Pseudomonas aeruginosa mediated apoptosis requires the ADP-ribosylating activity of ExoS

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primaril y infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were disco vered to induce apoptosis at high frequency in HeLa and other epithelial an d fibroblast cell sines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morpho logy, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, a nd (iv) activation of caspase-3 activity. Induction of the type III secreti on machinery, but not invasion of P. aeruginosa is required for induction o f apoptosis. The apoptosis phenotype is independent of the cytoskeletal rea rrangements that occur in the host cell early after infection. Mutants in P . aeruginosa exoS fail to induce apoptosis and complementation with wild-ty pe exoS restored the apoptosis-inducing capacity, demonstrating that ExoS i s the effector molecule. Analysis of exoS activity mutants shows that the A DP-ribosylating capacity of ExoS is essential for inducing the apoptotic pa thway.