Sure enough, after isolating the genomic DNA from each mutant strain, the researchers found that every single CRISPR region had expanded to include a new snippet of DNA spliced between the repeats. Furthermore, these new spacers perfectly matched the DNA of the phage to which that strain was now immune. What made this apparent mode of immunity so remarkable was that, because these changes were physically embedded in the bacteria’s CRISPR DNA, the new immunity was heritable and would be passed down every time the bacterial cells reproduced.
