Bikard et al. have developed sequence-specific antimicrobials based on the bacterial CRISPR–Cas immune system. The reprogrammed Cas9 nuclease, guided by small RNAs to specific virulence gene loci where it mediates DNA cleavage, was able to selectively kill virulent Staphylococcus aureus while leaving other members of the community (including avirulent strains of the same species) intact, which could then colonize the niche and limit the growth of pathogenic bacteria. This approach could be adapted to target plasmid-borne antibiotic resistance genes, thus preventing the spread of resistance in complex bacterial populations.