Team:Aachen/Project/2D Biosensor

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= 2D Biosensor =
= 2D Biosensor =
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With our 2D biosensor technology we are able to detect the pathogen ''Pseudomonas aeruginosa'' on solid surfaces. The sensor system is comprised of '''two distinct but inseparable modules''', a biological part and a technical part:
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With our 2D biosensor technology we are able to detect the pathogen ''Pseudomonas aeruginosa'' on solid surfaces. The sensor system is comprised of '''two distinct but inseparable modules''', a biological and a technical part:
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* Sensing chips containing '''''Cellocks''''', our '''engineered detective cells''' that fluoresce in the presence of the pathogen, make up the biological part of ''Cellock Holmes''.
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* Sensor chips containing '''''Cellocks''''', our '''engineered detective cells''' that fluoresce in the presence of the pathogen, make up the biological part of ''Cellock Holmes''.
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* Our '''measurement device ''WatsOn''''' and the complementary '''softare ''Measurarty''''' complete our sensing technology on the technical side.  
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* Our '''measurement device [https://2014.igem.org/Team:Aachen/Project/Measurement_Device ''WatsOn'']''' and the complementary '''software [https://2014.igem.org/Team:Aachen/Notebook/Software/Measurarty ''Measurarty'']''' complete our sensing technology on the technical side.  
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     <div class="team-item team-img" style="background: url(https://static.igem.org/mediawiki/2014/e/ef/Aachen_14-10-15_Medal_Cellocks_iNB.png); norepeat scroll 0% 0% transparent; background-size:100%;width:214px;height:214px;"> </div></a>
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<span class="anchor" id="biosensorpoo"></span>
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''Cellock Holmes'' is devised based upon a SynBio approach comprised of a '''two-dimensional biosensor and a measurement unit'''. The two-dimensional biosensor is designed to recognize quorum sensing molecules secreted by the pathogen cells and to generate a distinct fluorescence signal, while the measurement device recognizes and analyzes the produced signal. On the molecular side, we use the '''[https://2014.igem.org/Team:Aachen/Project/FRET_Reporter REACh construct]''' to turn regular ''E. coli'' cells into ''Cellocks''. We also developed another molecular approach as an alternative to our REACh approach: The '''[https://2014.igem.org/Team:Aachen/Project/Gal3 Galectin-3 construct]''' overcomes the limitation of the REACh method to bacteria that secrete quorum sensing molecules because it is based on tagging cells with a fluorescent reporter.
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''Cellock Holmes'' is designed upon a SynBio approach comprising a '''two-dimensional biosensor and a measurement unit'''. The two-dimensional biosensor is devised to recognize quorum sensing molecules secreted by the pathogen cells and to generate a distinct fluorescence signal; while the measurement device recognizes and analyzes the produced signal. On the molecular side, we use the '''[https://2014.igem.org/Team:Aachen/Project/FRET_Reporter REACh construct]''' to transform regular ''E. coli'' cells into ''Cellocks''.
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[graph quorum senising]
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{{Team:Aachen/Figure|Aachen 17-10-14 The basics of quorum sensing ipo.png||title=The principle of quorum sensing|subtitle=Microorganisms can sense the presence of their own kind based on quorum sensing which is a form of chemical communication. Depending on their cell density, quorum sensing allows these cells to activate or deactivate certain gene expression cascades (Waters and Bassler, 2005) for a specific function.|width=900px}}
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Our '''sensor cells are immobilized in agar chips'''. To make the chips, we mix the ''Cellocks'' with liquid LB agar.
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In the course of our project, we designed a casting mold specifically for the production of our agar chips. When the agar has cooled down, the chips are cut out of the mold and are ready to use. Storage of the readily usable sensor chips is possible for up to 2 days at 4 °C when using LB medium or up to 5 days if TB medium is used. A detailed description of the sensor chip manufacturing can be found in our [https://2014.igem.org/Team:Aachen/Notebook/Protocols/detection Protocols] section.
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{{Team:Aachen/Figure|Aachen 14-10-14 Flowsheet OD-device part1 ipo.png|title=How to use Cellock Holmes.|subtitle=This flow sheet shows the application of our 2D biosensor system.|width=1000px}}
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Our '''sensor cells, ''Cellocks'', are immobilized in agar chips'''. To make the chips, we mix the ''Cellocks'' with liquid LB agar.
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In the course of our project, we designed a casting mold specifically for the production of our agar chips. When the agar has cooled down, the chips are cut out of the mold and are ready to use. Storage of the readily usable sensor chips is possible for up to two days at 4°C when using LB medium or up to five days if TB medium is used. A detailed description of the sensor chip manufacturing can be found in our [https://2014.igem.org/Team:Aachen/Notebook/Protocols/detection Protocols] section.
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The application of ''Cellock Holmes'' for detection of ''P.&nbsp;aeruginosa'' cells is straightforward: Fist, a sampling chip is placed on a solid surface that is potentially contaminated with the pathogen. Second, the sampling chip is removed from the surface and put onto one of our sensor chips. Theorectically, the sensor chips could be directly used for sampling, however, this was avoided in our project to '''match  [https://2014.igem.org/Team:Aachen/Safety biosafety regulations]''' and to prevent the spread of GMOs into the environment. Third, the two layered chip-stack is put into a petri dish which is inserted into our measurement device [https://2014.igem.org/Team:Aachen/Project/Measurement_Device ''WatsOn''] for evalutation.
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{{Team:Aachen/Figure|Aachen 14-10-14 Flowsheet OD-device part1 ipo.png|title=Assay to detect ''P.&nbsp;aeruginosa'' using ''Cellock Holmes''|subtitle=This flow sheet shows the procedure to sample and detect ''P.&nbsp;aeruginosa'': A sampling chip is briefly put onto the potentially contaminated surface, added onto one of our sensor chips and inserted into ''WatsOn''.|width=900px}}
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Using ''Cellock Holmes'', we developed a simple assay to detect ''P.&nbsp;aeruginosa''. Initially, a so called sampling chip is placed on a solid surface that is potentially contaminated with the pathogen. Subsequently, the sampling chip is removed from the surface and put onto one of our sensor chips. Theorectically, the sensor chips could be directly used for sampling, however, this was avoided in our project to '''match  [https://2014.igem.org/Team:Aachen/Safety biosafety regulations]''' and to prevent the spread of genetically modified organisms (GMOs) into the environment. The two layered chip-stack is then put into a petri dish which is inserted into our measurement device [https://2014.igem.org/Team:Aachen/Project/Measurement_Device ''WatsOn''] for evalutation.
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{{Team:Aachen/Figure|Aachen 14-10-14 Flowsheet OD-device part2 ipo.png|title=Mode of action inside WatsOn.|subtitle=Chips are incubated and then illuminated with blue light. A picture is taken and analyzed for fluorescence signals using the software ''Measurarty''.|width=1000px}}
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{{Team:Aachen/Figure|Aachen 14-10-14 Flowsheet OD-device part2 ipo.png|title=Mode of action inside ''WatsOn''|subtitle=Chips are incubated at 37°C to stimulate cell growth and then illuminated with blue light to excite fluorescence. A picture is taken and analyzed for fluorescence signals using the software ''Measurarty''.|width=900px}}
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Inside ''WatsOn'', the chips are incubated at 37&nbsp;°C and populations of microorganisms stuck on the sampling chip start to multiply. ''P.&nbsp;aeruginosa'' secrets an increasing number of quorum sensing molecules in growing cell colonies which are taken up by our sensor cells where they induce a fluorescence signal. In case of ''P.&nbsp;aeruginosa'', we focused on a quorum sensing molecule called N-3-oxo-dodecanoyl-L-homoserine lactone, or 3-oxo-C-12-HSL for short, while our sensor cells comprise of genetically modified ''E. coli'' cells which are able to elecit a '''fluorescent response to the autoinducers'''. How we incorporate this 3-oxo-C-12-HSL detection system into the sensor cells is explained in more detail in the [https://2014.igem.org/Team:Aachen/Project/FRET_Reporter REACh Construct] section.
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Inside ''WatsOn'', the chips are incubated at 37°C and the sampled populations of microorganisms attached on the sampling chip start to grow and multiply. During incubation the chips can be '''illuminated with blue light''' at any time, and a '''photo of the chips''' is taken. The '''software ''Measurarty''''' then analyzes any fluorescent signal. ''P.&nbsp;aeruginosa'' secrets an increasing number of quorum sensing molecules that are recognized by ''Cellocks'', thereby producing a fluorescence signal. For detection of ''P.&nbsp;aeruginosa'', we focused on a quorum sensing molecule called N-3-oxo-dodecanoyl-L-homoserine lactone (for short: 3-oxo-C<sub>12</sub>-HSL), which is involved in virulence regulation of ''P.&nbsp;aeruginosa'' (Jimenez, Koch, Thompson et al., 2012). The incorporation of the 3-oxo-C<sub>12</sub>-HSL detection system into the ''Cellocks'' is explained in detail in the [https://2014.igem.org/Team:Aachen/Project/FRET_Reporter REACh Construct] section.
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During incubation the chips can be '''illuminated with blue light''' at any time, and a '''photo of the chips''' is taken. The '''software ''Measurarty''''' then analyzes any fluorescent signal. <!-- Depending on the intensity of the signal and the size of the spot, ''Cellock Holmes'' can '''calculate concentration and distribution of ''P.&nbsp;aeruginosa'' ''' on the sampled surface. -->
 
