Difference between revisions of "Team:Toulouse/Results"

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.

This test demonstrated that a solution of 4 % butyric acid attracts varraos. However, in the Cytotoxicity. part of the results, we also show 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.

We investigated if the bacteria can 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).

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.

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

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

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

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.

In the end, our objective is to have a bag which contains bacteria to produce alternately butyric acid and formic acid during at least ten days in order to be practical for beekeeper.

So we faced some biological questions:

In order to know better the E.coli strain we would use for our project, we made a culture in aerobic and micro-aerobic conditions. We sampled OD and supernatant to see what happened in it.

Micro-aerobic condition is obtained thanks to cultivation in specific falcons with holes covered with a membrane into the plug which let the oxygen pass through without opening the falcon. They were incubated at 37 °C without agitation to best correspond to our real condition.

For the medium, we use a minimal medium M9 because we want to follow acids production by NMR. And we chose a standard glucose concentration, 15mM.

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

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

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

Glucose is consumed approximately at the same rate for both conditions but it is not use for the same thing at all. In aerobic condition biomass reaches 3 g/L whereas in micro-aerobic condition there is six times less biomass. To the contrary, there are far less products in aerobic conditions, and bacteria consume them when there is not glucose anymore, than in micro-aerobic condition.

For our objective to produce acids in a microporous bag, it is a really interesting results to have naturally bacteria which have slow growth and fermentation products.

We can convert formate concentration into formic acid to know how much more we will have to produce to kill varroa. Indeed, the bacteria produce a base but it is the acid that interests us.
The formula below is used:

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

As it is said in the Eradicate> part, our goal is to produce 50 µM of formic acid to kill varroa, thanks to the equation (3) we know it corresponds to 77,7 mM of formate.

At the maximum the bacteria produces 32mmol/L of formate. It is necessary to add genes involved in formate production to regulate production and increase it by 240%. For a perfect regulation it would be necessary to delete pflB in E.coli genome to avoid formate production by day.

As it is explained here we plated bacteria on Petri dish to know if they were alive or not because OD measure cannot discriminate alive bacteria from dead. This test show us that wild type bacteria can easily survive during at least 15 days. So if we bring them a carbon source during this period they should survive even better.

Figure 20: Bacteria survival results from culture test with BW25113 on M9 with 15mM of glucose during 15 days to mime real survival condition.

En Presso B is a technology which enables production of a lot of recombinant proteins thanks to a low substrate delivering during 24 hours. This technology is based on polymer degradation by an enzyme leading to a control of the right quantity of substrate at each moment. We wanter to use this technology to cultivate our cells during one or two weeks in good conditions in order to produce butyrate and formate. The medium with the polymer was solid and contained in separate bags. To know which quantity of butyrate and formate we can produce, we had to know the quantity of substrate we could obtain with the polymer so we made a kinetic test with a high enzyme concentration (50 U/L).

Figure 21: Kinetic test of enzyme which degrades polymer from Biosilta kit. [Enzyme] = 50 U/L in order to have a complete degradation of polymer.

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

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

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

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

As we do not know any growth characteristic with Biosilta medium we tested different enzyme concentrations and not only the one which allow growth during 13 days. We made acquisition in two times because of software constraints, this is why there is a break at 5 days:

Except for 1.5 U/L enzyme, OD increase during 12 days so glucose releasing seems to function well. At the beginning there is an exponential growth because some glucose is directly available on medium. Since 2 days until the end growth is linear, only the slope change. It is higher between 2 and 4 days than after probably because bacteria were in worse condition after few days so they were not able anymore to consume the remaining glucose.

Thanks to those results we know it is possible to have continuous growth during at least 12 days. The only problem is that our control, without enzyme, grows also so either another substrate is available or bacteria could able to degrade polymer that could be a problem.

In order to answer these questions we did culture in falcon in order to analyze products and to see evolution of polymer quantity. We made culture without enzyme and we test concentration of enzyme of 0.72 U/L because it this one which allows to reach the highest OD in figure 22 and it is the one we calculated above to have glucose releasing during 13 days.

Figure 23 shows that polymer is not degraded, so it is only enzyme of Biosilta kit which released glucose. Enzyme concentration could be correlated to glucose releasing rate for further modeling. So, bacteria found another carbon source in Biosilta medium but we were not able to determine which one. Products concentrations are nearly identical to those in M9 medium.

In figure 24, polymer area decreased, so enzyme degraded it well. At the beginning, glucose concentration was almost constant so bacteria consumed it directly when enzyme releases it. At the end glucose concentration increases a bit, bacteria either did not consume it as fast as the beginning or they consumed formate because its concentration decreases. This could be a problem for us because we look for produce more formate so we would have to think about it.

