Growth of phage materials
The accomplishments of the natural world can inspire us to great engineering feats. Biomineralization is one particularly impressive trick nature pulls off. Vertebrates, invertebrates and plants all have ways to precisely position inorganic substrates into crystalline order. For example, calcium carbonate will form unstructured dust in the absence of genetically-programmed organizers, but the same material can be made into the hard and luminous shells of sea creatures. Similarly, diatoms organize silicon dioxide into intricate patterns that manufacturers of electronic components can’t begin to approach. In one more instance, bacteria align iron inside their cytoplasm to form magnetic rods on the submicron scale. These feats are accomplished without harsh chemicals, without extreme temperatures, and without noxious wastes that poison the nests of the organisms themselves. Humans have a lot to learn from nature’s successes. In the upcoming weeks we’ll use a virus that infects bacteria, namely the bacteriophage M13, and we'll rely on the self-assembling coat of this virus to template carbon nanotubes and TiO2. The interaction of these materials with a protein on the phage coat yields nanoscale-particles with useful energetic properties, as we’ll see.
The bacteriophage M13 is a member of the filamentous phage family. It has a long (~880 nm), narrow (~6 nm) protein coat that encases a small (~6.4 kb) single stranded DNA genome. The genome encodes 11 proteins, five of which are exposed on the phage’s protein coat and six of which are involved in phage maturation inside its E. coli host. The phage coat is primarily assembled from a 50 amino acid protein called pVIII (or p8), which is sensibly enough encoded by gene VIII (or g8) in the phage genome. For a wild type M13 particle, it takes about ~2700 copies of p8 to make the ~880 nm long coat. The coat's dimensions are flexible though and the number of p8 copies adjusts to accommodate the size of the single stranded genome it packages. For example, when the phage genome was mutated to reduce its number of DNA bases (from 6.4 kb to 221 bp) , then the p8 coat “shrink wraps" around the reduced genome, decreasing the number of p8 copies to less than 100. Electron micrographs of the resulting “microphage” and its wild type parent are shown below (image courtesy of Esther Bullitt, Boston University School of Medicine), where the black bar in each image is 50 nm long. And what about the upper limit to the length of the phage particle? Anecdotally, viable phage seems to top out at approximately twice the natural DNA content. However, deletion of a phage protein (p3) prevents full escape from the host E. coli, and phages that are 10-20X the normal length with several copies of the phage genome can be seen shedding from the E. coli host.
Electron micrographs of microphage described by Specthrie et al[
], images courtesy of Esther Bullitt
E. coli shedding M13 with p3 mutation, image courtesy of M. Russel and schematic of M13 genome, image courtesy of M. Blaber
The general stages to a viral life cycle are: infection, replication of the viral genome, assembly of new viral particles and then release of the progeny particles from the host. Filamentous phages use a protein at their tip, namely p3, to contact a bacterial structure known as the F pilus to infect E. coli. The phage genome is then transferred through the pilus to the cytoplasm of the bacterial cell where resident proteins convert the single stranded DNA genome to a double stranded replicative form (“RF”). This DNA then serves as a template for expression of the phage genes and produces new phage particles that shed off the surface of the infected cell. Other phage are known to lyse their host cells but in the case of M13 and E. coli, they co-exist in a balanced way, allowing the growth of both host and virus, though the infection does slow down the doubling time of the E. coli, causing "plaques" to form in a bacterial lawn.
has been used for decades as a tool for discovery. This technique exploits natural selection and identifies functional peptide sequences that can be fused to the phage coat. Most often it’s the p3 protein at the phage tip that’s used for phage display because, despite the limited number of displayed peptides per phage (on the order of 5), there is enough flexibility to accommodate peptides of 20 to 30 amino acids
. The other protein used for phage display, p8, is presented at much higher copy number per phage (on the order of 2700) but it has limited flexibilty. The semi-crystalline packing of p8 on the phage coat restricts fusions to only 4 to 6 neutral or negatively charged amino acids. For scientists who can tolerate a mix of p8 proteins on the phage coat for their work, there are phage-display variations that mix and match fusion and wild-type proteins on a phage coat, but for those who want phage of a particular form, the options are limited.