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<span class="anchor" id="biosensordevelopment"></span>
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=== Medium ===
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{{Team:Aachen/FigureFloat|Aachen_ILOV_GFP_HM_1,5h.png|title=iLOV and GFP in the Gel Doc<sup>TM</sup>|subtitle=Sensor cells producing iLOV (K1319042, A) and GFP (B) 1.5&nbsp;h after induction.|left|width=500px}}
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Prior to using our own device for detection of fluorescence emitted by the sensor chips we used equipment readily available in the lab. A Molecular Imager&reg; Gel Doc<sup>TM</sup> XR+ from BIO-RAD was available which uses UV and white light illuminators. However, only two different filters were available for the excitation ligth wavelength, which resulted in very limitted possibilities for the excitation of fluorescent molecules. For example, it was possible to detect the expression of iLOV in our sensor chips but the detection of GFP was not possible. It was thus determined that the Gel Doc<sup>TM</sup> was not suitable for our project.
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=== Equipment and medium selection ===
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Our first approach (before developing our own device) was to use the Molecular Imager&reg; Gel Doc™ XR+ from BIO-RAD in our lab to detect fluorescence. This device uses UV and white light illuminators. However, only two different filters were available for the excitation light wavelength, which resulted in very limited possibilities for the excitation of fluorescent molecules. For example, it was possible to detect the expression of iLOV (K1319042) in our sensor chips, but not the expression of GFP. Hence, the '''Gel Doc™ was not suitable for our project'''.
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{{Team:Aachen/FigureFloat|Aachen_Chip_medium_geldoc.png|title=Differend medium in the Gel Doc™|subtitle=complex media exhibited high background fluorescence while less back- ground fluorescence was observed with the minimal media (HM, M9, NA).|right|width=500px}}
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{{Team:Aachen/FigureFloat|Aachen_5days_K131026_neb_tb_1,5h.jpg |title=Testing our chips' shelf-life|subtitle= Chips of [http://parts.igem.org/Part:BBa_K131026 K131026] in NEB were stored five days at 4°C. The right chip was induced with 0.2&nbsp;µL of 500&nbsp;µg/mL HSL and an image was taken after 1.5&nbsp;h.|left|width=500px}}
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We tested different media (LB, TB, M9, NA and HM) for the preparation of our sensor chips. The medium compositions can be found in the [https://2014.igem.org/Team:Aachen/Notebook/Protocols Protocols] section. We screened for an optimized medium composition to minimize background fluorescence and to enhance cell growth. The results of the analysis are presented in the table below. Due to the low background fluorescence in ''WatsOn'' and the excellent cell growth, we '''chose LB&nbsp;medium''' over the other tested media for sensor chip manufacturing.
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{{Team:Aachen/Figure|Aachen_ILOV_GFP_HM_1,5h.png|title=iLOV and GFP in the Gel Doc<sup>TM</sup>|subtitle=Sensor cells producing iLOV (A) and GFP (B) 1.5&nbsp;h after induction.|width=900px}}
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{| class="wikitable"
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Regarding the medium used for our sensor chips, LB medium showed a high background fluorescence when exposed to UV light. Surprinsingly, the background fluorescence resulting from the LB medium was to high to detect a signal emitted by our sensor cells. Instead we tried using minimal media (NA, M9, Hartman (HM)) in order to minimize background fluorescence. The appliction of minimal media was sufficient to minimize the background fluorescence, but this approach came with the drawback of minimal to zero growth of our sensor cells.
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! !! LB !! TB !! NA !! M9 !! HM
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{{Team:Aachen/Figure|Aachen_Chip_medium_geldoc.png|title=Differend medium in the Gel Doc<sup>TM</sup>|subtitle=Full media (LB) exhibited high background fluorescence while less background fluorescence was observed with the minimal media (HM, M9, NA).|width=900px}}
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| Growth of ''Cellock'' || <div style="text-align: center;">'''+'''</div> || <div style="text-align: center;">'''+'''</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div>
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| Background fluorescence in GelDoc || <div style="text-align: center;">'''+'''</div> || <div style="text-align: center;">'''+'''</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div>
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| Background fluorescence in ''WatsOn'' || <div style="text-align: center;">-</div> || <div style="text-align: center;">'''+'''</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div> || <div style="text-align: center;">-</div>
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|}
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{{Team:Aachen/FigureFloat|Aachen_5days_K131026_neb_tb_1,5h.jpg |title=Testing our chips' shelf-life.|subtitle= [http://parts.igem.org/Part:BBa_K131026 K131026] in NEB induced after 5 days of storage at 4&nbsp;°C. The right chip was induced with 0.2&nbsp;µL of 500&nbsp;µg/mL HSL, and the image was taken 1.5&nbsp;h afterwards.|width=300px}}
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Another set of experiments were conducted to test the '''long-time storage''' of the sensor chips. We varied the glycerol content of the chips as well as the storage temperature. Storage at -20°C resulted in the loss of our sensor cells. Adding 5-10% (v/v) glycerol ensured survival of the sensor cells, but resulted in the loss of fluorescence ability. Hence, we concluded that long-time storage of the sensor chips at -20°C is not possible under the tested conditions. However, the 'ready-to-use' sensor chips can be kept at at 4°C for two days when using LB medium, and storage at this temperature for 5 days is possible with chips made from TB medium.
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<!--Regarding the medium used for our sensor chips, LB medium showed a high background fluorescence when exposed to UV light in the Gel Doc. Surprisingly, the background fluorescence resulting from the LB medium was too high to detect a signal emitted by our sensor cells. Hence, minimal media (NA, M9, Hartman (HM)) was used to minimize background fluorescence, but this approach resulted in less to no growth of our sensor cells. In our device ''WatsOn'', optimized wavelengths of 450&nbsp;nm and 480&nbsp;nm were used for excitation of iLOV and GFP, respectively. When exposed to either excitation wavelength TB medium, which is basically an improved LB medium and highly supports cell growth, showed strong background fluorescence in our own device. High background fluorescence was also observed for TB medum when using the Gel Doc. In contrast to the Gel Doc LB medium showed minimal fluorescence in our device ''WatsOn'' and no difficulties in cultivation of our ''Cellocks'' were observed. Because of the reduced fluorescence compared to TB medium when using ''Watson'' for sensor chip evaluation and because of sufficient cultivation conditions for our 'Cellocks'' LB medium was chosen over TB mediium for sensor chip manufacturing. -->
