CRISPR/Cas system and resistance to bacteriophage infection

Bacteria and phages co‐evolve in their environments through an arm‐race with bacteria developing strategies to combat infection by diverse phages, while phages finding ways to circumvent them. Clustered regularly interspaced short palindromic repeats (CRISPR) loci, along with several Cas (CRISPR‐associated) proteins, represents a form of immune system widespread in Bacteria and Archaea. The CRISPR loci evolve through the incorporation of short deoxyribonucleic acid (DNA) sequences (spacers), derived mostly from extrachromosomal DNA such as phage or plasmid sequences, between two partially palindromic repeats. A CRISPR transcript is produced and cleaved within the repeats by Cas protein(s) with or without other host proteins to produce smaller ribonucleic acid fragments (RNAs). These small mature RNAs and Cas proteins target and cleave through base complementarity the invading nucleic acids to ensure cell defence. Phages can also evade the CRISPR/Cas system through point mutations or deletions, forcing the host to adapt by either acquiring new spacers or relying on other defence systems. Hence, phage/host interactions can be appreciated at a microbial population‐wide level through the dynamism of CRISPR/Cas loci