Type III CRISPR-Cas systems and the roles of CRISPR-Cas in bacterial virulence

Type III CRISPR-Cas systems constitute nearly a quarter of all known CRISPR systems and have been found to reside in both archaea and bacteria, including important bacterial pathogens such as staphylococci and mycobacteria. By blocking the horizontal transfer of bacteriophage and conjugative plasmids, CRISPR-Cas systems not only protect against foreign invaders, but also prevent the acquisition of virulence factors and antibiotic resistance cassettes that are encoded in these mobile genetic elements. For this reason, these systems can have a broad impact on the evolution of bacterial pathogens. This chapter explores in-depth our current understanding of the molecular mechanisms of CRISPR interference in the type III systems in particular, and more generally, the existing data supporting the role of CRISPR-Cas systems in the emergence of bacterial pathogens.