Francisella Tularensis is a particularly devious bacteria, using our own immune system against us. We rely on our immune cells, called macrophages (macro=big, phage=eater), to recognise and eat bacteria. Once the bacteria is ingested by the macrophage they are trapped within phagosomes where the bacteria are broken down and destroyed. Francisella Tularensis is wise to this process and acts like a Trojan horse; it allows itself to be recognised and eaten by the macrophages. Once inside its escapes the phagosome to get into the cytosol, with access to the cells juicy underbelly thus is the perfect place to replicate. Once the Francisella Tularensis has sucked its macrophage dry of all its nutrients to support many many more bacteria the macrophage bursts open to release all the bacteria ready to the whole process again but on a much larger scale causing Tularemia, a potentially lethal infection.
Depicted is the life cycle of Francisella Tularensis reviewed by Audrey Chong and Jean Celli in Frontiers in Microbiology in 2010