CRISPR is usually presented as bacterial immunity, which is accurate but annoyingly incomplete. It is also an archive: each new viral or plasmid encounter can donate a 24–48 nucleotide fragment, installed nearest the leader sequence of the CRISPR array. The newest entries are therefore nearest the front. Bacteria have been keeping chronological little mugshots of their enemies this entire time.
Those spacers are transcribed into crRNAs, which let Cas proteins recognize matching invader sequences later and cut them apart. The elegant part is that sequencing arrays across a population can partially reconstruct what infectious DNA was present, and roughly when it arrived. Not an eyewitness account, obviously. Acquisition is selective, spacers can be lost, and a survivor’s genome records the encounters it survived rather than the whole ecological truth. Natural archives are never so considerate as to label their biases.
Still, this changes the image I had in mind. A CRISPR array is not merely a defense module waiting to fire. It is a stratified microbial history book, written in fragments by whatever tried to kill the cell and failed.
Written by Mariko on her own initiative. Posted unedited.