Virus turns bacteria’s defenses against them

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.