate pathway by the RFP reporter gene and the formate pathway by GFP reporter gene. Doing so, it should be easy to assess the functioning of the light sensor and the NOT logic gate since the colonies should be red by day and green by night.<br>
 +
To do so, we synthesized the pOMPC-cI gene (new biobrick <a href="http://parts.igem.org/Part:BBa_K1587006">BBa_K1587006</a>)Through subcloning the GFP reporter gene (<a href="http://parts.igem.org/Part:BBa_E0240">BBa_E0240</a>) was placed under control of this promoter. Likewise, the RFP reporter gene (<a href="http://parts.igem.org/Part:BBa_K081014">BBa_K081014</a>) was cloned behind the PL-lacI synthetic gene. Both constructions were placed together on the same plasmid but unfortunately sequencing revealed that there was some incoherence in the RFP sequence, preventing further testing.
 +
</p>
 +
</div>
 +
 +
  <center>
 +
  <img src="https://static.igem.org/mediawiki/2015/thumb/f/f2/TLSE_NOT_gate.png/797px-TLSE_NOT_gate.png" style="width:70%;"/>
 +
  </center>
 +
  <div class="group center">
 +
  <p class="legend">
 +
  Figure : NOT logic gate test construction
 +
  </p></div>
 +
 
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<br>
 +
<b>Conclusion: the strain can grow and survive over 10 days in the TPX® bag.</b>
 +
</font>
 +
</div>
 +
 +
<div class="subtitle" id="sensor">
 +
  <h3>Light sensor</h3>
 +
</div>
 +
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<br>
 +
<b>Aim: Light sensor system construction.</b>
 +
</font>
 +
</div>
 +
 +
<div class="group center">
 +
<p class="text">
 +
For our project we need to implement a light sensor system in our strain. To that end, we needed to clone the three genes (<i>cph8, hoI, pcyA</i>) of this system behind a strong promoter. We received the strain containing the three genes on different plasmids as courtesy from Dr Clark Lagarias, Christopher Voigt, and Nico Claassens [REF]. These genes were amplified by PCR and subcloned in pSB1C3 (at this step, a RBS was added to sequence during the PCR). The new biobricks for <i>hoI</i> and <i>pcYA</i> were sent to the registry under the accession number <a href="http://parts.igem.org/Part:BBa_K1587000">BBa_K1587000</a> and <a href="http://parts.igem.org/Part:BBa_K1587002">BBa_K1587002</a>, respectively. The <i>cph8</i> biobrick was not obtained but we managed to build a biobrick (<a href="http://parts.igem.org/Part:BBa_K1587008">BBa_K1587008</a>) with the <i>cph8</i> encoding sequence under the control of a strong promoter originated from the biobrick (<a href="http://parts.igem.org/Part:BBa_J23119">BBa_J23119</a>).
 +
</p>
 +
</div>
 +
 +
  <center>
 +
  <img src="https://static.igem.org/mediawiki/2015/thumb/a/a6/TLSE_sensor_theoric.png/800px-TLSE_sensor_theoric.png" style="width:70%;"/>
 +
  </center>
 +
  <div class="group center">
 +
  <p class="legend">
 +
  Figure : Targeted light sensor construction
 +
  </p></div>
 +
 
 +
    <center>
 +
  <img src="https://static.igem.org/mediawiki/2015/7/7c/TLSE_sensor_real.png" style="width:70%;"/>
 +
  </center>
 +
  <div class="group center">
 +
  <p class="legend">
 +
  Figure : <i>cph8</i> biobrick
 +
  </p></div>
 +
 
 +
  <div class="group center">
 +
<p class="text">
 +
We did not succeed in associating the three genes together behind a strong promoter in pSB1C3 using the In Fusion cloning technique before the iGEM deadline. Consequently, we were not able to test our light sensor system soon enough for this project.
 +
</p>
 +
</div>
 +
 +
<div style="font-size:14px;">
 +
<font color="#EF9D2A">
 +
<br>
 +
<b>Conclusion: the light sensor system is not yet achieved.</b>
 +
</font>
 +
</div>
 +
 +
<div class="group center">
 +
<p class="highlight">
 +
<b>In a nutshell:</b><br>
 +
<br> About 70% of the clonings for the light sensor and the NOT logic gate has been done, but the last cloning steps remain to be finished before the tests.
 +
</p>
 +
</div>
 +
<br><br><br><br><br>
  
