Team:ShanghaitechChina/Biofilm
Biofilm Introduction
Biofilms are ubiquitous as they can be found both in human and some extreme environments. They can be formed on inert surfaces of devices and equipment, which will be hard to clean and cause dysfunction of the device.
However, we view it through different lenses to transform this ill impact into merits. We envision to establish the Solar Hunter system on E.Coli’s biofilm. Biofilm can substantially increase the resistance of bacteria to adverse conditions like acid or oxidative stress and form a stable and balanced system. These traits can elevate its adaptability to application to industry for they do not need to be meticulously taken care of and are capable to withstand harsh conditions. Therefore, it will be a good practice to reduce the production cost.
What’s more, biofilm can automatically grow by static adherence, which facilitate s regeneration and recycling in mass production in industry. Startlingly, biofilms can also serve as a synthetic nonconductive biological platform for self-assembling function materials. The amyloid protein CsgA, which is the dominant component in E.Coli, can be programmed to append small peptide domain and successfully secreted with biological functions. Also, it has been tested that CsgA subunits fused with not too large peptide can be tolerated by curli export machinery and maintain the self-assembly function as always. (Neel S. Joshi, 2014)
Motivation
For the reasons above, biofilm becomes our best candidate to engineer and would be equipped with some additional functions we want. Here, we conceive the semiconductor-enzyme system linked to the E.Coli’s biofilm, whose subunits are engineered respectively with PolyHistidine tags and SpyTag and SpyCatcher system from FbaB protein to provide binding sites for quantum dots and enzymes.
Based on these ideas, we constructed:
- CsgA-Histag
- His-CsgA-SpyCatcher-Histag
- His-CsgA-SpyCatcher
We envision two ways to utilize the biofilm display to establish the whole biohydrogen platform:
Plan 1. Strains secrected CsgA-his or His-CsgA-SpyCatcher-(Histag) biofilms for binding nanorods + Strain producing hydrogenase HydA
Through this approach, we want to realize producing hydrogen by attach nanorods onto biofilms. The electrons from nanorods excited by sunlight can transfer into engineered hydrogenase-producing strain through mediator solution and accepted by hydrogenases which are not secreted. Since anaerobic hydrogenase will not be exposed to oxygen directly in this way, we view it as a practical and promising way to conduct in lab and consequently realize biohydrogen.
Plan 2. Strain secreted His-CsgA-Spycatcher-(Histag) biofilms for binding nanorods + purified hydrogenase HydA-SpyTag
Based on this concept, we want to construct a catalytic system outside cells. After extracted and purified from strain which produce hydrogenase, the HydA-Spytag engineered enzyme could covalently bind with SpyCatcher protein on the Strain secrected His-CsgA-Spycatcher-(Histag) biofilms. At the meanwhile, nanorods are firmly attach to biofilm as well for there are histags on biofilm subunits. Electrons from nanorods excited by light thus transfer directly to purified HydA due to short spacial distance and achieve hydrogen production in vitro.
Our ultimate goal is to harness this bio-abiotic hybrid system to efficiently convert solar energy into alternative energy or other high value-added industrial products.
Mechanism
We focused on the bacterial amyloid curli structure. The curli consists of two kinds of amyloid proteins bound together and extending on the cell membrane. CsgA, the main subunit, can self-assemble in the extracellular space creating an amyloid nanowire while CsgB is the part which anchors to the membrane, nucleating CsgA and facilitate s extension of nanowire. CsgA is about 13-kDa, whose transcription needs to be regulated by CsgD and expression are processed by CsgE, F and secreted with the assistance of CsgC, G (these all belong to curli genes cluster. After secretion, CsgA assembles automatically to form amyloid nanofibers, whose diameter is around 4-7 nm and length varies(Neel S. Joshi, 2014). CsgA subunits secreted by different bacteria individuals will not have trouble in assembling and bridging each other, therefore finally achieving the goal as extensive as an organized community network.
We constructed a family of CsgA biobricks (see Parts) which are respectively modified with different small peptide domain, endowing the biofilm with designed functions. The expression of CsgA is strictly controlled by inducer anhydrotetracycline (aTc) and its biomass can be tuned by the concentration of inducer (see later for pictures) so that the biofilm is only formed when we need it and is conductive to be well operated when our system is industrialized. Next, we demonstrate the experiments we conducted to test the expression, quantify the biomass, and analyze the viability of different CsgA biobricks.
Construction and Characterization
Principles of methods of characterization
Congo Red
Congo Red dye is a classic method to detect amyloid protein (Alan Marcus, 2012). Amyloid can be visualized and quantified through the staining of Congo Red because Congo Red molecules obtain an oriented arrangement on amyloid fibrils. This property can be ascribed to the hydroxyl groups on the amyloid and hydrogen bonding on the Congo Red (Puchtler, 1962). It only takes approximately 20 minutes to dye so it is indeed a good practice in lab to crudely test the expression of biofilm.