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Further experiments were conducted to test long-time storage of the sensor chips. Storage at -20 °C resulted in the loss of our sensor cells. Adding 5-10% (v/v) glycerol ensured survival of the sensor cells, but resulted in an expression stop of fluorescence proteins. Thus, the idea of long time storage of the sensor chips had to be passed on. However, it was possible to store ready-to-use sensor chips for 2 days at 4&nbsp;°C when using LB medium and storage for 5 days was possible with chips made from TB medium.
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{{Team:Aachen/FigureFloat|Aachen_2_chipform.jpg|title=Sensor chip manufacturing using the closed mold|subtitle=When injecting the liquid agar into a closed mold we encounter problems due to frequent bubble formation.|left|width=500px}}
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In our device ''WatsOn'', optimized wavelengths of 450&nbsp;nm and 480&nbsp;nm were used for excitation of iLOV and GFP, respectively. When exposed to either excitation wavelength the TB medium showed minimal background fluorescence and no difficulties were observed in cultivation of our ''Cellocks''. Furthermore, a reduction of background fluorescence compared to TB medium was observed when using LB medium for sensor chip manufacturing in combination with fluorescence evaluation using ''WatsOn''.
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=== Optimal Agarose Concentration for Sensor Chip Manufacturing ===
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For the sensor chip manufacturing, agarose was preferred over agar because of the uniform linkage between molecules that results in a better chip homogeneity. In addition, agarose reduced diffusion of the inducer molecules through the chip. A reduction in diffusion is essential for the formation of distinct fluorescent spots on the sensor chips.
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=== Agar Concentration ===
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{{Team:Aachen/FigureFloat|Aachen_Final_chipform.jpg|title=The finalized chip mold|subtitle=An open casting mold was found to be optimal for sensor chip manufacturing, because this approach was fast, easy to handle and generated a reproducible chip quality.|left|width=500px}}
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For the sensor chip manufacturing, a concentration of 1.5&nbsp;% agarose was found to be optimal. When agarose concentrations below 1.5&nbsp;% (w/v) were used the sensor chips were easily damaged and were not transportable. Agar concentrations above 1.5&nbsp;% (w/v) had to be avoided, because the agarose started to solidify before it could be poured into the chip casting mold.
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=== Optimal Chip Configuration ===
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Agarose was chosen above agar, because of more even linkage resulting in a better chip homogenity. In addition, agarose reduced diffusion of inducer molecules through the chip. A reduced diffusion was desired in order to achieve distinct fluorescent spots on the sensor chips.
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Several approaches were tested for the production of agarose-based sensor chips with reproducible quality. The first approach was to cast every sensor chip individually. To achieve a plain chip surface, a requirement for high quality images, we casted the sensor chips between two microscope slides. However, this approach was not adequate because the agar was too liquid and leaked from the microscope slides. In a second approach, we designed a closed mold into which liquid agar is injected using a pipette, but we encountered a high number of bubbles in the resulting chips. Bubbles in the sensor chips interfered with fluorescence evaluation. Finally, we tried an open casting mold. Once solidified, we cut the agar along precast indentations in the casting mold to form the chips. An advantage of the open mold is the ability to simultaneously produce nine sensor chips while the surface tension of the liquid agar ensures a plane chip surface.
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=== Induction of the Sensor Chips ===
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To test our molecular constructs, we simulated the presence of ''P.&nbsp;aeruginosa'' by using IPTG or 3-oxo-C<sub>12</sub>-HSL. Initial experiments showed that diffusion of the inducers hinder the formation of distinct fluorescent spots. Through this set of experiments we determined that the best compromise between diffusion and spot intensity is an induction volume of 2.0&nbsp;µL for IPTG and 0.2&nbsp;µL for HSL. Furthermore, detection of growing ''P.&nbsp;aeruginosa'' based on secreted HSLs was possible using the [http://parts.igem.org/Part:BBa_K131026 K131026] construct. The experiments for optimizing the induction of our sensor chips are described in more detail in the [https://2014.igem.org/Team:Aachen/Project/2D_Biosensor#biosensorachievements Achievements] section.
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=== Negative Control ===
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To ensure that the fluorescence signal resulted from the sensor construct and not from the medium or ''E. coli'' cells themselves, [http://parts.igem.org/Part:BBa_B0015 B0015] in NEB10β cells was used as negative control during sensor chip induction with IPTG, HSL and ''P.&nbsp;aeruginosa''.
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=== Chip Form ===
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{{Team:Aachen/Figure|Aachen_B0015_IPTG_HSL_Pseudomonas.png|title=B0015 in NEB10β was used as a negativ control|subtitle=Induction with A) 0.2&nbsp;µL of 100&nbsp;mM IPTG, image taken after 2.5&nbsp;h; B) 0.2&nbsp;µL of 500&nbsp;µg/mL 3-oxo-C<sub>12</sub>-HSL, image after 2.5&nbsp;h; C) with five spots of ''Pseudomonas&nbsp;aeruginosa'' liquid culture on the left and one big spot on the right, image taken after 2&nbsp;h.|width=900px}}
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Various approaches were tried for production of sensor chips with reproducable quality. The first approach was to cast every sensor chip individually. In order to achieve a plain chip surface, which was required for high quality images, we tried to cast the sensor chips between four microscope slides. This approach had to be rejected, because the agar was too liquid. In a second try, we produced a closed mold into which liquid agar was injected using a pipette, but we encountered a high number of bubbles in the chips when using this method. Bubbles in the sensor chips resulted in problems during fluorescence evalutaion.
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{{Team:Aachen/Figure|Aachen_2_chipform.jpg|title=Chips made using the closed mold|subtitle=With this method we encounter problems due to frequent bubble formation.|width=600px}}
 