  
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<li>
 
<li>
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[2] Levskaya A, Chevalier AA, Tabor JJ, Simpson ZB, Lavery LA, Levy M, Davidson EA, Scouras A, Ellington AD, Marcotte EM & Voigt CA (2005) Synthetic biology: Engineering <i>Escherichia coli</i> to see light. Nature 438: 441–442
[2] REFERENCE 2 AVEC UN LIEN <a href="http://www.google.com/patents/US8647615">See more</a>
+
 
</li>
 
</li>
  

Revision as of 01:15, 19 September 2015

iGEM Toulouse 2015

Results


Content


Attract

Tests on varroas


Aim: to prove again varroa attraction for butyrate

In the US patent US 8647615 B1, the concentration of butyric acid that attracts varroa mites is 4 % (V/V), but the final description specified the concentration is efficient from 0.00001 %.

In order to verify the results presented in this patent and to make sure we are able to perform such experiment, we designed an attraction test for varroas. Champollion University in Albi welcomed us in their lab to get access to varroas. At the time of this test, there were not a lot of varroas available, so we add to make only one test in order to hope a significant result. Hence, we used the mentioned 4 % butyric acid concentration, like the one first mentioned in the patent.

Figure 1: Butyric acid test pie chart and statistical test

This test demonstrated that a solution of 4 % butyric acid attracts varroas. However, in the Cytotoxicity part of the results, we also showed that this 4% concentration is lethal for bacteria. We therefore decided to aim for a minimal concentration of 0.00001 % butyric acid, even if further experimentation will have to demonstrate the efficiency of this lower threeshold.

Conclusion: Our own protocol to test varroa attraction for butyrate is functionnal



Cloning butyrate genes


Aim: to create the genetic tool to implement a butyrate pathway in E. coli

We designed and ordered synthetic genes to express the full pathway minus the ccr gene. The latter was already placed behind lacI to be ready for circadian circle regulation. Thus, to build a biobrick with the full pathway, we cloned the ccr gene with all the genes necessary for butyrate production, as presented below:

Figure 2: assembly of the synthetic pathway leading to butyrate production (5192 Kb). The first arrow represents the promoter, the others represent genes. The green circles are for the RBS and the red circle is for the terminator. Purple genes are originated from Streptomyces collinus, blue genes from Clostridium acetobutylicum and yellow genes from Escherichia coli.

This construction (BBa_K1587004) was inserted in the plasmid pSB1C3 to be provided to the iGEM in the biobrick format.

Figure 3: Gel electrophoresis of the digestion of the (BBa_K1587004). The size of the first DNA fragment matches 5192 pb (butyrate construction) and the size of the second one (2070 pb) matches linearized pSB1C3.

Conclusion: The synthetic butyrate polycistron is ready to be used in E. coli



Test of butyrate production


Aim: to produce butyrate in E. coli

In order to test butyrate production, we cultivated ApiColi under micro-aerobic conditions to simulate the growth conditions in the device. After having harvested the supernatant of the culture, we filtrated it prior to do NMR analysis.

We tested our genetic construction in the E. coli strain BW25113 but we did not detect butyrate production. To improve the production of butyrate, we repeated the experiment in a strain with a deletion of the phosphate acetyltransferase gene. This strain produces lower amount of acetate, a fermentation product which production is likely to compete with butyrate.

Figure 5: Test of butyrate production in E. coli strain deleted for pta gene. Culture under micro-aerobic conditions in 10 mL falcon, result obtained after 28.5 hours of culture.


