Enterococcus faecalis senses target cells and in response expresses cytolysin.

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.