Nonetheless, peptides with remarkably diverse functions have been isolated with phage display. Once the fusion site is chosen, a library of sequences encoding random peptides can be synthesized and cloned. In this way a pool of phage, each with different fusions, can be made. Finally, the phage pool can be screened for interesting behaviors or properties. Peptide-fusion proteins to p8 or p3 that include stop codons or intolerable sequences are largely lost from the population after the first round of “panning.” Other phage that can bind to a substance of interest or show enzyme activity or glow green…, these remain and can be directly isolated from the pool or further enriched by a second, third, fourth round of panning. Ultimately anywhere from 10 to 1000 candidate sequences may remain from a starting pool of 1 billion .
Despite phage display techniques being available for more than a generation, this tool has been applied only recently to the search for novel materials. Largely it’s been Angela Belcher and her lab who highlighted and then demonstrated the usefulness of this search tool for finding peptides that interact with materials to meet human needs. That M13 could interact with inorganic materials could not have been predicted from the original genetic studies on the phage, but there was also no one who had tried it. Phage that can bind to cobalt oxide, gold, iridium and indium tin oxide are all in-hand thanks to their work (e.g see reference ). Today you will harvest a phage that can bind to single-walled carbon nanotubes (SWNTs) and TiO2 since these can be used to build nanocomposites that will be assembled into a photovoltaic device before this module is over.
In preparation for this lab, a bacterial host ( XL1-blue) was infected with the M13 phage clone named "DSPH." A second batch of host cells was infected with the M13 phage clone named "p8#9". These phage clones are modified with an 8 amino acid addition in the p8 protein, =DSPHTELP in the case of "DSPH" and =VSGSSPDS in the case of the p8#9 modification. Both phage clones were obtained by panning, starting with a library of p8 mutants and isolating phage that could bind materials. The DSPH clone can bind single walled carbon nano tubes, "SWNTs" (doi:10.1038/nnano.2011.50) while the p8#9 phage can bind thin gold films (PMID: 16178252).
Today, you will isolate phage from the infected bacterial culture and measure its concentration using the spectrophotometer. Finally, you will bind the phage to SWNTs or gold nanoparticles. These phage will be used as a template for TiO2 nanotube assembly for the dye-sensitized solar cell, "DSSC," in future lab sessions.
Part 1: Phage purification
- Divide the overnight culture (~80 ml volume) into 2 x 50 ml conical tubes.
- Label the tubes with your group color.
- Spin 10, 000 rpm, 10 minutes using a fixed angle rotor. You will be shown where down the hall you can find a centrifuge to spin this volume.
- Transfer the supernatant to new 50 ml conical tubes, splitting the supernatant between them. The transfer should be done with a plastic pipet and a bulb so you can measure the volume of supernatant.
- Add a 1/6th that volume of 20% PEG-8000/2.5M NaCl solution.
- Invert to mix then incubate on ice 60 minutes. During this time, you should get started on your research pre-proposal. Read the instructions for this assignment that are found here.
- Spin at 11,000 rpm for 15 minutes. A white pellet may be visible...these are your precipitated phage. If you can't see a pellet keep going, but be aware of where the pellet you can't see is in the tube and don't scrape a tip against it or you will accidentally remove it.
- Remove the supernatant by pouring most down the sink and the rest with aspiration (carefully so as not to disturb the pellet).
- Resuspend the pellet in 3 ml sterile H2O. This is best done by adding 3 ml of H2O to one of the conical tubes, washing the water up and down the side of the tube with the phage pellet, and then moving the 3 ml of phage solution to the second tube and dissolving that pellet as well by washing the water up and down the side of the tube.
- Split the phage solution between three eppendorf tubes. For this part of the protocol, you will be given special eppendorf tubes that can hold 2 ml.
- Spin tubes in a room temperature microfuge for 1 minute to remove residual bacterial residues. Transfer supernatant to fresh eppendorf tubes.
- Add a 1/6th volume of 20% PEG-8000/2.5M NaCl solution.
- Invert to mix. Then incubate on ice for 15 minutes.
- Spin the tubes full speed in a microfuge for 10 minutes.
- Aspirate the supernatant and resuspend the pellets (if you can see them) in 0.2 ml TBS--using 0.2 ml to resuspend one pellet and moving that volume to resuspend the next pellet, and then moving that volume again to resuspend the third pellet. This is your phage stock (yay!).
- If the solution looks at all cloudy, spin in a room temperature microfuge for 1 minute more and move supernatant with the phage to a new tube.