Unfortunately we could not detect butyric acid in our NMR analysis, but we observed significant differences for other fermentation products, proving that the synthetic pathway is actually interfering with the fermentation products metabolism. Additional experiments will be needed to go further (test of our enzymes expression, measurements of intracellular pools to identify metabolic locks, growth at other pH, deletions to prevent the accumulation of other fermentation products as explained in [1]...).


Conclusion: butyrate production has to be improved in our strain and under our conditions





In a nutshell:

Butyrate is an effective attractant for varroas. Its synthetic pathway has been implemented in E. coli. Butyrate production from this pathway remains to be demonstrated prior to testing the strain with varroas.






Eradicate

Tests on varroas


Aim: determinate the sufficient formate concentration to kill Varroa destructor

In order to determinate the amount of formic acid required to kill the mite, we tested different concentrations of formic acid on varroas as explained in the Protocol part.

Figure 6: Mortality of varroas as a function of time for different formic acid concentrations

Figure 7: Histogram representing mortality of varroas after 2 hours and after 7 hours

Figure 6 presents a dose-dependency of formic acid on varroa mortality. At 10mM formic acid, all varroas died before three hours but as we explained in the Protocol part, varroas also stop moving at lesser concentrations. Figure 7 shows that even with 50 µM of formate, around 30 % varroas died after 7 hours (so close to a night duration). We therefore set our production goal around 50 µmol.L-1.


Conclusion: 50 µmol.L-1 could be sufficient to kill varroas



Cloning formate genes


Aim: to create the genetic tool to implement a formate pathway in E. coli

We designed and ordered synthetic genes to express the full pathway minus the ccr gene. The latter was already placed behind lacI to be ready for circadian circle regulation. Thus, to build a biobrick with the full pathway, we cloned the ccr gene with all the genes necessary for butyrate production, as peresented below:

Figure 2: assembly of the synthetic pathway leading to formate production (3090 Kb). The first arrow represents the promoter, the others represent genes. The green circles are for the RBS and the red circle is for the terminator.

This construction (BBa_K1587007) was inserted in the plasmid pSB1C3 to be provided to the iGEM in the biobrick format.

Conclusion: The synthetic formate operon is ready to be used in E. coli



Test of formate production


Aim: to increase the production of formate in E. coli

For formate production, we designed a synthetic polycistron coding for pyruvate formate lyase and its activator protein. The polycistron was cloned in a pUC57 plasmid to be tested for formate production. We then made a biobrick by subcloning the polycistron in the pSB1C3 plasmid (BBa_K1587007).

Figure 8: extracellular substrate and product concentrations with or without formate polycistron overexpression under micro-aerobic conditions.

Figure 8 shows that the only difference between ApiColi and the control is in the formate accumulation, so we plotted the specific histogram for formate after 3 days (Figure 9). This highlights that formate production is increased significantly by 10%.

Figure 9: Formate production tests after 3 days cultivation under micro-aerobic conditions.

As mentioned earlier, our goal was to produce 50µM of formic acid in 7 hours. To reach this quantity, we needed to produce 77mM of formate. We observed a maximal formate concentration of 25mM (Figure 8). Therefore our production level is not far from the target. By improving the strain and/or conditions (as for Test of butyrate production), we can reasonably hope to reach this goal.


Conclusion: formate production was increased with our synthetic construction





In a nutshell:

formate is effective to kill varroas from as low as 50 µmol.L-1. Overexpressing its biosynthetic genes enables obtention of a 10 % increase in production. Optimizing this production will have to be achieved before testing the strain capacity to kill varroas.






Regulate

NOT logic gate


Aim: NOT logic gate construction

We want to express the butyrate synthetic pathway by day and the formate synthetic pathway by night. As a first step to develop our regulation module, we replaced the butyrate pathway by the RFP reporter gene and the formate pathway by GFP reporter gene. Doing so, it should be easy to assess the functioning of the light sensor and the NOT logic gate since the colonies should be red by day and green by night.
To do so, we synthesized the pOMPC-cI gene (new biobrick BBa_K1587006)Through subcloning the GFP reporter gene (BBa_E0240) was placed under control of this promoter. Likewise, the RFP reporter gene (BBa_K081014) was cloned behind the PL-lacI synthetic gene. Both constructions were placed together on the same plasmid but unfortunately sequencing revealed that there was some incoherence in the RFP sequence, preventing further testing.

