Enterococcus faecalis Senses Target Cells and in Response Expresses Cytolysin

Abstract

Many virulent strains of Enterococcus faecalis produce a two-subunit toxin, termed cytolysin. Cytolysin expression is regulated by one of the subunits (CylL S ″) through a quorum-sensing autoinduction mechanism. We found that when target cells are absent, the other subunit (CylL L ″) forms a complex with CylL S ″, blocking it from autoinducing the operon. When target cells are present, however, CylL L ″ binds preferentially to the target, allowing free CylL S ″ to accumulate above the induction threshold. Thus, enterococci use CylL L ″ to actively probe the environment for target cells, and when target cells are detected, allows the organism to express high levels of cytolysin in response.