Fermentation products have high concentrations in comparison to culture in M9 with 15mM of glucose, around 20 times more for lactate, 3 times more for acetate and 2 times more for ethanol. So if we delete production ways for lactate, acetate and ethanol and degradation way of formate we would able to produce enough formate and butyrate.

With the rate of glucose 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).

Our goal was to produce at least 50µmol/L and with this graph the maximum production could be 6µmol/L. So we had to produce nearly 10 times more formic acid. In order to reach our goal we could see which rate of glucose we needed with a reverse thought.

To produce 50 µmol/L of formic acid different strategies were available. Either a low growth rate could be chosen so that a low glucose rate would be necessary, or a high growth rate could be chosen and a high glucose rate would be necessary. 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 rate needed.

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

According to modeling results in figure 10, we would have been able to produce around 100µmol/L of butyric acid that corresponds to 0.0092% (V/V). As we explained in "Results" part, "attract" section, our objective was to produce at least 0.00001%, so with modeling we reached it.

Nevertheless, in order to have this right glucose rate it was necessary to calculate how much polymer is needed at the beginning and which enzyme concentration.

With the same equations as we used in Characteristics of Biosilta kit we could determine which quantity of glucose is needed in total during 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 had to concentrate the medium 7 times. Concerning the glucose rate, the 0.3 g.L^-1.h^-1 value correspond to 5 U/L of enzyme. Thus, we tested different concentrations of Biosilta medium with different enzyme concentrations.

As we did not know the exact composition of Biosilta medium, we are not able to say if there is a molecule which could be toxic at high concentrations. We could only have a global analysis on our results :

There was hardly any growth during three first days, bacteria probably adapted themselves to the medium. During 3 days until the end, OD increased up to 1 for 1.5 U/L enzyme but it was still slow. Moreover, bacteria grew better with 0.72 and 1.5 U/L than with 3 or 4 U/L. It could be explained by an excess of glucose that inhibits bacteria growth. Indeed, enzyme could release too much glucose, that bacteria would not consume this fast, then glucose accumulated itself in medium. We tested with a less concentrated medium in order to see if latency period could be reduce.

With a 4 times concentrated medium, there was no latency period anymore but enzyme concentration did not seem to affect bacteria growth. Bacteria probably consumed all free glucose in medium and then enzyme did not have enough time to degradate the polymer. A longer period test would have been necessary to know if bacteria were able to consume glucose as fast as the enzyme released it. Maybe by testing a twice concentrated medium, we would have been able to answer it.

Decline of curve for 0.72U/L was not expected because in “normal” Biosilta medium, figure 5, bacteria grew during 12 days. We cannot explain this result, but it shows that it is complicated to work with a medium with an unknown composition.

Curves for 3 and 4 U/L enzyme or very similar so it seems that bacteria are not able to consume all glucose released by enzyme. As we only measured OD we do not know if bacteria would assimilate glucose for another way that growth metabolism.

Thanks to figure 11, 12 and 13 we know that it would not be possible to have enough polymer in our medium. As a solution we think to use a dialysis system: in one side there will be bacteria and in the other side there will be the polymer with enzyme. Membrane which separates them will allow only small molecules to pass like glucose. Thanks to this system our device will have enough substrate for two weeks.

Regarding the rate of glucose assimilation we could do additional tests where we would measure glucose in medium to determine maximum rate of assimilation. An optimization of this assimilation could be essential.

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

In M9 medium, growth was slower at the beginning in both figures but OD max was almost the same for both medium.

For formic acid, the only significant difference was for 10mM with a slower growth in LB than in M9. For butyric acid the difference was stronger because in LB bacteria did not grow anymore with 109mM, whereas in M9 there was growth.

In fact, M9 was buffered and not LB so we measured pH in both medium with different acids concentrations in order to see if there were a correlation.

It is clear that in M9 medium pH stayed at pH 7 for higher acids concentrations than LB medium. Moreover, thanks to previous figures, it is possible to see that bacteria did not grow anymore when pH was around 5. This results show that bacteria are sensitive to acid pH, but they may have resisted to higher acids concentrations if the medium were better buffered. We will now see if it would be interesting or not to have buffered better our medium.

Figure 34 shows a dose/response relationship between formic acid concentration and bacteria growth. We wanted to produce at least 50µmol/L of formic acid in order to kill varroas and bacteria growth normally up to 1mM. So, there should not have been toxicity problems during the treatment.

The higher butyric concentration was, the less was bacteria growth, as our previous results with formic acid. However we had an intermediate result with 109mM of butyric acid. We did not have a specific butyric acid concentration to produce and modeling showed us that we could produce around 15mM with all optimizations, so we would not have any butyric acid toxicity during our treatment.

Note: For our conclusions about acids toxicity, we consider that acids evaporate during day for formic acid and night for butyric acid, so there would not be a lot of acid accumulation in the medium.