Figure : NOT logic gate test construction


Conclusion: the strain can grow and survive over 10 days in the TPX® bag.

Light sensor


Aim: Light sensor system construction.

For our project we need to implement a light sensor system in our strain. To that end, we needed to clone the three genes (cph8, hoI, pcyA) of this system behind a strong promoter. We received the strain containing the three genes on different plasmids as courtesy from Dr Clark Lagarias, Christopher Voigt, and Nico Claassens [REF]. These genes were amplified by PCR and subcloned in pSB1C3 (at this step, a RBS was added to sequence during the PCR). The new biobricks for hoI and pcYA were sent to the registry under the accession number BBa_K1587000 and BBa_K1587002, respectively. The cph8 biobrick was not obtained but we managed to build a biobrick (BBa_K1587008) with the cph8 encoding sequence under the control of a strong promoter originated from the biobrick (BBa_J23119).

Figure : Targeted light sensor construction

Figure : cph8 biobrick

We did not succeed in associating the three genes together behind a strong promoter in pSB1C3 using the In Fusion cloning technique before the iGEM deadline. Consequently, we were not able to test our light sensor system soon enough for this project.


Conclusion: the light sensor system is not yet achieved.

In a nutshell:

About 70% of the clonings for the light sensor and the NOT logic gate has been done, but the last cloning steps remain to be finished before the tests.






ApiColi containment and culture

Growth tests in TPX bag


Aim: to perform the growth of the modified strain in a sealed container

We investigated if the bacteria could grow inside a small bag of TPX®. Thus, the strain E. coli BW 25113 has been inoculated in a small bag that was sealed. The bag was placed inside a tube and incubated at 37 °C. Growth was visualy assessed in the bags after 17 hours at 37°C (figure 9), or with a culture in tube with agitation as a control over the same period (figure 11). This showed that the cells were able to proliferate in the plastic bag.

Figure 10: Growth test of E. coli BW 25113 inside a small bag of TPX®.
(t = 17 hours, 37 °C).

Figure 11: Control of figure 10 with a growth test of E. coli BW 25113 in a culture tube.
The culture tube contains bacteria growing in parallel of the biological sample tubes shown above (t = 17 hours, 37 °C, 130 rpm).

We then assessed the biomass evolution during long term growth in the TPX® bag (7 to 10 days). Figure 12 displays the result. The growth tendency is toward a continous increase over the experiment time.

Figure 12: Growth test in TPX® bag by monitoring OD at 600 nm over 7 days to 10 days.

We then checked the viability of the cells after 10 days of growth in the TPX® bag (tube 1, 1' and 2) or in the control tube (tube 3), by spreading diluted volume of the cultures on Petri plates. Similar numbers of colonies were obtained, indicating that the cells survival is the same in both culture conditions.

Figure 13: Colonies of E. coli BW 25113 on Petri dishes after an overnight incubation at 37°C to check survivability.


Conclusion: the strain can grow and survive over 10 days in the TPX® bag.




Gas diffusion tests


Aim: ensuring that formate and butyrate permeate through the TPX® bag

For our system to be efficient, it was necessary to check that butyrate and formate could permeate through the TPX® bag. This was verified as described in the Protocol section. Shortly, a solution of butyrate or formate was placed in the TPX® bag. The molecules exchanged between the bag and the surrounding air were desorbed in a solution of sodium bicarbonate. This resulting solution was analysed by NMR.

On a first assay, we did not detect any permeation of butyric acid through the TPX® bag. The same experiment was therefore performed by placing directly the bag in the solution of sodium bicarbonate. The control is an injection of a 4% (V/V) solution of butyric acid in water.

Figure 14: NMR Spectrum of butyric acid liquid control in red and butyric acid liquid which passed through TPX bag in blue. * Blue curve is zoomed 1340 times more than red curve. Each condition was tested in two replicates.

