Why Should You Care How Bacteria Fight Viruses?

Regular readers have been learning a great deal about the human immune system thanks to our ongoing series on allergies by Julia van Rensburg. But did you know that bacteria have an immune system of sorts, too? Yes, even germs get germs!* Bacteria are susceptible to a group of viruses called bacteriophages, or phages for short. Phages resemble early spacecraft and “land” on the surface of bacteria in order to inject their DNA/RNA, much like a syringe ejects its contents.

Houston, we have a problem! A phage has just injected its DNA into our cell!

Bacteria, which have been on Earth for some 3.5 billion years, have had plenty of time to evolve defense mechanisms against predatory phages. Just like human viruses, phages are a most unwelcomed guest. They barge into the cell unannounced, “borrow” cellular components without asking, and then use them to make baby viruses until the cell becomes so engorged with viral progeny that it explodes, releasing the huge viral family so that it can invade more bacteria and repeat the process all over again. Phages that burst the bacterium like this are called “lytic”, but there are other types that don’t blow the house up. These are referred to as “lysogenic” phages and can insert their genetic material into the bacterial genome, becoming a permanent resident of that bacterium. Even more sinister, the incorporated viral genome is copied like all the other bacterial genes when the bacterium divides, so it is inherited by the daughter cell!

Lytic phages will replicate until they blow the infected bacteria apart. In contrast, lysogenic phages can stick around forever, even getting passed on to future generations since the viral genome was inserted into the bacterial genome.

So that sucks – imagine if you had uninvited viral DNA shoved into your DNA – such viruses basically transform you into a GMO. Sorry to inform you, but up to 8% of your genome is already littered with lots of viral DNA. If you oppose GMOs, I hope you can still stand to be in your own skin!

Presently, we don’t know how to remove foreign DNA from our own. But bacteria have figured out a way to get rid of incoming phage DNA, which provides the basis for a type of bacterial immune system.

 

Some combinations work great together, like chocolate and peanut butter. But getting viral DNA stuck into your own DNA, a strategy used by many viruses including HIV, is not a welcome combination.

In 1987, scientists uncovered unusual repeat sequences in the genome of E. coli bacteria, which were later named “clustered regularly interspaced short palindromic repeats”, or CRISPR. In the early 2000s, scientists identified bacterial proteins interacting with CRISPR sequences (now called CRISPR-associated (Cas) proteins) and discovered that they provide resistance to phage infection. Through the efforts of many laboratories, it is now known that bacteria can use a phage invasion as a vaccination by incorporating some of the foreign DNA between CRISPR repeat sequences. This provides the bacteria with a “catalogue” – a memory system, if you will – of foreign DNA that it can pass along to future generations.

But CRISPR is not just a storage system. The bacteria can retrieve these sequences and hook them to Cas9, a nuclease enzyme that can cut DNA. When foreign DNA enters that bacteria, its CRISPR-Cas9 system can specifically target the invasive element and neutralize it.


Foreign DNA, such as that injected by a phage, can be neutralized by CRISPR/Cas9, which serves as a type of bacterial immune system. Bacteria can store foreign DNA sequences in its genome and express them as crRNAs that bind to Cas9. If the bacterium encounters foreign DNA that matches any of the sequences stored in its CRISPR array, the crRNA will deliver Cas9 to that invading sequence to chop it up.

Pretty clever for tiny bacteria, huh? But here is where things get really interesting, or worrisome, depending on your appetite for paranoia. Scientists have adapted CRISPR/Cas9 to work in all sorts of cell types, including human. Cas9 acts as DNA shears that can cut wherever we tell it to by directing it with a “guide RNA” (analogous to how a crRNA operates in bacteria). This provides us with an unprecedented means to easily “edit” the genome of virtually any living thing, including stem cells and embryos. Furthermore, Cas9 has been modified to do more than just cut DNA; versions exist now that can insert new DNA sequences or switch out bad (mutated) DNA with good DNA.

In the hit TV show, Orphan Black, a group of clones discover that their DNA has been “barcoded” to designate them as intellectual property by their maker. Theoretically, CRISPR technology could have been used to tag DNA in this fashion.

The power of genome editing can be used for good. Several diseases, such as cystic fibrosis and sickle-cell anemia, are caused by a single mutation in one gene. CRISPR/Cas9 is a plausible tool that may be able to repair this defect. However, tinkering with one gene can have unforeseen repercussions on other genes, so this exciting technology could have adverse effects. In March, 2015, a group of scientists proposed a ban on editing the human genome, arguing that a greater understanding of how CRISPR/Cas9 works is required before we even consider applying it clinically.

Gene editing using CRISPR/Cas9 can be used to modify the genome of virtually any creature. One recent application is the creation of wheat that is resistant to a fungus that causes mildew.

Here is a video that shows how CRISPR/Cas9 works and some of the applications it may have down the road:

 

 

Contributed by:  Bill Sullivan

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*It should be noted that not all bacteria are “germs”; in fact, many species of bacteria inhabit our bodies to constitute our “microbiome” and provide important services to us. Learn more about your microbiome here.
 

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