To make sure that the OmpX-T18 and OmpX-T25 are expressed in the external membrane, OmpX fusion proteins have been muted to be able to integrate an unnatural amino acid in one of their extracellular loops by implementing the amber stop codon TAG. A specific tRNA can then add an azido-modified amino acid to the protein, these modified proteins are called COMPs.
The azido group of the protein reacts with a DIBO group, the reaction allows to click the extracellular DIBO to the functionnalized biosensor (COMP) protein.
COMPs are fused with T18 or T25 subparts and have to be expressed at the external membrane of the bacteria.
To ensure this, microscopy observations have been done with an Dalexia 488 conjugated DIBO group.
Fluorescent microscopy observations of the COMP, COMP-T18 and COMP-T25 clickable proteins show surface labelled bacteria indicating that a the recombinante proteins are expressed at the external membrane of E. coli.

The bioluminescence intensity produced by the NanoLuciferase enzyme is determined.
Several experimental conditions are tested using decreasing amount of bacterial culture (100µL, 25µL, 5µL and 1µL) at OD600nm = 0.6 : respectively 48E+06 CFU, 12E+06 CFU, 24E+05 CFU and 48E+04 CFU .
In addition, times of induction are tested from 0 to 360 minutes with 30 minutes increments.
Cultures of the different recombinant bacteria are incubated overnight at 18°C under shaking in order to induce an optimal COMPs proteins production [http://2015.igem.org/Team:TU_Eindhoven cf Team Eindhoven 2015].
The low temperature allows a native protein folding and membrane insertion to avoids as much as possible the formation of inclusion bodies.

Then cultures are diluted at OD600nm = 0,4 and let to grow to OD 600nm = 0.6 before induction.
The induction is performed by addition of 0,5 mM IPTG and 2mM of ATP for different periods of time. Bacteria are incubated at 37°C under shaking (180 rpm) to allow an optimal NanoLuciferase production.

After induction, 1, 5, 25 or 100µL of bacteria are distributed in a 96 wells black NUNC plate (ThermoFisher) and the Nano-Glo® Luciferase Assay assay from Promega® is performed (More informations) :
“Prepare the desired amount of reconstituted Nano-Glo® Luciferase Assay Reagent by combining one volume of Nano-Glo® Luciferase Assay Substrate with 50 volumes of Nano-Glo® Luciferase Assay Buffer.For example, if the experiment requires 10 mL of reagent, add 200μl of substrate to 10 mL of buffer.”
Then the amount of bioluminescence is measured using a luminometer by recording Relative Luminescence Units (RLU).

Several measures are made in the same well in order to reduce incertitude induced by the luminometer.
In order to test the reproducibility of our measures the means of 3 differents experiments with 3 measurements per well are calculated.
Data are expressed as the mean +/- standard deviation.

Several controls are performed:
∅ IPTG, ∅ ATP : To check the promoter leakage without any induction.
∅ IPTG, 2 mM ATP :To check if the addition of extracellular ATP helps the production of cAMP and to check if addition of ATP modifies the promoter leakage.
0.5 mM IPTG, ∅ ATP : To check if adding extracellular ATP is needed for protein expression.
0.5 mM IPTG, 2 mM ATP : Is the experimental condition, it correspond to the measure at 360min.

Results

The mBACTH following results are obtained with 5µL of bacteria at OD600nm = 0.6 : 24E+05 CFU.
With 1µL (48E+04 CFU), the bioluminescence intensity was too low and the measurement were not discriminant enough.
Above 25µL of bacteria (12E+06 CFU), the signal was quickly saturated when the induction time increased and the luminometer could not record workable measures.
5µL (24E+05 CFU) is a good compromise, it’s enough to have a discriminant signal and sensitive enough to work as a small drop in our NeuroDrop device.
iGEM Grenoble-Alpes device NeuroDrop is designed for the use of small volumes of biological sample like drops. Proving that 5µL of bacteria are enough to detect a significant difference in bioluminescence intensity between negative and positive conditions result was a challenge that we have overcome. Other reagents (see the full system) will be added to the drop of bacteria and its volume should not exceed 20µLworkto allow its automatic moving on the surface of the device.


Means of measurements obtained through 3 differents experiments with 3 measurements per well for each condition of the mBACTH generated with either
BBa_K3128018 and BBa_K3128017 : AC sub-parts fused to OmpX  : negative condition,
or BBa_K3128026 and BBa_K3128027 : Leucine Zipper mediated reconstitution of AC : positive condition.
Blank was done with 24E+05 CFU of untransformed BTH101 (RLU = 300) and subtracted to each measurements.

Using positive control strain, we measured 1.48E+06 RLU of bioluminescence produced in the 0.5 mM IPTG condition compared to 9.02E+05 in the condition without IPTG and without ATP, indicating that IPTG increase slightly the transcription.
Additionally, with 2.55E+0,6 RLU of bioluminescence produced in the condition without IPTG and 2mM ATP condition compared to 9.02E+05 in the without IPTG and without ATP condition, it seems that ATP have a significant* effect on transcription.
This was expected because of the lack of ATP in the periplasm of the bacteria. Thereby, adding a great amount of ATP in the medium able to diffuse in the periplasm help the cAMP production by the periplasmic adenylate cyclase.
Obviously, those observations do not prove anything but give clues on the way the system operates.
* A T test was done for the values of time above 90 min and led to a p-value below 0.01.


<i>Luminescence production over time of induction for the negative condition strain (red curve) and the positive condition strain of the mBACTH assay (blue curve).
Area of the significant* difference between both curves is highlighted in yellow.
Blank was done with 24E+05 CFU of untransformed BTH101 (RLU = 300) and subtracted to each measurements.
* A T test was done for the values of time above 210 min and led to a p-value below 0.05.

From 0 to 120 minutes of induction time, the bioluminescence produced by the two strains is similar.
At 120 minutes, the two curves start to split and give rise to a significant difference between the free sub-parts : negative condition and the Leucine Zipper: positive condition from around 210 minutes.

The discrepancy keeps increasing upon time of induction, thus highlighting the efficiency of the amplification signal thanks to the signalling cascade and the strong reporter gene.

From 0 to 120 minutes of induction time, the bioluminescence produced by the two strains is similar.
At 120 minutes, the two curves start to split and give rise to a significant difference between the two strains from around 210 minutes the negative condition strain compared to the Leucine Zipper_positive condition .

The discrepancy keeps increasing upon induction time, thus highlighting the efficiency of the amplification signal thanks to the signalling cascade and the strong reporter gene.

Conclusion

There is a significant difference between the negative and the positive condition of the mBACTH assay,
suggesting that a Bacterial Adenylate Cyclase Two-Hybrid can be successfully performed in the periplasm of bacteria which property is required for the sensing and detection of extracellular molecules.

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