-
 
-
Finally, we used an open mold into which the agar was poured right after mixing with the sensor cells. When the agar had solidified the chips were cut out along precast indentations in the casting mold. An advantage of the open mold was the ability to simultaneously produce nine sensor chips while the surface tension of the liquid agar ensured a plane chip surface.
 
-
 
-
{{Team:Aachen/Figure|Aachen_Final_chipform.jpg|title=Final chip mold.|subtitle=We found the above shown casting mold to be ideal for our purposes.|width=600px}}
 
-
 
-
=== Induction ===
 
-
For artificially induction of our molecular detection constructs we simluated the presence of ''P.&nbsp;aeruginosa'' by use of IPTG or 3-oxo-C12 HSL.
 
-
A minimal pipetting volume was desired for induction, because initial experiments showed that diffusion of the inducers through the chip hindered detection of distinct spots on the chips. The lowest volume we could pipett was limitted to 0.2 &nbsp;µL due to the pipetts available.
 
-
Sensor cells based on ''E. coli'' BL21, which incorporated the [https://2014.igem.org/Team:Aachen/Parts#partsK1319042 K1319042] construct were able to detect IPTG concentrations down to 1&nbsp;mM (0.2&nbsp;µL), so were sensor cells based on ''E. coli'' BL21, which incorporated the REACh constructs.
 
-
Sensor cells based on ''E. coli'' BL21, which incorporated the [http://parts.igem.org/Part:BBa_K131026 K131026] construct were able to detect HSL concentrations down to 500&nbsp;µg/mL (0.2&nbsp;µL). Further more, detection of growing ''Pseudomonas aeruginosa'' cells based on secreted HSLs was possible using the [http://parts.igem.org/Part:BBa_K131026 K131026] construct. A detailed description including pictures of the experiments leading to the findings mentioned above can be found in the [https://2014.igem.org/Team:Aachen/Project/2D_Biosensor#biosensorachievements Achievements] section.
 
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We are able to detect IPTG, 3-oxo-C{{sub|12}} HSL and ''Pseudomonas aeruginosa''. To prove that the sensor constructs produce the flourescence signal and not the medium or ''E. coli'' in its own we have B0015 in NEB as a negativ control for IPTG, HSL and ''Pseudomonas aeruginosa'' induction.
+
We developed and optimized a 2D biosensor, which is able to detect IPTG, 3-oxo-C{{sub|12}}-HSL and living ''Pseudomonas&nbsp;aeruginosa'' cells.  
 +
{{Team:Aachen/FigureFloat|Aachen_K1319042_Platereader.gif|title=Testing K1319042 in our sensor chips|subtitle=Sensor chips based on [http://parts.igem.org/Part:BBa_K1319042 K1319042] were investigated for fluorescence using a plate reader. Blue color indicates the absence of fluorescence, while red color indicates fluorescence. The upper chip was not induced, while the lower chip was induced with IPTG (2.0&nbsp;µL, 100mM).|width=260px}}
 +
=== Testing our Sensor Chips in a Plate Reader ===
-
{{Team:Aachen/Figure|Aachen_B0015_IPTG_HSL_Pseudomonas.png|title=Negativ control |subtitle=B0015 in NEB as negativ control induced with A) 0.2&nbsp;µl of 100&nbsp;mM IPTG, image taken after 2.5&nbsp;h; B) 0.2&nbsp;µl of 500&nbsp;µg/ml HSL (3-oxo-C12), image after 2.5&nbsp;h; C) with 5 spots of ''Pseudomonas aeruginosa'' on the left and one big spot on the right, image taken after 2&nbsp;h|width=900px}}
+
To establish a prove-of-principle for our sensor chip design, we used our construct [http://parts.igem.org/Part:BBa_K1319042 K1319042], an IPTG inducible iLOV. ''E. coli'' cells carrying the this construct were introduced into sensor chips and fluorescence was measured every 15&nbsp;minutes after induction with 2&nbsp;µL of 100&nbsp;mM IPTG. The results are displayed on the left.
 +
We observed a distinct difference in fluorescence between the non-induced chip (top) and the induced chip (bottom). The middle of the bottom chip started to exhibit a clear and visible fluorescence that increased over time and spread outwards. The top chip, however, also showed an increase in the measured fluorescence over time which was primarily due to the leaky promoter and background fluorescence.
-
=== Testing our Sensor Chips in a Platereader ===
+
{{Team:Aachen/FigureFloatRight|Aachen_K131026_Platereader.gif|title=Testing K131026 in our sensor chips|subtitle=Sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026] were investigated for fluorescence using a plate reader. Blue color indicates the absence of fluorescence, while red color indicates fluorescence. The lower chip was induced with with 3-oxo-C{{sub|12}}-HSL (0.2&nbsp;µL, 500 µg/mL).|width=360px}}
-
{{Team:Aachen/FigureFloat|Aachen_K1319042_Platereader.gif|title=Testing K1319042 in our sensor chips|subtitle=K1319042 in our sensor chip induced with 2&nbsp;µL IPTG and measured with a plate reader. Blue color indicates no fluorescence, red color indicates fluorescence. Top chip is not induced, bottom chip is induced with IPTG.|width=300px}}
+
-
To establish a proof of principle we used our construct [http://parts.igem.org/Part:BBa_K1319042 K1319042] an IPTG inducible iLOV. They were introduced into our sensor chips and then fluorescence was measured every 15 minutes after an induction with 2&nbsp;µl 100&nbsp;mM IPTG.
+
=== Detecting 3-oxo-C{{sub|12}}-HSL with Sensor Chips ===
-
There is a clear difference in fluorescence between the not induced chip (top) and the induced chip (bottom). It is distinctively visible that the middle of the bottom chip start to exhibit fluorescence and then the fluorescence increases over time and spreads outward. The top chip also shows a slight increase in measured fluorescence but it is nowhere near the level of the induced chip and is primarily attributable to a leaky promoter and the background fluorescence.  
+
In an initial attempt to detect 3-oxo-C{{sub|12}}-HSL, we incorporated the [http://parts.igem.org/Part:BBa_K131026 K131026] construct generated by the 2008 iGEM Team Calgary in our sensor chips. This construct generates a fluorescent signal based on GFP in presence of 3-oxo-C{{sub|12}}-HSL molecules produced by ''P.&nbsp;aeruginosa'' during quorum sensing (Jimenez, Koch, Thompson et al., 2012). First, we tested the construct by direct induction with 3-oxo-C{{sub|12}}-HSL (0.2&nbsp;µL, 500&nbsp;µg/mL). The fluorescence measurement was taken every 15&nbsp;minutes with an excitation wavelength of 496&nbsp;nm and an emission wavelength of 516&nbsp;nm. The results of this test are shown on the right.
-
This demonstrates a general proof of principle of the sensor chip design. Therefore the next was testing the detection of 3-oxo-C{{sub|12}} HSL.
+
A distinct fluorescence signal was observed on the induced chip (bottom) compared to the non-induced chip (top).  
-
<html></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br>
+
Fluorescence started in the middle of the chip, the point of induction, and then extended outwards while growing in intesnity. The basal level of fluorescence was attributed to leakiness of the promoter and general background fluorescence of growing ''E. coli'' cells. In the induced chip (bottom), the background fluorescence was lower than in the non-induced chip (top) because the signal masked the noise. The difference between the induced and non-induced chips indicates a clear response to the HSL and proofed the ability of our 2D sensor chip design to detect HSLs produced by ''Pseudomonas&nbsp;aeruginosa''.
-
</html>
+
 +
{{Team:Aachen/FigureFloat|Aachen_I746909_slower_reduced.gif|title=IPTG-inducible superfolder GFP (I746909) in sensor chips|subtitle=Expression of superfolder GFP ([http://parts.igem.org/Part:BBa_I746909 I746909]) was induced by the addition of IPTG (2&nbsp;µL,&nbsp;100mM) on the right chip. The left chip was not induced.|width=480px}}
-
=== Detecting 3-oxo-C{{sub|12}} HSL with sensor chips ===
+
=== Detecting IPTG with Sensor Chips ===
 +
The clip displayed on the left side shows the construct [http://parts.igem.org/Part:BBa_I746909 I746909] from the 2007 iGEM Team Cambridge which expresses super folder GFP under the control of a T7 promoter in combination with our 2D sensor chip technology. The [http://parts.igem.org/Part:BBa_I746909 I746909] construct was introduced into BL21(DE3) cells to make the expression IPTG-inducible since the genome of BL21(De3) contains the T7 RNA Polymerase under the control of a lacI promoter.
-
{{Team:Aachen/FigureFloatRight|Aachen_K131026_Platereader.gif|title=Testing K131026 in our sensor chips|subtitle=K131026 in our sensor chip induced with 0.2&nbsp;µL 3-oxo-C{{sub|12}} HSL and measured with a plate reader. Blue color indicates no fluorescence, red color indicates fluorescence.|width=300px}}
+
While the left chip does not show visible fluorescence, the right chip exhibits a strong fluorescence signal. This proves the ability of our sensor chip technology to detect IPTG. The fluorescence response is also high enough to be detected and analyzed by our measurement device ''WatsOn''.
-
As a next step, we used [http://parts.igem.org/Part:BBa_K131026 K131026] from the 2008 iGEM Team Calgary in our sensor chips to detect 3-oxo-C{{sub|12}} HSL which is produced by ''Pseudomonas&nbsp;aeruginosa'' during quorum sensing. First, we tested them by direct induction with purified 3-oxo-C{{sub|12}} HSL (0.2&nbsp;µL, 500&nbsp;µg/mL). A fluorescence measurement was taken every 15&nbsp;min with an excitation wavelength of 496&nbsp;nm and an emission wavelength of 516&nbsp;nm (for GFP).
 