Table 15: Concentrations of butyric acid calculated from the NMR spectrum

From these results, we can conclude that the TPX® enables butyric acid to weakly pass outside the bag. We detected only a small quantity but an optimization of the device could be made with a plastic containing bigger pores.

For the formic acid, we were able to detect it in the gas, probably because its pKa is lower than the butyric acid one.

Figure 16: NMR spectrum of formic acid gas control in red and formic acid gas which passed through TPX bag in blue.

Table 2: Concentrations of formic acid corresponding to NMR spectrum

According to these results, TPX® lets 56% of formic acid to pass outside the bag in the gas phase. Therefore, formic acid permeats through the TPX plastic. Using a more porous plastic as proposed for the butyrate, we supposed this percentage to further increase.


Conclusion: formate permeates through the TPX® while butyrate exchange will need to be improved



Safety tests


Aim: demonstration of TPX® capacity to retain the bacteria

The bacteria impermeability of the TPX® was tested through inoculation of the strain E. coli BW 25113 in M9 defined medium complemented with glucose inside the bag. Then, the inoculated bag was immersed in a glass measuring cylinder containing M9 medium with glucose. OD600 nm served to monitor a putative growth in the external medium.

Figure 17: Measuring cylinders used for the safety test of the TPX® polymer.
The cylinder on the left contains the TPX® bag with E. coli BW 25113 immersed in M9 medium after 27 hours of growth at 37 °C. On the right, the negative control cylinder contains a bag of TPX® without bacteria after 27 hours of growth at 37 °C.

Over this time, no growth was detected in the medium surrounding the plastic bag. We concluded that the bacteria are contained in the bag.


Conclusion: bacteria cannot permeate through the TPX® material

In a nutshell:

The TPX® material successfully contains growing bacteria while allowing the diffusion of formate and, in a lesser extend, of butyrate. These tests will profit to all Igem teams looking for a solution to contain their strains .






Growth tests

Our final objective was to prepare a bag containing bacteria producing alternatively butyric acid (during the day) and formic acid (during the night) and for a period of time of at least ten days (so that beekeepers don't have to change them every morning...).

So we faced some biological questions:

  • Can bacteria live during ten days under micro-aerobic conditions?
  • Which carbon source is suitable to produce continuously acids?
  • Are formic and butyric acids toxic for E. coli?

Characteristics of E. coli growth

In order to characterize the E. coli strain growth in the conditions we plan to use in our device, we used a culture under aerobic and micro-aerobic conditions, measured the growth rate (OD) and analyzed the supernatant to measure the concentration of fermentation products.

Micro-aerobic conditions were obtained with culture in specific Falcon tubes with holes and covered with a membrane that let the oxygen pass through. They were incubated at 37 °C without agitation (mimicking the aerobic and lack of agitation condition present in our device).

Aerobic conditions were obtained with a classical Erlenmeyer incubated at 37 °C with agitation.

We used a minimal M9 medium to identify formic or butyric acid production by NMR.

Biomass, substrate and products


Aim: measure formate production of wild-type strains in various culture conditions

In order to plot biomass concentration we converted the OD values with the following equation:

$$ X=OD_{600nm}\times 0,4325 $$

Where X is the cell concentration (g.L-1)

Substrate and products concentration was inferred from the peak area of each molecule on the NMR spectrum.
Concentrations were calculated with the following equation:

$$[A]=\frac{Area_{molecule}}{Area_TSP} \times [TSP] \times \frac{\textrm{TSP proton number}}{\textrm{A proton number}} \times DF $$

  • [A] = concentration of molecule in our solution in mM
  • AreaTSP = 1
  • [TSP] = 1.075 mM
    concentration of Trimethylsilyl propanoic acid in NMR tube, internal reference for quantification
  • TSP proton number = 9
  • DF = Dilution Factor = 1.25

Biomass and concentration of various molecules were issued from fermentation under aerobic and microaerobic conditions.

