Bacteriostatic antibiotics promote the evolution of CRISPR-Cas immunity

Phage therapy can be used in combination with antibiotics to combat infections with bacterial pathogens 1–3 . However, bacteria can rapidly evolve phage resistance via receptor mutation, or using their CRISPR-Cas adaptive immune systems 4 , which insert short phage-derived sequences into CRISPR loci in the bacterial genome 5 to guide sequence-specific cleavage of cognate sequences 6 . Unlike CRISPR-Cas immunity, mutation of the phage receptor leads to attenuated virulence when the opportunistic pathogen Pseudomonas aeruginosa is infected with its phage DMS3vir 7 , which underscores the need to predict how phage resistance evolves under clinically relevant conditions. Here, using eight antibiotics with various modes of action, we show that bacteriostatic antibiotics (which inhibit cell growth without killing) specifically promote evolution of CRISPR-Cas immunity in P. aeruginosa by slowing down phage development and providing more time for cells to acquire phage-derived sequences and mount an immune response. Our data show that some antimicrobial treatments can contribute to the evolution of phage-resistant pathogens with high virulence.