Isolating DNA from your Microbes
One of the big advantages of the molecular revolution in microbial ecology is that we can actually find, based on 16S rRNA gene sequences, the identity of your isolates. In order to do that, we will follow the protocol below which first involves liberating the DNA in your isolate, then amplifying the 16S rRNA gene, then sequencing it. The bioinformatics analysis of this sequence will be covered next time we meet. For this protocol, you will need to start with fresh overnight cultures from your isolates so remember to come in the previous day!
Isolate Genomic DNA From Your Sample
Please wear gloves during this protocol
1. Transfer 1 mL of an overnight culture for each isolate to a labeled, 1.5 mL tube.
2. Spin in the centrifuge at 10,000 g for 5 minutes and decant the spent media or suction off with your pipette.
3. To each of the cell pellets, add 200 uL of PBS and 20 uL of proteinase K.
4. Add 200 uL of buffer AL to each 1.5 mL tube and mix by vortexing. Your pellet should resuspend. If it does not, use a 1000 uL pipette to dislodge it.
5. Incubate your mixture at 56C for 10 minutes (the water bath in the back).
6. Once the incubation is done, add 200 ul of ethanol (100%) to the suspension and mix by vortexing.
7. For each isolate, label an individual DNeasy mini spin column in a 2mL collection tube.
8. Pipet the entire mixture into the appropriately labeled column.
9. Spin the column at 6,000 g for 1 minute. Your DNA will now have adhered to the column. Discard the flow through and place the column in a new 2 mL collection tube.
- the next steps are all washes *
10. Add 500 ul AW1 to the column. Centrifuge for 1 minute at 6,000 g. Discard the flow through and place the column in a new 2 mL collection tube
11. Add 500 ul AW2 to the column and centrifuge for 3 minutes at 20,000 g (max speed). Discard the flow through and place the column in a labled, 1.5 mL tube
12. Elute the DNA from the column by adding 200 ul of buffer AE to the center of the spin column. Incubate the column for 1 minute at room temperature and spin at max speed for 1 minute.
- Your DNA is now in the eluate!*
We will next attempt to amplify only bacterial rRNA genes by using "universal" bacterial primers :a forward primer, Eub27F (5′–3′:AGA GTT TGA TCC TGG CTC AG) , and a reverse primer, Eub1492R (5′–3′: ACG GCT ACC TTG TTA CGA CTT). These primers are short sequences of single stranded DNA that are complementary in sequence to areas of the 16s rRNA gene. The 16S rRNA gene sequence is particularly good target gene for amplification because this gene (encoding a ribosomal subunit) contains conserved sequences of DNA common to all bacteria (to which the primers are directed) as well as divergent sequences unique to each species of bacteria (allowing identification of the bacterial species from sequence databases and sequence identifying software). Our "universal" primers will anneal to most bacterial DNA and initiate an exponential amplification of the 16s rRNA gene from the template DNA. After 30 cycles of polymerase chain reaction in a thermal cycler, the result will be a pcr product containing hundreds, if not thousands, of the 16s rRNA gene.
PCR Amplification of 16s rRNA genes from Universal Bacterial Primers
To review how the polymerase chain reaction works and how it exponentially amplifies specific sequences of DNA, go to the following web site:
All PCR reactions require a thermal cycler to elevate and reduce the reaction temperature quickly and keep it at a specific temperature for a prescribed amount of time. There is a basic pattern to these temp. cycles, but there are differences, so you must be sure to program the cycler with the correct time and temperature for your specific amplification. Traditionally, pcr used Taq polymerase, a heat stable DNA polymerase originally found in a extremophilic bacterium, Thermus aquaticus, that lives and reproduces in boiling hot springs. We are not using Taq for our pcr but a different polymerase, Finnzyme's Phusion High-Fidelity Polymerase, a proprietary reagent that uses a novel heat-stable Pyrococcus-like enzyme. Phusion DNA Polymerase generates long templates with a greater accuracy and speed than with Taq. The error rate of Phusion DNA Polymerase in Phusion HF Buffer is determined to be 4.4 x 10-7, which is approximately 50-fold lower than that of Thermus aquaticus DNA polymerase, and 6-fold lower than that of Pyrococcus furiosus, another proof-reading DNA polymerase.
Therefore, our pcr product DNA will have far fewer "mistakes" in the sequences that are replicated from template DNA. Our polymerase will also work much faster so our ~20 cycles will require less time than conventional Taq based pcr.
Protocol for PCR
Obtain a tiny 0.2ml pcr tube from your instructor. All of the ingredients listed below in the table, except the template DNA, have been added together previously and kept on ice for you in these tubes.
Label it with a fine tipped Sharpie on the top and side with the code name for your isolate. Do not use tape.
you will add 4 μL of lysate to your PCR tube with master mix. Since your pcr tube already has 10μL master mix, 4μL DNAase free water, and 1μL of each of 2 primers, the total reaction volume for everyone will be 20μL.
