Which of the following is the reason for Legionella pathogenicity?

Intracellular survival of the Legionella attributed to following factors: Reduced oxidative burst Failure of phagosome to acidify Failure of phagosome to fuse with lysosome Bacterial resistance to lysosomal contents In fact, the bacterial growth within infected macrophages has been estimated at 100- to 1000-fold within 48-72 hours of infection, which is considered remarkable compared to that of other intracellular oppounistic bacteria (e.g., Salmonella, Mycobacterium, Listeria).