-
The measured fluorescence again showed a distinct signal on the induced chip (bottom) compared to the uninduced chip (top). The fluorescence clearly starts in the middle of the chip (point of induction) and then extends outwards, still showing an ever increasing signal of fluorescence. The base level of fluorescence, even though a lot lower than the bottom chip, is attributed to leakiness of the promoter and general background fluorescence of growing ''E. coli'' cells. The difference between the induced and non-induced chips indicates a clear response to the HSL and a proof for the ability of our sensor chip design to being able to detect the HSL produced by ''Pseudomonas&nbsp; aeruginosa''.
 
-
<html></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></br></html>
 
-
===Detecting the 3-oxo-C{{sub|12}} HSL with K131026 in our sensor chip with WatsOn===
 
-
{{Team:Aachen/FigureFloat|Aachen_K131026_HSLdetection_slow.gif|title=Detection of 3-oxo-C{{sub|12}} HSL with K131026|subtitle=0.2 µl of 3-oxo-C{{sub|12}} HSL were put in the middle of the chip and then incubated at 37 °C in our WatsOn device.|width=480px}}
 
-
The next step for the eventual goal to detect ''Pseudomonas aeruginosa'' was to replicate the detection of 3-oxo-C{{sub|12}} HSL, which was established in the platereader, in our own [https://2014.igem.org/Team:Aachen/Project/Measurement_Device WatsOn] device. Therefore we again used K131026 as our construct in ''E. coli'' BL21(DE3) cells and induced with 0.2&nbsp;µl 3-oxo-C{{sub|12}} HSL with a concentration of 500&nbsp;µg/ml. The right chip was induced and the left chip was not induced as a negative control. Pictures were taken every 4 minutes.
 
-
The result clearly replicated the success of the platereader experiment. The induced chip shows a clear fluorescence response eminating from the center where the induction with HSL took place. This demonstrates the ability of not only our sensor chips but also our measurement device WatsOn to successfully detect 3-oxo-C{{sub|12}} HSL which is produced by ''Pseudomonas aeruginosa''.
+
{{Team:Aachen/FigureFloatRight|Aachen_K131026_HSLdetection_slow.gif|title=Detection of 3-oxo-C{{sub|12}}-HSL with K131026|subtitle=0.2&nbsp;µL of 3-oxo-C{{sub|12}}-HSL was placed in the middle of a sensor chip based on [http://parts.igem.org/Part:BBa_K131026 K131026] followed by incubation at 37°C in ''WatsOn''.|width=480px}}
-
<html></br></br></br></br></br></br></br></br>
+
===Detecting the 3-oxo-C{{sub|12}}-HSL with K131026 in our Sensor Chips using ''WatsOn''===
-
</html>
+
 +
The next step towards the final goal of detecting living cells of ''Pseudomonas&nbsp;aeruginosa'' was to reproduce the detection of 3-oxo-C{{sub|12}}-HSL, already established in the plate reader, in our own [https://2014.igem.org/Team:Aachen/Project/Measurement_Device ''WatsOn''] device. Therefore, we again used ''E. coli'' BL21(DE3) cells carrying [http://parts.igem.org/Part:BBa_K131026 K131026] and induced with 0.2&nbsp;µL 3-oxo-C{{sub|12}}-HSL of a concentration of 500&nbsp;µg/mL. In the clip displayed on the left, the right chip was induced and - as a negative control - the left chip was not induced. Pictures were taken every four&nbsp;minutes.
-
===Detecting Pseudomonas aeruginosa with K131026 in our sensor chip with WatsOn===
+
The result was a clear replication of the success of the plate reader experiment. The induced chip showed a clear fluorescence response eminating from the center, where the induction with HSL took place. This demonstrated the ability of not only our sensor chip technology but also our measurement device ''WatsOn to successfully'' detect 3-oxo-C{{sub|12}}-HSL.
-
{{Team:Aachen/FigureFloatRight|Aachen_K131026_Pseudomonas_aeruginosa_detection.gif|title=Detection of ''Pseudomonas aeruginosa'' with K131026|subtitle=Direct detection of ''Pseudomonas aeruginosa'' on our sensor chips. Sensor cell used were K131026.|width=480px}}
+
-
After establishing the successful detection of 3-oxo-C{{sub|12}} with our sensor chips the next step was the detection of ''Pseudomonas aeruginosa'' with our measurement device WatsOn. Therefore sensor chips with K131026 were again prepared and the right chip was induced with 0.2&nbsp;µl of a ''Pseudomonas aeruginosa'' culture while the left chip was not induced.
 