TEM
In order to visualize the formation and different appearance of biofilm nanowire network, we utilize transmission electron microscope to directly look into the microscopic world. TEM can visualize nano-structure with the maximal resolution of 0.2nm which is beyond the range of optical microscope. In using TEM, samples must be prepared accordingly. The first step is to apply UAc on objects. After object is covered by UAc, the certain area would absorb or cause scattering of electrons and therefore the detector cannot receive transmissive electrons through copper grid, thus leaving a dark shadowy appearance of sample in the image.
Crystal Violet Assay
Crystal violet is a triarylmethane dye used as a histological stain to classify biomass. This is a simple assay practical and useful for obtaining quantitative data about the relative quantity of cells which adhere to multi-wells cluster dishes. After solubilization, the amount of dye taken up by the monolayer can be quantitated in a plate reader. (Soares, n.d.)
Construction of CsgA-Histag
CsgA-HisTag is a part from the previous year iGem competition. It is documented by team TU_Delft with the Part ID BBa_K1583003(链接). However, its status not released. Luckily, we obtained the sequence from Allen Chen at Harvard. The two shared the same amino acid sequence, with some difference in the DNA sequence, possibly modified due to the PARTS Standards. We used the Histag on the CsgA-Histag as the binding site of quantum dots, meanwhile, we applied methods described previously to characterize CsgA.
Characterization
1. Congo Red：successful secretion and expression
The series of Congo Red assay are aim to visualize the expression of biofilm. To produce curli, we spread the CsgA-Histag mutant E.coli onto a low-nutrition culture medium, YESCA- CR plates (Neel S. Joshi, 2014), containing 10 g l-1 of casmino acids, 1 gl-1 of yeast extract and 20 gl-1 of agar, supplemented with 34 μg ml-1 of chloromycetin, 5 μg ml-1 of Congo Red and 5 μg ml-1 of Brilliant Blue. (Details in protocol 链接) Red staining indicates amyloid production.
The figures shown above point out that the CsgA-Histag mutant induced by 0.25 μg ml-1 of aTc will produce amyloid structures which are dyed to red by CR in comparison to the negative control. This assay indicates the success in expression of the self-assembly to curli fibers.
2. Crystal Violet Assay： quantification test of biofilm
Further, we use crystal violet assay to simply obtain quantitative information about the relative density of cells and biofilm adhesion to multi-wells cluster dishes. As illustrated in pictures, CsgA-Histag mutant distinguishes itself in absorbance after applying standard crystal violet staining procedures (See protocal 链接) in comparison to strain ΔCsgA and 30% acetic acid negative control. There’s certain amount of background absorption of strain ΔCsgA because the dye can stain the remaining E.coli adhering to the well. This difference between induced strains secreted CsgA-Histag and ΔCsgA manifest a distinct extracellular biofilm production in the modified strain.
3. Quantum dots fluorescence test: successful binding test of Histag with nanomaterials
new characterization of the PART BBa_K1583003
In order to test the effect of binding between CsgA-Histag mutant and inorganic nanoparticles, we apply same amount of suspended QDs solution into M63 medium which has cultured biofilm for 72h. After 30-min incubation, we used PBS to mildly wash the well, and the result was consistent with our anticipation: On the left, CsgA-Histag mutant were induced and thus secreted biofilm, and firmly attached with QDS and thus show bright fluorescence. Therefore, we ensure the stable coordinate bonds between CsgA-Histag mutant and QDs can manage to prevent QDs from being taken away by liquid flow. The picture was snapped by ChemiDoc MP,BioRad, false colored.
4. TEM: visualization of binding test
Finally, transmission electron microscopy(TEM) visualize the binding effect of CsgA-Histag mutant E.coli with QDs in comparison with image of pure nanofiber composed by CsgA-Histag and one without inducer. As can be clearly seen from the figures, with inducer, there’s distinct nanofibers outside the bacteria contrast to the third picture in which E.coli are not induced. From the first picture, it shows biofilm areas are densely covered by QDs and we ulteriorly confirm the viability of bio-abiotic hybrid system.
Construction of His-CsgA-SpyCatcher-Histag/ His-CsgA-SpyCatcher
PARTS：BBa_K2132001，BBa_K2132002
In light of the immunization platform of biofilm for enzymes, we need some tags acting like glues or stickers that could be connected to the tags on the enzyme. The SpyCatcher and SpyTag system seem like a good choice for us. The SpyCatcher on the biofilm will mildly bind the SpyTag on the enzyme. Note that it is not the other way around, given that the huge size (138 amino acids) may impair the normal function of some delicate enzyme, hydrogenase in our case. For more details for the principles of SpyCatcher and SpyTag and our motivation on this system, see Linkage System. On top of the linkage to the enzyme, we would like to equip the biofilm the ability to bind quantum dots. This goal makes the construction of His-CsgA-SpyCatcher-Histag or His-CsgA-SpyCatcher necessary. The two sequences are submitted as our first two original parts. See webpage of the parts here: BBa_K2132001, BBa_K2132002.
