EVIDENCE FOR PORE-FORMING ABILITY BY LEGIONELLA-PNEUMOPHILA

Legionella pneumophila is the cause of Legionnaires' pneumonia. After internalization by macrophages, it bypasses the normal endocytic pathw ay and occupies a replicative phagosome bound by endoplasmic reticulum . Here, we show that lysis of macrophages and red blood cells by L. pn eumophila was dependent on dotA and other loci known to be required fo r proper targeting of the phagosome and replication within the host ce ll. Cytotoxicity occurred rapidly during a high-multiplicity infection , required close association of the bacteria with the eukaryotic cell and was a form of necrotic cell death accompanied by osmotic lysis. Th e differential cytoprotective ability of high-molecular-weight polyeth ylene glycols suggested that osmotic lysis resulted from insertion of a pore less than 3 nm in diameter into the plasma membrane. Results co ncerning the uptake of membrane-impermeant fluorescent compounds of va rious sizes are consistent with the osmoprotection analysis. Therefore , kinetic and genetic evidence suggested that the apparent ability of L. pneumophila to insert a pore into eukaryotic membranes on initial c ontact may play a role in altering endocytic trafficking events within the host cell and in the establishment of a replicative vacuole.