Pore-forming toxins (such as HlyA / ClyA / SheA) are secreted by enterohemorrhagic bacteria. These toxins form transmembrane pores in human cells, causing them to lyse. These proteins are hazardous to handle in the lab.
Note that the Hemolysin (Hly) operon is a Type I Secretion System. Proteins can be secreted from E. coli by fusing them to the C-terminal signal sequence from HlyA. The signal sequence by itself is not harmful.
For review chapter see Hunt, Green, & Artymiuk