In constructing the sequence, we simply used Gibson Assembly to assemble the clips of CsgA, SpyCatcher, and the plasmid backbone together at one single reaction. For more details and the experiment data, please download the pdf here(此处设置超链接).
In constructing the parts, we had been worried about whether the huge SpyCatcher will interfere with the CsgA secretion and whether they will secret together. Careful characterization of each subunit proves that the two parts work excellently, in consistence with previous findings. (Citation)
Characterization
Since the sequence is actually a fusion protein, we identify each unit individually in characterization.
1. Congo Red：successful secretion and expression
His-CsgA-SpyCatcher-Histag
After CR dye, the figure indicates that the His-CsgA-SpyCatcher-Histag mutant induced by 0.25 μg ml-1 of aTc successfully secreted a thin-layer biofilm on the plate which are stained to brown-red color by CR, compared to the negative control with no inducer. (Because the ratio between Congo Red dye and Brilliant Blue dye is not in the best state which leads to the unapparent phenomenon through the lens, the brown red biofilm is easy to be identified visually.) This assay also proved that the new and challenging construction of appending a large protein onto CsgA subunits will work accurately and effectively.
His-CsgA-SpyCatcher
After 72h culture, we scratch the biofilm down from the well and apply 25 μg ml-1 of Congo Red into solution. Then centrifuged and washed by PBS for several times, we get the result: newly Histag-CsgA-SpyCatcher mutant induced by 0.25 μg ml-1 of aTc was stained to bright-red color by CR, compared to the negative control with no inducer and the color can’t be washed away. This assay also manifested the success in construction of Histag-CsgA-SpyCatcher mutant and add versatility to our biofilm platform.
2. Quantum dots fluorescence test: successful binding test of Histag with nanomaterials
Then comes to the next part: we want to check if SpyCatcher protein will be too large to cause steric hindrance effect on Histag peptide. The best approach to verify is the fluorescence assay of binding with nanomaterials.
His-CsgA-SpyCatcher-Histag
After applying the same steps as introduced above, the bottom of left well show a large area of bright fluorescence, manifesting His-CsgA-SpyCatcher-Histag mutant secreted biofilms under the control of inducer and Histag on it is not blocked, what is more, it is firmly attached with QDs. From this assay, we assure that the SpyCatcher will not impose negative effect on the binding between inorganic material and biofilm. The picture was snapped by ChemiDoc MP, BioRad.
His-CsgA-SpyCatcher
Using the same approach, we also conducted binding assay of Histag-CsgA-SpyCatcher with QDs to characterize the expression of biofilm and the visual result shows vividly that Histag-CsgA-SpyCatcher can bind successfully with the QDs with the existence of inducer aTc, which shows the functional similarity in his-tag. The picture was snapped by BioRad ChemiDoc MP.
Linkage System
SpyTag and SpyCatcher (Zsofia Botyanszki, 2015)
A widely applied linkage system, SpyTag and SpyCatcher, originally discovered from Streptococcus pyogenes. By splitting its fibronectin-binding protein FbaB domain, we obtain a relatively small peptide SpyTag with 13 amino acids and a bigger protein partner, SpyCatcher, with 138 amino acids (Bijan Zakeria, 2012). The advantage of this system lies in the following three aspects. Firstly, they can spontaneously form a covalently stable bond with each other which guarantee the viability of the permanent linkage. The second point is quick reaction within 10 min, which will stand out by its efficiency in industrial application. Besides, the whole process proceeds in mild condition (room temperature), thus set lower requirement for reaction both in lab and future practice. Therefore, we design to leverage this advantageous system to achieve the binding of biofilm with specific enzyme.
Appending SpyTag to CsgA subunit is a traditional and hackneyed approach to modify biofilm posttranscriptionally. Here, we challenge to attach larger part, SpyCatcher, to CsgA to enrich the versatility of biofilm platform. For one thing, we intend to pioneer new approach. For another aspect is that we concern SpyCatcher is too large that might jeopardize the biological activity and function of the enzyme. After comprehensive consideration, we decide to append SpyTag and SpyCatcher respectively to CsgA subunit and enzyme, and successfully prove their feasibility and stability.