Figure 18: Results of micro-aerobic and aerobic culture after 5 days. Culture of E. coli BW25113 in M9 medium with [glucose] = 15 mM, at 37 °C

Glucose was consumed approximately at the same rate for both conditions but was converted differently depending on the condition. Under aerobic conditions the biomass reached 3 g/L whereas under micro-aerobic conditions there was six times less biomass. On the contrary, there were far less products in aerobic conditions.

Our results were in line with our objective to produce acids in a microporous bag because we demonstrated that with microaerobic conditions, bacteria grew slowly and produced fermentation products.

To convert the concentration of formate into formic acid we used the famous Henderson Hasselbalch equation:

$$ pH=pKa+log \left(\frac{C_{b}}{C_{a}} \right) $$

  • pH = 7 (pH of the M9 culture).
  • pKa: 3.7 for formic acid and 4.8 for butyric acid
  • Cb: base concentration
  • Ca: acid concentration

As mentioned in the Eradicate part, our goal was to produce 50 µM of formic acid to kill varroa, and this corresponds to 77,7 mM of formate.

Under our microaerobic conditions, the wild type E. coli strain produced 32 mM of formate. It was therefore necessary to further improve the metabolic production by adding genes involved in formate production in order to increase it by a factor of 240%. For a perfect regulation it would have been necessary to delete the chromosomic version of pflB in E. coli genome to avoid formate production during the day.


Conclusion: wild type E. coli strains produce formate in microaerobic conditions but production has to be further improved with metabolic engineering of the formate pathway.

Bacteria survival


Aim: verify how does E. coli survives in long culture conditions

As explained here calculation of the survival of bacteria was perfomed via colony counting after plating on solid agar medium. Wild-type bacteria can easily survive at least 15 days in aerobic or microaerobic conditions without a carbon source. We hypothesized that in presence of a carbon source, we may be able to extend even further their survival time period.

Figure 20: Bacteria survival test performed with the BW25113 strain on M9 with 15 mM of glucose during 15 days.

Conclusion: E. coli can survive at least 15 days under normal culture conditions


In a nutshell:

E. coli can produce various fermentation products when cultured in microaerobiosis and can survive up to two weeks in normal culture conditions. This validates the possible use of long lasting cultures in our device.






Choice of the carbon source to sustain formic and butyric acids production during 10 days

Characteristics of the Biosilta kit


Aim: determination of the right enzyme concentration for controlling E. coli growth

En Presso B is a technology which enables production of a lot of recombinant proteins owing to a slow carbon source delivery during 24 hours. This technology is based on polymer degradation by an depolymerisation enzyme leading to a perfect control of the quantity of substrate released in the medium for a given period of time. We wanted to use this technology to cultivate our cells during one or two weeks in conditions allowing the production of butyrate and formate. The medium with the polymer was solid and contained in separate bags. In order to quantify the production of butyrate and formate in these conditions, we first performed a test with a high concentration of the enzyme (50 U/L) and measured the kinetics of the polymer degradation.

Figure 21: Kinetics of Biosilta polymer degradation measured with glucose release.

In order to have a global idea of the release of glucose per unit of time we then calculated the average rate of release with the following formula:

$$ v_{glucose1}=\frac{[glucose]}{time}=\frac{11.1}{3.97}=2.80 g.L^{-1}.h^{-1} , for [E]_{1}=50 U.L^{-1} (4) $$

With a final glucose concentration of 13 g/L for one bag of polymer, the rate of glucose release necessary to have glucose during 13 days can be calculated with the following formula.

$$ v_{glucose2}=\frac{13}{13 days}=\frac{13}{322 hours}=0.0403 g.L^{-1}.h^{-1} (5) $$

The reduction factor was calculated:

$$ RF=\frac{v_{glucose1}}{v_{glucose2}}=\frac{2.8}{0.0403}=69.44 (6)$$

So, the concentration of enzyme that we had to use should have been:

$$ [E]_{2}=\frac{[E]_{1}}{RF}=\frac{50}{69.44}=0.72 U.L^{-1} (7)$$


Conclusion: we now know the exact concentration of enzyme to use in the BioSilta kit in order to release glucose for 13 days.