-
The results clearly demonstrate our ability to detect ''Pseudomonas aeruginosa'' with our measurement device WatsOn. On the induced chip a definite fluorescence response is visible in response to ''Pseudomonas aeruginosa''. The fluorescence eminates outward from the induction point and shows a significant difference to the non induced chip. Therefore detection of ''Pseudomonas aeruginosa'' is possible with our sensor chip technology in our measurement device WatsOn!
 
-
<html></br></br></br></br></br></br></br></br>
 
-
</html>
 
-
=== Comparing the REACh construct with K731520 and I746909 ===
 
-
More information about the kinetic differences between these construct in our sensor chips, look under
+
{{Team:Aachen/FigureFloat|Aachen_K131026_Pseudomonas_aeruginosa_detection.gif|title=Detection of ''Pseudomonas&nbsp;aeruginosa'' with K131026|subtitle=Direct detection of ''Pseudomonas&nbsp;aeruginosa'' on sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026].|width=480px}}
-
[https://2014.igem.org/Team:Aachen/Project/FRET_Reporter The REACh Construct] Achievements.
+
 
 +
 
 +
===Detecting ''Pseudomonas&nbsp;aeruginosa'' with K131026 in our Sensor Chip with ''WatsOn''===
 +
 
 +
After establishing the successful detection of 3-oxo-C{{sub|12}}-HSLs with our sensor chips the next step was to detect living cells of ''Pseudomonas aeruginosa'' with our measurement device ''WatsOn''. Therefore sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026] were prepared and the right chip was induced with 0.2&nbsp;µL of a ''Pseudomonas aeruginosa'' culture while the left chip was not induced (Detection of 3-oxo-C12 HSL with K131026, displayed below). On the induced chip, a clear fluorescence signal was visible in response to ''P. aeruginosa''. The fluorescence signal emerged outward from the induction point and showed a significant difference to the non-induced chip. The results clearly demonstrate the ability of our sensor chip technology and our measurement device ''WatsOn'' to detect ''P. aeruginosa''!
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The next step in developing our sensor chip platform further after successfully detecting ''P. aeruginosa'' is an improvement of the sampling chip. The current technique of using a simple agarose chip is limited in its ability to transfer as many colonies as possible from a hard surface onto our sensor chips. Therefore the aim is to improve the sampling chip by trying different more adhesive surfaces on our sampling chips.  
+
We are '''committed to improve''' our sensor chip platform. The current technique of using a simple agarose chip is not sufficient to collect all microorganisms from the sampled surface. Therefore, the aim is to improve the sampling chip by using a different, more adhesive material.
 +
 
 +
Furthermore, diffusion in the sensor chips will be reduced to '''limit the spread of the fluorescence signal'''. Currently, the fluorescence spot grows beyond the point of induction and makes it difficult to differentitate between multiple points of induction. By introducing diffusion barriers into our chips, the growth of the fluorescence spots might be reduced, thus enabling the detection of multiple sources of fluorescence lying close together.
 +
 
 +
Additionally, the application of our sensor chips in combination with our ''WatsOn'' device is currently being evaluated for the detection of signals other than fluorescence. '''Detecting bio- and chemiluminescence''' has been identified and will be investigated as an area of potential future application.
 +
 
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{{Team:Aachen/BlockSeparator}}
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==References==
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* Waters, C. M., & Bassler, B. L. (2005). QUORUM SENSING: Cell-to-Cell Communication In Bacteria. Annual Review of Cell and Developmental Biology, 21(1), 319-346. Available online at http://www.annualreviews.org/doi/full/10.1146/annurev.cellbio.21.012704.131001.
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Furthermore the diffusion in the sensor chips will be reduced going forward to try and limit the spread of the fluorescence signal. Currently the fluorescence spot grows outward from the point of detection a lot and makes it hard to successfully differentitate between multiple points of induction. By introducing different diffusion barriers into our chips the aim is to limit the growth of the fluorescence spots and enabling the detection of multiple sources of fluorescence which are close to each other.  
+
* Jimenez, P. N., Koch, G., Thompson, J. A., Xavier, K. B., Cool, R. H., & Quax, W. J. (2012). The Multiple Signaling Systems Regulating Virulence in ''Pseudomonas aeruginosa''. Microbiology and Molecular Biology Reviews, 76(1), 46-65. Available online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3294424/#B63.
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Additionally the application of our sensor chips is currently being evaluated for the use in other areas then fluorescence in combination with our WatsOn device.Detecting Bio- and Chemiluminescence is identified as a potential area of application going forward.
 
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{{Team:Aachen/Footer}}

Latest revision as of 03:43, 18 October 2014

2D Biosensor

With our 2D biosensor technology we are able to detect the pathogen Pseudomonas aeruginosa on solid surfaces. The sensor system is comprised of two distinct but inseparable modules, a biological and a technical part:

  • Sensor chips containing Cellocks, our engineered detective cells that fluoresce in the presence of the pathogen, make up the biological part of Cellock Holmes.
  • Our measurement device WatsOn and the complementary software Measurarty complete our sensing technology on the technical side.


Aachen 15-10-14 Principle of operation 2D sensor ipo.png

Principle of Operation

Cellock Holmes is designed upon a SynBio approach comprising a two-dimensional biosensor and a measurement unit. The two-dimensional biosensor is devised to recognize quorum sensing molecules secreted by the pathogen cells and to generate a distinct fluorescence signal; while the measurement device recognizes and analyzes the produced signal. On the molecular side, we use the REACh construct to transform regular E. coli cells into Cellocks.

Aachen 17-10-14 The basics of quorum sensing ipo.png
The principle of quorum sensing
Microorganisms can sense the presence of their own kind based on quorum sensing which is a form of chemical communication. Depending on their cell density, quorum sensing allows these cells to activate or deactivate certain gene expression cascades (Waters and Bassler, 2005) for a specific function.

Our sensor cells, Cellocks, are immobilized in agar chips. To make the chips, we mix the Cellocks with liquid LB agar. In the course of our project, we designed a casting mold specifically for the production of our agar chips. When the agar has cooled down, the chips are cut out of the mold and are ready to use. Storage of the readily usable sensor chips is possible for up to two days at 4°C when using LB medium or up to five days if TB medium is used. A detailed description of the sensor chip manufacturing can be found in our Protocols section.

Aachen 14-10-14 Flowsheet OD-device part1 ipo.png
Assay to detect P. aeruginosa using Cellock Holmes
This flow sheet shows the procedure to sample and detect P. aeruginosa: A sampling chip is briefly put onto the potentially contaminated surface, added onto one of our sensor chips and inserted into WatsOn.