Characterization
As figure illustrated, his-CsgA-SpyCatcher-his mutant incubated with mcherry-SpyTag show a clear biofilm-associated mcherry fluorescence signal, which indicating the accurate conformation and function of the SpyTag and SpyCatcher linkage system. The third figure is merged by the first and second figures of each sample are snapped respectively under green laser field with 558 nm wavelength and bright field of fluorescence microscopy, Zeiss Axio Imager Z2. As for controls, strains secreted CsgA–histag and ΔCsgA both are unable to specifically attach to SpyTag thus no distinct localization highlight of red fluorescence on E.coli. That to a large extent prove the specificity of our desired linkage between SpyTag and SpyCatcher system.
Results and Optimization
The new CsgA mutants we obtained or newly constructed, and applied in our Solar Hunter project are as follows:
- CsgA-Histag
- His-CsgA-SpyCatcher-Histag
- His-CsgA-SpyCatcher
Crystal Violet Assay of the peptide fusion mutants library transformed into ΔCsgA strains. Firstly, we apply 0.1% crystal violet solution to all mutants to gain quantitative data about their relative expression and secretion performance. The result was read and exported by BioTek CYTATION5. CsgA-His mutant stands out as the highest peak in absorbance after washing. (See protocal 链接) The reason why His-CsgA-SpyCatcher-Histag and His-CsgA-SpyCatcher mutant doesn’t perform as well as CsgA-His might be ascribed to size hindrance of SpyCatcher, which impedes the transport process of CsgA mutant subunits from inner area to extracellular environment by CsgG outermenbrane exporter, whose pore size is around 2 nm. Yet, the differences between induced CsgA-His, His-CsgA-SpyCatcher-Histag, His-CsgA-SpyCatcher mutant and negative control exhibit a success extracellular biofilm production in all our constructed and modified strains.
Nanoparticle binding assay of all constructs library. QDs are templated by CsgA-His mutant incubated in M63 solution. We added equivalent amount of QDs solution into M63 medium with mutant E.coli which have been cultured for approximately 72h and all mutants were induced by aTc. After 15min incubation, we apply PBS washing 3 times to wash away the unbinding quantum dots. Pictures demonstrate CsgA-His were produced by three mutants we constructed and QDs are templated on biofilms .
We cultured all E.coli mutants in multi-wells with increasing inducer gradient. The result demonstrated in accordance that 0.25 μg ml-1 of aTc will induce the best expression performance of biofilm. The possible reason for higher concentration of inducer strangely leading into low production of biofilm might lie in that aTc, a kind of antibiotic, can be harmful to protein synthesis in bacteria. We speculate there’s an antagonism between the effect of promoting expression and impeding growth brought by aTc and 0.25 μg ml-1 of aTc just reach the optimal point.
Achievements
Above all, we tested and proved that all the strains we constructed work well:
1.Strains with engineered CsgA subunits : 1) CsgA-Histag
2) His-CsgA-SpyCatcher-Histag 3) His-CsgA-SpyCatcher can successfully expressed, secreted and realized self-assembly outside cell membrane.
2.Small peptide histag on CsgA subunits can function well and attach to the ligands on nanorods and quantum dots.
3.Large protein SpyCatcher on CsgA subunits are also able to be secreted by transporter machinery and successfully form nanofibers. We also prove the biological function of SpyCatcher after appending on CsgA subunits, thus provide potential for our second plan mentioned above.
For Fun
We bought a personalized sticky paper with ShanghaiTech University logo. Initially, we paste the sticky paper on the bottom of the plate and add 25ml M63 minimum medium to culture strain secreted CsgA-His. After 72h culture, QDs with red emission light is applied to the plate and incubated for another 24h. Then solution is removed and we visualize the effect under UV light.
Reference
Alan MarcusEvita Sadimin, Maurice Richardson, Lauri Goodell,and Billie Fyfe,. (2012). Fluorescence Microscopy Is Superior to Polarized Microscopy for Detecting Amyloid Deposits in Congo Red–Stained Trephine Bone Marrow Biopsy Specimens. Am J Clin Pathol.
Bijan Zakeria, J. O.-L. (2012, February 24). Peptide tag forming a rapid covalent bond to a protein, through engineering a bacterial adhesion. PNAS.
Neel S. Joshi, P. Q. (2014, September 17). Programmable biofilm-based materials from engineered curli nanofibres. nature communications.
Soares, M. J. (n.d.). Crystal Violet Assay. Retrieved from KU MEDICAL CENTER: http://www2.kumc.edu/soalab/LabLinks/protocols/cvassay.htm
Zsofia Botyanszki, 1. P. (2015, May 20). Engineered Catalytic Biofilms: Site-Specific Enzyme Immobilization onto E. coli Curli Nanofibers. Biotechnology and Bioengineering.
Puchtler, H. S. (1962). On the binding of Congo red by amyloid. . Cytochem.
