Lower their shields! —

Virus turns bacteria’s defenses against them

The enemy of my enemy is my friend.

The “evolutionary arms race” that is immunity extends even to bacteria and the viruses that infect them.

It is hardly news that viruses change to evade our immune systems; we know this all too well from HIV and influenza. It turns out that bacteriophages—viruses that infect bacteria—can do the same thing. Bacteria have many innate immune strategies to protect themselves against phage, but we only know of one that can adapt to attack different viruses. Turns out that some phages "know" about it too—and can change to avoid it, thus destroying the bacteria and ensuring the production of more phage.

The only documented bacterial adaptive immune system is the CRISPR/Cas system (That stands for Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated proteins).  These are regions of DNA consisting of a short sequence repeated a number of times, with the repeats separated by a spacer region of DNA. The spacers contain copies of fragments of DNA from phage that have attacked the bacteria in the past.

This is what is supposed to happen is that when phages invade: the repeat elements are made into RNA that matches the phage’s nucleic acids, signaling the bacteria to the presence of the foreign genetic material and inducing the destruction of the phage “through an unknown mechanism.” But some phage have found away around that.

Vibrio cholera bacteria has a phage called ICP1 that was isolated from the diarrhea of cholera patients in Bangladesh. ICP1 uses its own CRISPR/Cas system to destroy the bacteria’s version.  And when researchers mutated either the phage’s or the bacteria’s CRISPR/Cas sequences so the length of the spacers didn’t match, succeeding generations of phage mutated to incorporate an appropriate spacer so they could successfully knock out the bacteria's protections and infect it.

CRISPR-like arrays aren't limited to cholera-causing bacteria. Clostidium difficile, a strain of bacteria that causes severe diarrhea, also has one. If we could find a bacteriophage to fight C. difficile, it might provide a more palatable therapy than fecal transplants.

Nature, 2013. doi: 10.1038/nature11927  (About DOIs).

Channel Ars Technica