Using Cellock Holmes, we developed a simple assay to detect P. aeruginosa. Initially, a so called sampling chip is placed on a solid surface that is potentially contaminated with the pathogen. Subsequently, the sampling chip is removed from the surface and put onto one of our sensor chips. Theorectically, the sensor chips could be directly used for sampling, however, this was avoided in our project to match biosafety regulations and to prevent the spread of genetically modified organisms (GMOs) into the environment. The two layered chip-stack is then put into a petri dish which is inserted into our measurement device WatsOn for evalutation.

Aachen 14-10-14 Flowsheet OD-device part2 ipo.png
Mode of action inside WatsOn
Chips are incubated at 37°C to stimulate cell growth and then illuminated with blue light to excite fluorescence. A picture is taken and analyzed for fluorescence signals using the software Measurarty.

Inside WatsOn, the chips are incubated at 37°C and the sampled populations of microorganisms attached on the sampling chip start to grow and multiply. During incubation the chips can be illuminated with blue light at any time, and a photo of the chips is taken. The software Measurarty then analyzes any fluorescent signal. P. aeruginosa secrets an increasing number of quorum sensing molecules that are recognized by Cellocks, thereby producing a fluorescence signal. For detection of P. aeruginosa, we focused on a quorum sensing molecule called N-3-oxo-dodecanoyl-L-homoserine lactone (for short: 3-oxo-C12-HSL), which is involved in virulence regulation of P. aeruginosa (Jimenez, Koch, Thompson et al., 2012). The incorporation of the 3-oxo-C12-HSL detection system into the Cellocks is explained in detail in the REACh Construct section.


Aachen 14-10-16 Iterative Process iNB.png

Development & Optimization

Aachen ILOV GFP HM 1,5h.png
iLOV and GFP in the Gel DocTM
Sensor cells producing iLOV (K1319042, A) and GFP (B) 1.5 h after induction.

Equipment and medium selection

Our first approach (before developing our own device) was to use the Molecular Imager® Gel Doc™ XR+ from BIO-RAD in our lab to detect fluorescence. This device uses UV and white light illuminators. However, only two different filters were available for the excitation light wavelength, which resulted in very limited possibilities for the excitation of fluorescent molecules. For example, it was possible to detect the expression of iLOV (K1319042) in our sensor chips, but not the expression of GFP. Hence, the Gel Doc™ was not suitable for our project.

Aachen Chip medium geldoc.png
Differend medium in the Gel Doc™
complex media exhibited high background fluorescence while less back- ground fluorescence was observed with the minimal media (HM, M9, NA).
Aachen 5days K131026 neb tb 1,5h.jpg
Testing our chips' shelf-life
Chips of [http://parts.igem.org/Part:BBa_K131026 K131026] in NEB were stored five days at 4°C. The right chip was induced with 0.2 µL of 500 µg/mL HSL and an image was taken after 1.5 h.

We tested different media (LB, TB, M9, NA and HM) for the preparation of our sensor chips. The medium compositions can be found in the Protocols section. We screened for an optimized medium composition to minimize background fluorescence and to enhance cell growth. The results of the analysis are presented in the table below. Due to the low background fluorescence in WatsOn and the excellent cell growth, we chose LB medium over the other tested media for sensor chip manufacturing.

LB TB NA M9 HM
Growth of Cellock
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+
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-
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Background fluorescence in GelDoc
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Background fluorescence in WatsOn
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Another set of experiments were conducted to test the long-time storage of the sensor chips. We varied the glycerol content of the chips as well as the storage temperature. Storage at -20°C resulted in the loss of our sensor cells. Adding 5-10% (v/v) glycerol ensured survival of the sensor cells, but resulted in the loss of fluorescence ability. Hence, we concluded that long-time storage of the sensor chips at -20°C is not possible under the tested conditions. However, the 'ready-to-use' sensor chips can be kept at at 4°C for two days when using LB medium, and storage at this temperature for 5 days is possible with chips made from TB medium.

Aachen 2 chipform.jpg
Sensor chip manufacturing using the closed mold
When injecting the liquid agar into a closed mold we encounter problems due to frequent bubble formation.

Optimal Agarose Concentration for Sensor Chip Manufacturing

For the sensor chip manufacturing, agarose was preferred over agar because of the uniform linkage between molecules that results in a better chip homogeneity. In addition, agarose reduced diffusion of the inducer molecules through the chip. A reduction in diffusion is essential for the formation of distinct fluorescent spots on the sensor chips.

Aachen Final chipform.jpg
The finalized chip mold
An open casting mold was found to be optimal for sensor chip manufacturing, because this approach was fast, easy to handle and generated a reproducible chip quality.

Optimal Chip Configuration

Several approaches were tested for the production of agarose-based sensor chips with reproducible quality. The first approach was to cast every sensor chip individually. To achieve a plain chip surface, a requirement for high quality images, we casted the sensor chips between two microscope slides. However, this approach was not adequate because the agar was too liquid and leaked from the microscope slides. In a second approach, we designed a closed mold into which liquid agar is injected using a pipette, but we encountered a high number of bubbles in the resulting chips. Bubbles in the sensor chips interfered with fluorescence evaluation. Finally, we tried an open casting mold. Once solidified, we cut the agar along precast indentations in the casting mold to form the chips. An advantage of the open mold is the ability to simultaneously produce nine sensor chips while the surface tension of the liquid agar ensures a plane chip surface.

Induction of the Sensor Chips

To test our molecular constructs, we simulated the presence of P. aeruginosa by using IPTG or 3-oxo-C12-HSL. Initial experiments showed that diffusion of the inducers hinder the formation of distinct fluorescent spots. Through this set of experiments we determined that the best compromise between diffusion and spot intensity is an induction volume of 2.0 µL for IPTG and 0.2 µL for HSL. Furthermore, detection of growing P. aeruginosa based on secreted HSLs was possible using the [http://parts.igem.org/Part:BBa_K131026 K131026] construct. The experiments for optimizing the induction of our sensor chips are described in more detail in the Achievements section.

Negative Control

To ensure that the fluorescence signal resulted from the sensor construct and not from the medium or E. coli cells themselves, [http://parts.igem.org/Part:BBa_B0015 B0015] in NEB10β cells was used as negative control during sensor chip induction with IPTG, HSL and P. aeruginosa.

Aachen B0015 IPTG HSL Pseudomonas.png
B0015 in NEB10β was used as a negativ control
Induction with A) 0.2 µL of 100 mM IPTG, image taken after 2.5 h; B) 0.2 µL of 500 µg/mL 3-oxo-C12-HSL, image after 2.5 h; C) with five spots of Pseudomonas aeruginosa liquid culture on the left and one big spot on the right, image taken after 2 h.


Aachen 14-10-15 Medal Cellocks iNB.png

Achievements

We developed and optimized a 2D biosensor, which is able to detect IPTG, 3-oxo-C12-HSL and living Pseudomonas aeruginosa cells.

