Exploiting CRISPR-Cas to manipulate Enterococcus faecalis populations

Abstract

CRISPR-Cas provides a barrier to horizontal gene transfer in prokaryotes. It was previously observed that functional CRISPR-Cas systems are absent from multidrug-resistant (MDR) <i>Enterococcus faecalis</i>, which only possess an orphan CRISPR locus, termed CRISPR2, lacking <i>cas</i> genes. Here, we investigate how the interplay between CRISPR-Cas genome defense and antibiotic selection for mobile genetic elements shapes in vitro <i>E. faecalis</i> populations. We demonstrate that CRISPR2 can be reactivated for genome defense in MDR strains. Interestingly, we observe that <i>E. faecalis</i> transiently maintains CRISPR targets despite active CRISPR-Cas systems. Subsequently, if selection for the CRISPR target is present, toxic CRISPR spacers are lost over time, while in the absence of selection, CRISPR targets are lost over time. We find that forced maintenance of CRISPR targets induces a fitness cost that can be exploited to alter heterogeneous <i>E. faecalis</i> populations.