Legionella pneumophila, the agent of human Legionnaire's disease is a Gram-negative, rod-shaped bacterium. During infection, the bacteria invade human cells and replicate intracellularly. L. pneumophila can induce apoptosis in human myeloid and epitheloid cells and this may contribute to the development of pathology and disease. However, the molecular mechanism of apoptosis induction is still uncertain. Here we investigate this process. Legionella efficiently induced apoptosis in myeloid cells, T cells and fibroblasts. Induction of apoptosis involved activation of the initiator caspase-9 and effector caspases. Caspase activity was required for cell death. Analysis of mutant cells showed that the death receptor pathway was not involved in Legionella-induced apoptosis. Surprisingly, caspase activity was found almost exclusively in cells that did not harbor bacteria. Infection with Legionella caused the activation of the pro-apoptotic protein Bax and the release of cytochrome c. Mouse embryonic fibroblasts deficient for Bax and/or Bak were protected from Legionella-induced caspase activation. These results show a clear contribution of the mitochondrial pathway to Legionella-induced apoptosis.
