Pathogenic bacteria enter host cells basically for "food and shelter", and to hide from the host's immune system. Under normal circumstances, the phagocytic cells of the immune system engulf the bacteria and/or tag them for destruction by other means. This keeps the bacteria away from other cells. Studies indicate however, that pathogenic bacteria enter non-phagocytic cells via receptor-mediated endocytosis. The bacterium has a surface protein that a protein on the host cell recognizes. The cell is then fooled into believeing that the bacterial protein is one that needs to be destroyed or recycled. Since the cell's protein destruction machinery is inside the cell, it brings the bacterium into the cell via endocytosis. And the rest is history!