A novel means of self-protection, unrelated to toxin activation, confers immunity to the bactericidal effects of the Enterococcus faecalis cytolysin.

Enterococcus faecalis has become a pervasive clinical problem due to the emergence of resistance to most antibiotics. The cytolysin of E. faecalis is a novel bacterial toxin that contributes to the severity of disease. It consists of two structural subunits, which together possess both hemolytic and bactericidal activity. Both toxin subunits are encoded in a complex operon frequently harbored on pheromone-responsive plasmids. E. faecalis strains lacking such plasmids are susceptible to the bactericidal effects of the cytolysin. A novel cytolysin immunity determinant at the 3' end of the pAD1 cytolysin operon is described in the present study. Deletion analysis and specific mutagenesis isolated the immunity function to a single open reading frame. Specific mutagenesis experiments demonstrate that cytolysin immunity is unrelated to cytolysin activator (CylA) expression as previously proposed. Cytolysin immunity is, however, encoded on the same transcript as and 3' to CylA, and previous associations between immunity and CylA can be ascribed to the polar behavior of Tn917 insertion.