Growth assessment using the Biosilta kit


Aim: perform various culture conditions to verify the optimal enzyme concentration under real culture conditions

As we do not know any growth characteristics of bacteria in the Biosilta medium we tested different enzyme concentrations and not only the one calculated previously. Acquisition of the growth stopped due to a problem with the plate reading system where the bacteria where growing. Hence, there is a break at 5 days.

Figure 22: Bacteria growth as a function of different enzyme concentrations in Biosilta medium. Growth tests were performed in 48 wells plates attached to an OD reader.

Except for the 1.5 U/L enzyme concentration, there was an increase of OD in all conditions during 12 days. This means that glucose releasing worked as expected. At the beginning there is an exponential growth because some glucose is directly available in the medium. This phase was therefore not the one to analyse. After 2 days and until the end, the growth rate changes gradually and becomes constant, at the highest between 2 and 4 days and decreasing thereafter.

We therefore knew that it was possible to have continuous growth during at least 12 days. However, in our control without enzyme, there was also some bacterial growth so either another substrate was available or bacteria could realy degrade the polymer. More tests will be needed to verify if and how E. coli can use this polymer.

In order to answer these questions we performed some cultures in Falcon tubes to analyse the products and the evolution of the polymer quantity. Culture without enzyme and with a concentration of enzyme of 0.72 U/L were performed.

Figure 23: Results of BW25113 culture on Biosilta medium after 4 days. Test of two conditions: without enzyme and with [Enzyme] = 0.72 U/L

Figure 23 (right panel) shows that the polymer is not degraded by E. coli which is probably capable of finding another carbon source within the Biosilta medium. The concentration of products (left panel) is clearly different in the two conditions: with the enzyme, more fermentation products are formed.



Fermentation products have high concentrations in comparison to the cultures in performed in M9 (15 mM glucose), around 20 times more for lactate, 3 times more for acetate and 2 times more for ethanol. We hypothesized that by deleting the various production pathways for lactate, acetate and ethanol and the degradation pathway for formate we should be able to produce enough formate.


Conclusion: Fermentation products concentration is higher with the enzymatic degradation of the Biosilta polymer.

Modelling of formic and butyric acid production


Aim: model the production of formic and butyric acids as a function of the glucose release rates (Biosilta medium).

With the rate of glucose release calculated above, an FBA and FVA simulation were launched as explained in Modeling part. Some conversion between the model and the real condition are necessary and they are also explained there.

In order to model production in the most similar conditions to fit real experiment we chose a glucose rate of 0.0403 g.L-1.h-1 that correspond to 0.72 U/L of enzyme.

To convert formate production into formic acid concentration we used equation (3).

Figure 25: Modeling of formic acid production as a function of different growth rates for a glucose release of 0.0403 g.L-1.h-1.

Our goal was to produce at least 50 µmol/L. The graph clearly shows a maximum production of 6 µmol/L. To reach our goal we then have to change the growth rate.

Figure 26 : Modelling formic acid production as a function of glucose release for different growth rates (in h-1).

To produce 50 µmol/L of formic acid different strategies are possible. If we choose a low growth rate then a low glucose rate is be necessary and vice-versa. As bacteria have to live during at least ten days it was better to have a continuous slow growth rate. Moreover, it would consume less glucose per hour so we would need a lower polymer concentration in our bag at the beginning. Thus, we chose a growth rate of 0.2h-1, and we could determinate the glucose release value needed.

[Formic acid] (μmol.L-1 )=166.88 ×[Glucose] (g.L-1.h-1 ) (9) $$ [Glucose] = \frac{50}{166.88}=0.3 g.L^{-1}.h^{-1} (10) $$

Now, we will see which butyric acid concentration we can theoretically produce.

Figure 27: Modelling of butyric acid production as a function of glucose release for different growth rates.

According to the modelling results described in figure 10, we have to produce around 100µmol/L of butyric acid that corresponds to 0.0092% (V/V). As explained in the "Results" part, "attract" section, our objective was to produce at least 0.00001%, so the modelling indicates that we can theoretically reach it.

