Like humans,bacteria arrive under attack from viruses and rely on an immune system to defend themselves. A bacterial immune system known as CRISPR helps microbes "remember" the viruses they encounter and more easily fend them off in the future. Since researchers first discovered CRISPR in the mid-2000s, they have noticed something peculiar: It records confrontations with viruses sequentially, and placing the most recent attack first in a series of genetically encoded memories.
Source: phys.org