The endless battle between phages and CRISPR–Cas systems in Streptococcus thermophilus

Abstract

This review describes the contribution of basic research on phage–bacteria interactions to the understanding of CRISPR–Cas systems and their various applications. It focuses on the natural function of CRISPR–Cas systems as adaptive defense mechanisms against mobile genetic elements such as bacteriophage genomes and plasmids. Some of the advances in the characterization of the type II-A CRISPR–Cas system of Streptococcus thermophilus and Streptococcus pyogenes led to the development of the CRISPR–Cas9 genome-editing technology. We mostly discuss the 3 stages of the CRISPR–Cas system in S. thermophilus, namely the adaptation stage, which is unique to this resistance mechanism; the CRISPR RNA biogenesis; and the DNA-cutting activity in the interference stage to protect bacteria against phages. Finally, we look into applications of CRISPR–Cas in microbiology, including overcoming limitations in genome editing.