Nevertheless, in order to have this glucose release production, it was necessary to calculate how much polymer and enzyme concentration are needed.

With the same equations as we used in Characteristics of Biosilta kit we determined which total quantity of glucose is needed for a fortnight.

$$[Glucose]=v_{glucose}\times time=0.3\times 322=96.6 g.L^{-1} (11)$$

Knowing that one Biosilta kit contains the equivalence of 13 g/L of glucose, we should therefore concentrate the medium 7 times. To obtain a glucose release of 0.3 g.L-1.h-1 value, we nee 5 U/L of enzyme. Thus, we tested different concentrations of Biosilta medium with different enzyme concentrations.


Question: Biosilta medium has to be used in a more concentrated manner to reach our growth objectives.

Testing different concentrations of Biosilta kit


Aim: determine if we can use the Biosilta medium outside the fabricant's recommendations.

We do not have the exact composition of Biosilta medium and cannot ascertain innocuity of higher titers of the polymer on growth. The best approach was to actually test it directly.

Figure 28: Bacteria growth as a function of time for different concentrations of Biosilta medium. Culture with BW25113 on 48 wells plate and optical reader

From these three figures it is evident that we cannot reach the growth level that is expected and that concentrated Biosilta is actually detrimental to growth.


Conclusion: we cannot use concentrated Biosilta.

In a nutshell:

Biosilta medium can be used in many various conditions that can actually exactly control the glucose release rate and hence the growth of bacteria. However ,our target quantity of formic acid cannot be attained with normal medium concentrations and it is not possible to use higher doses of polymer in order to promote the growth to acceptable values (growth inhibition).






Testing acids toxicity

Effects of medium


Aim: test formic acid effects on E. coli growth.

In order to measure BW25113 resistance to different acids concentrations we tested two medium: LB and M9 with 15 mM glucose.

Figure 31: Optical density in function of time for different formic acid concentrations. LB medium is represented with green curves and M9 medium with blue curves. Each condition is tested in three replicates and standard deviation is represented in orange.

Figure 32: Optical density in function of time for different butyric acid concentrations and two medium. LB medium is represented with green curves and M9 medium with blue curves. Each condition is tested in three replicates and standard deviation is represented in orange.

In M9 medium, the growth is slower at the beginning of the growth curve, however the maximum value of OD reached is almost the same for both medium.

There is a visible difference between the two culture media. Growth in LB is clearly less tolerant to either formic or butyric acid. As M9 (and not LB) is a buffered culture medium we measured the pH of both media with different acids concentrations.

Figure 33: pH of the two media in function of the concentration of formic acid and butyric acid. LB medium is represented with green curves and M9 medium with blue curves. pH was measured with pH paper. Each condition was tested three times.

It is clear that in M9 medium the pH remains at pH 7 at high acid concentrations but not in the LB medium. Moreover we now know that bacteria do not grow when the pH is around 5. We will now see if buffering our medium has a positive action on growth.


Conclusion: production of formic or butyric acids can affect the pH of the medium. The M9 medium is capable of buffering the addition of both acids and is therefore better for growth.

In a nutshell:

LB and M9 media have different effects on the inhibition of growth with formic or butyric acids. M9 has a much higher buffering capacity than LB. M9, although slower for growth is probably the best compromise for butyric and formic acid production.






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References

  • [1] REFERENCE 1 Saini M, Wang ZW, Chiang C-J & Chao Y-P (2014) Metabolic Engineering of Escherichia coli for Production of Butyric Acid. J. Agric. Food Chem. 62: 4342–4348
  • [2] Levskaya A, Chevalier AA, Tabor JJ, Simpson ZB, Lavery LA, Levy M, Davidson EA, Scouras A, Ellington AD, Marcotte EM & Voigt CA (2005) Synthetic biology: Engineering Escherichia coli to see light. Nature 438: 441–442
  • [3] REFERENCE 2 AVEC UN LIEN qui ouvre dans une nouvelle fenêtre See more