Delivering CRISPR-Cas9 for targeted killing and plasmid removal. Left: phages are engineered to encode the Cas9 nuclease, a trans-activating crRNA (tracrRNA) and an array of plasmid-targeting or genome-targeting CRISPR RNAs. The CRISPR RNAs are designed to target unique sequences in the bacterial chromosome or in harbored plasmids. Right: injection of the phage DNA into a mixed population of bacteria leads to removal (here depicted with broken lines) of targeted strains or plasmids without impacting the rest of the population. With further development, this strategy has the potential to treat multidrug-resistant infections without impacting beneficial microbes, to remove contaminating microbes from industrial fermentations and to provide further insights into microbial communities.