It is very important to pipet these tiny volumes accurately. Use the P10 or P20 pipettes. Look at the tip after you draw up your measured volume to make sure you have liquid there.
Dispense the template DNA into the liquid directly, watching to make sure that the liquid has left the pipette tip.
Tap the bottom of the tube (VERY GENTLY!) and flick the tube to mix. Do not treat these tubes roughly as they are quite thin-walled and can break or crack.
Bring your tube to your instructor; they will show you where the thermal cycler is located in JH 022. Your instructor will start the reaction when everyone's tubes are loaded.
|| amt. in a 20 μl
| Final Conc.
| 4 μL already in tube.
Want to achieve
total of 20 μl reaction vol.
Add from 0 - 3μl
| 2x Phusion Master Mix
|| 10 μl
| 27F primer
|| 0.5 μMolar
| 1492R primer
|| 0.5 μMolar
| template DNA
|| 4 μl
|| optimum is 100ng of DNA/reaction
The cycling program is shown below.
Thermal Cycler Program:
3 step program
| Cycle Step
|| # of Cycles
| Initial Denaturation
|| 5 min.
| 10 sec
| Final Extension
| 10 min
While the 16S rRNA genes from all of the bacterial species in your DNA are being amplified in the thermal cycler, you will have about an hour to work on any other parts of your project.
After the PCR reactions are complete, you will need to complete a "Clean-Up" of your pcr products (remove the unused dNPTs, primer dimers, salts, etc. The instructions for using a kit to purify your pcr products and get them ready for sequencing next time. You will also need to set up a gel to assess the purity of your pcr product and the success of your amplification.
You can store your DNA at -20C in the freezer at the front of the lab in the box labeled M465.
Agarose Gel Electrophoresis of Clean PCR product
To see if you successfully amplified the 16s rRNA gene and not anything else, you will "run a gel" on your cleaned pcr products. To run a gel means that we will perform an electrophoretic separation of the DNA fragments in your cleaned up pcr product, using a small fraction of the volume of your pcr product applied to a 1% agarose gel stained with Sybr Safe DNA stain. Your instructor will photograph the gel, label it with your amplicon id from the template and post the gel photo on OnCourse so you can evaluate your success at 16S rRNA gene amplification. You should see a single band of ~1.5kb indicating that the only dsDNA in your pcr product came from amplification of a ~1500bp gene fragment. Can you explain how we know the size of our amplified gene fragment?
Protocol for making a 1% agarose gel
In order to run out your PCR products you will first pour a 1% agarose gel. The recipe for the gel is 1.0% agarose (w/v) in 1x TBE buffer (10x=890mM Tris, 890mM Boric Acid, 20mM EDTA) with SybrSafe™ stain. Your instructor will add the SybrSafe stain to your molten agarose after you prepare it. Before you start, prepare your agarose gel, comb, and reservoir as you are instructed
1. Using a graduated cylinder, fill a glass, 250 mL flask with 100 mL of 1x TBE buffer.
2. Using the top loading balance, measure out 1 gram of Agarose
3. Pour the agarose into the flask containing the TBE buffer and stir to mix. The agarose won't go into solution here but needs to be heated
4. Place the flask inside the microwave and heat for 1 minute, maximum power. At the end of the minute, swirl the solution to mix and place back in the microwave.
5. Heat for 1 minute again, this time watching closely to make sure it does not boil. At the end, swirl to mix and bring to your instructor for the sybersafe dye
6. After your instructor adds the SyberSafe, pour your gel into the mould. It will take ~ 25 minutes to set.
DNA is uniformly negatively charged and will,therefore, move toward the positive electrode. The separation is determined by the size or mass of the molecule or fragments of DNA.
Procedure for Agarose Gel Electrophoresis of PCR products
1. Load 5 microliters of your PCR product on your gel by placing the 5 microliters of your pcr product as a spot on a small piece of parafilm and adding 5 microliters of loading dye (0.25% XC, 30% glycerol, 0.1mg/ml RNAase). Mix the loading dye by pipetting up and down on the spot, on the parafilm, before loading all 10 microliters into a lane of the 1% agarose gel.
2. Record which lane is loaded with which product. Be sure to leave the first two lanes and the last lane empty for the 1kbp ladder, the positive control and the negative water control.
- Note that Loading dye contains glycerol to keep our sample in the lane rather than floating away and will have one of 3 marker dyes (bromophenol blue, xylene cyanol, or orange G) that facilitate estimation of DNA migration distance and optimization of agarose gel run time. 1x TBE buffer is used in this electrophoretic separation (89mM Tris, 89mM Boric acid, 2.0mM EDTA. The gel will be run at 120V for approximately 30 minutes.