The CRISPR-Cas systems were selectively inactivated during evolution of Bacillus cereus group for adaptation to diverse environments.

CRISPR-Cas systems are considered as barriers to horizontal gene transfer (HGT). However, the influence of such systems on HGT within species is unclear. Also, little is known about the impact of CRISPR-Cas systems on bacterial evolution at the population level. Here, using Bacillus cereus sensu lato as model, we investigate the interplay between CRISPR-Cas systems and HGT at the population scale. We found that only a small fraction of the strains have CRISPR-Cas systems (13.9% of 1871), and most of such systems are defective based on their gene content analysis. Comparative genomic analysis revealed that the CRISPR-Cas systems are barriers to HGT within this group, since strains harboring active systems contain less mobile genetic elements (MGEs), have lower fraction of unique genes and also display limited environmental distributions than strains without active CRISPR-Cas systems. The introduction of a functional CRISPR-Cas system into a strain lacking the system resulted in reduced adaptability to various stresses and decreased pathogenicity of the transformant strain, indicating that B. cereus group strains could benefit from inactivating such systems. Our work provides a large-scale case to support that the CRISPR-Cas systems are barriers to HGT within species, and that in the B. cereus group the inactivation of CRISPR-Cas systems correlated with acquisition of MGEs that could result in better adaptation to diverse environments.