260px
Testing K1319042 in our sensor chips
Sensor chips based on [http://parts.igem.org/Part:BBa_K1319042 K1319042] were investigated for fluorescence using a plate reader. Blue color indicates the absence of fluorescence, while red color indicates fluorescence. The upper chip was not induced, while the lower chip was induced with IPTG (2.0 µL, 100mM).

Testing our Sensor Chips in a Plate Reader

To establish a prove-of-principle for our sensor chip design, we used our construct [http://parts.igem.org/Part:BBa_K1319042 K1319042], an IPTG inducible iLOV. E. coli cells carrying the this construct were introduced into sensor chips and fluorescence was measured every 15 minutes after induction with 2 µL of 100 mM IPTG. The results are displayed on the left. We observed a distinct difference in fluorescence between the non-induced chip (top) and the induced chip (bottom). The middle of the bottom chip started to exhibit a clear and visible fluorescence that increased over time and spread outwards. The top chip, however, also showed an increase in the measured fluorescence over time which was primarily due to the leaky promoter and background fluorescence.

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Testing K131026 in our sensor chips
Sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026] were investigated for fluorescence using a plate reader. Blue color indicates the absence of fluorescence, while red color indicates fluorescence. The lower chip was induced with with 3-oxo-C12-HSL (0.2 µL, 500 µg/mL).

Detecting 3-oxo-C12-HSL with Sensor Chips

In an initial attempt to detect 3-oxo-C12-HSL, we incorporated the [http://parts.igem.org/Part:BBa_K131026 K131026] construct generated by the 2008 iGEM Team Calgary in our sensor chips. This construct generates a fluorescent signal based on GFP in presence of 3-oxo-C12-HSL molecules produced by P. aeruginosa during quorum sensing (Jimenez, Koch, Thompson et al., 2012). First, we tested the construct by direct induction with 3-oxo-C12-HSL (0.2 µL, 500 µg/mL). The fluorescence measurement was taken every 15 minutes with an excitation wavelength of 496 nm and an emission wavelength of 516 nm. The results of this test are shown on the right.

A distinct fluorescence signal was observed on the induced chip (bottom) compared to the non-induced chip (top). Fluorescence started in the middle of the chip, the point of induction, and then extended outwards while growing in intesnity. The basal level of fluorescence was attributed to leakiness of the promoter and general background fluorescence of growing E. coli cells. In the induced chip (bottom), the background fluorescence was lower than in the non-induced chip (top) because the signal masked the noise. The difference between the induced and non-induced chips indicates a clear response to the HSL and proofed the ability of our 2D sensor chip design to detect HSLs produced by Pseudomonas aeruginosa.

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IPTG-inducible superfolder GFP (I746909) in sensor chips
Expression of superfolder GFP ([http://parts.igem.org/Part:BBa_I746909 I746909]) was induced by the addition of IPTG (2 µL, 100mM) on the right chip. The left chip was not induced.

Detecting IPTG with Sensor Chips

The clip displayed on the left side shows the construct [http://parts.igem.org/Part:BBa_I746909 I746909] from the 2007 iGEM Team Cambridge which expresses super folder GFP under the control of a T7 promoter in combination with our 2D sensor chip technology. The [http://parts.igem.org/Part:BBa_I746909 I746909] construct was introduced into BL21(DE3) cells to make the expression IPTG-inducible since the genome of BL21(De3) contains the T7 RNA Polymerase under the control of a lacI promoter.

While the left chip does not show visible fluorescence, the right chip exhibits a strong fluorescence signal. This proves the ability of our sensor chip technology to detect IPTG. The fluorescence response is also high enough to be detected and analyzed by our measurement device WatsOn.





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Detection of 3-oxo-C12-HSL with K131026
0.2 µL of 3-oxo-C12-HSL was placed in the middle of a sensor chip based on [http://parts.igem.org/Part:BBa_K131026 K131026] followed by incubation at 37°C in WatsOn.

Detecting the 3-oxo-C12-HSL with K131026 in our Sensor Chips using WatsOn

The next step towards the final goal of detecting living cells of Pseudomonas aeruginosa was to reproduce the detection of 3-oxo-C12-HSL, already established in the plate reader, in our own WatsOn device. Therefore, we again used E. coli BL21(DE3) cells carrying [http://parts.igem.org/Part:BBa_K131026 K131026] and induced with 0.2 µL 3-oxo-C12-HSL of a concentration of 500 µg/mL. In the clip displayed on the left, the right chip was induced and - as a negative control - the left chip was not induced. Pictures were taken every four minutes.

The result was a clear replication of the success of the plate reader experiment. The induced chip showed a clear fluorescence response eminating from the center, where the induction with HSL took place. This demonstrated the ability of not only our sensor chip technology but also our measurement device WatsOn to successfully detect 3-oxo-C12-HSL.



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Detection of Pseudomonas aeruginosa with K131026
Direct detection of Pseudomonas aeruginosa on sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026].


Detecting Pseudomonas aeruginosa with K131026 in our Sensor Chip with WatsOn

After establishing the successful detection of 3-oxo-C12-HSLs with our sensor chips the next step was to detect living cells of Pseudomonas aeruginosa with our measurement device WatsOn. Therefore sensor chips based on [http://parts.igem.org/Part:BBa_K131026 K131026] were prepared and the right chip was induced with 0.2 µL of a Pseudomonas aeruginosa culture while the left chip was not induced (Detection of 3-oxo-C12 HSL with K131026, displayed below). On the induced chip, a clear fluorescence signal was visible in response to P. aeruginosa. The fluorescence signal emerged outward from the induction point and showed a significant difference to the non-induced chip. The results clearly demonstrate the ability of our sensor chip technology and our measurement device WatsOn to detect P. aeruginosa!

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Outlook

We are committed to improve our sensor chip platform. The current technique of using a simple agarose chip is not sufficient to collect all microorganisms from the sampled surface. Therefore, the aim is to improve the sampling chip by using a different, more adhesive material.

Furthermore, diffusion in the sensor chips will be reduced to limit the spread of the fluorescence signal. Currently, the fluorescence spot grows beyond the point of induction and makes it difficult to differentitate between multiple points of induction. By introducing diffusion barriers into our chips, the growth of the fluorescence spots might be reduced, thus enabling the detection of multiple sources of fluorescence lying close together.

Additionally, the application of our sensor chips in combination with our WatsOn device is currently being evaluated for the detection of signals other than fluorescence. Detecting bio- and chemiluminescence has been identified and will be investigated as an area of potential future application.


References

  • Waters, C. M., & Bassler, B. L. (2005). QUORUM SENSING: Cell-to-Cell Communication In Bacteria. Annual Review of Cell and Developmental Biology, 21(1), 319-346. Available online at http://www.annualreviews.org/doi/full/10.1146/annurev.cellbio.21.012704.131001.
  • Jimenez, P. N., Koch, G., Thompson, J. A., Xavier, K. B., Cool, R. H., & Quax, W. J. (2012). The Multiple Signaling Systems Regulating Virulence in Pseudomonas aeruginosa. Microbiology and Molecular Biology Reviews, 76(1), 46-65. Available online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3294424/#B63.