The plasma membrane is selectively permeable. It allows some substances to enter and leave the cell unrestricted while others are restricted. Some toxins produced by certain bacteria cross the plasma membrane, interfering with normal chemical reactions, and ultimately killing the affected cell. Why are cells unable to stop the flood of these harmful substances into the cell

There are specific receptors present on the surface of the normal cell membrane to recognize the molecules to allow them to pass or not. Most of the bacterial toxin are water-soluble molecules, and bound to the cell surface and interact with them as some time antibody-antigen interaction or cell recognition does not occur that form a pore in the cell membrane and toxins move into the cell.

Once toxins enter the cell they attack intracellular targets through endocytic vesicles. Ultimately these ends in cell death.