Part 2: Measuring concentration of phage
With this technique you will calculate the concentration of phage in your stock using the spectrophotometer. This method can approximate the number of phage based on the ability of the virions to absorb ultraviolet light. The number of phage is calculated by the formula:
Number of phage particles/ml = (6x10^16)*(A269 - A320)/(#DNA Bases in the genome of the phage)
- the molar extinction coefficient of the phage and the average size of a DNA base are used collected into the constant
- the absorbance at 269 nm reflects the protein and DNA content in the solution
- the absorbance at 320 nm corrects for the naturally high baseline value of the solution
- the number of DNA bases in DSPH is ~7220.
This method for titering the phage stock is less informative than the traditional plaque method (known as titering) since materials other than phage might be contributing to the absorbance readings. Thus, the number of infectious particles isn't truly known. Since infectivity is not critical for the synthesis of SWNT-TiO2 nanowires,however, we will be using spectrophotometry only.
- Dilute the phage stock you have 1:10 by adding 70 ul of the phage to 630 ul of TBS, vortex to mix and then move this solution to a quartz (not plastic!) cuvette.
- A few things to be aware of when using quartz cuvettes:
- They are very expensive.
- The lab has very few.
- When you are done using your cuvette, you should carefully clean it by shaking out the contents into the sink and rinsing it once with 70% EtOH, then two times with water. Quartz cuvettes get most of their chips and cracks when someone is shaking out the contents since it is so easy for the cuvette to slip from wet fingers or be hit against the sink. Don’t let this happen to you.
- Read the absorbances of your phage dilution at 269 and 320, using TBS in a second quartz cuvette to blank the spectrophotometer at each wavelength.
- Calculate the number of phage particles/ml using the formula shown above.
Part 3: Binding phage to solar cell enhancers: Complexing phage with SWNTs / Complexing phage with gold nanoparticles
The class will be optimizing the solar cell's performance by comparing the influence of gold nanoparticles versus SWNT’s on efficiency metrics. In order to make the best use of our time, each team will bind their phage to one of the material/concentration parameters in the table below. Performance results between all groups and conditions will be compared after solar cell assembly at the end of the module.
Part 3a: Complexing phage with SWNTs
The table below indicates the appropriate mass and phage particles to use, assuming a phage concentration of about 4x10^13 phage/ml. If the phage stock you have isolated is lower or higher, you will have to adjust the mass of SWNTs accordingly.
||phage (# of particles)*
- Volume of phage should be between 0.5 and 5 ml to give this number of phage particles
- Prepare the tubing you'll need for dialysis. We will use dialysis tubing with a molecular weight cutoff of 12,000-14,000 against a pH'd solution of NaCl in order to change the electrostatic properties of the phage inside the bag. You should always wear gloves when handling the dialysis tubing.
- Cut a length of tubing that is 20 cm long for the narrow tubing, or 7 cm for the wide tubing.
- Soak the tubing in 50 ml dH2O in a falcon tube. It is recommended that the tubing soaks for 20 minutes, but a shorter time is fine too.
- Clips for the tubing are numbered in pairs and you should note which pair you have. Next time, this number will be the only way to distinguish your sample from the others.
- Open both clips. Remove the tubing from the water with a gloved hand and close one end with a clip, leaving ~0.5 cm overhang.
- Use a P1000 to transfer the phage solution to the tubing, being careful not to drop the tubing (it's slippery!) or let the liquid squirt out (so fill the tubing slowly with the tip low in the tubing).
- Use a P1000 to transfer the appropriate volume of SWNTs to the bag.
- Clamp the other end and dialyze the bag in 1 liter of 10mM NaCl, pH 5.3, with the other samples from the class. You may need to tie some teflon tape around the end of one of your clips and secure the other end of the teflon tape out of the liquid in order to keep your tubing from getting bashed around by the magnetic stir bar. After 2 hours, the NaCl will be replaced with fresh 10mM NaCl, pH 5.3. Tomorrow, one of the teaching faculty will refresh the beaker with clean 10 mM NaCl, pH 10 and the samples will remain in the beaker at room temperature until you return to lab.
Part 3b: Complexing phage with Gold nanoparticles
- The concentration of gold nanoparticles is 5x10^13 particles/ml. Calculate the amount of solution you require to complex at the correct ratio of nanoparticles to phage.
- Obtain a glass scintillation vial and mix phage and nanoparticles together. Store in the fridgerator and leave it until you return to lab.
- The quartz cuvettes can be returned, clean, to the instructors bench at the front of the room.
For next time
20% PEG 8000, 2.5 M NaCl
- 50 mM Tris
- 150 mM NaCl
- pH 7.6
- Stock prepared by Belcher lab (thank you!!) = 20 ug/ml
- 0.5844 g/liter, pH to 5.3 or 10