Biological warfare between two bacterial viruses in a genomic archipelago sheds light on the spread of CRISPR-Cas systems

CRISPR-Cas systems are acquired immunity systems of bacteria and archaea that prevent infection by phages and other mobile genetic elements. It is currently known that this defense system has also been co-opted by viruses. These viruses could use CRISPR-Cas systems to compete against other rival viruses. We have discovered a virus in the bacterium Acinetobacter baumannii that incorporates a CRISPR-Cas system into an integration hotspot of the host genome. Once integrated, this could prevent the infection of the most frequent viruses in this bacterial species, especially one that competes with the CRISPR-Cas system itself for the same integration site. This latter virus is prevalent in strains of the species belonging to the so-called Global Clone 2, which causes the most frequent outbreaks worldwide. Knowledge of this new viral warfare using CRISPR-Cas systems, known to limit the entry of antibiotic resistance genes into bacteria, could be useful in the fight against the infections they cause. But it would also shed light on the way in which these defense systems expand in bacteria. ### Competing Interest Statement The authors have declared no competing interest.