I found this article which explains BCG, what it is and how it works:
TURB is usually followed by treatment administered within the bladder to rid the organ of any tumor cells that broke off during TURB. Called intravesical therapy, this type of immunotherapy uses the body’s own immune system to fight cancer. The agent most often used for intravesical treatment is bacillus Calmette-Guérin (BCG), a form of bacteria that is sometimes used as a tuberculosis vaccine. The treatment is typically given once a week for six weeks, and although much of the drug is washed out during urination, a good amount remains in the bladder.
Researchers believe BCG works by alerting the cells lining the bladder of a brewing infection, which results in infection-fighting white blood cells rushing to the bladder. But unlike the normal chain of events that occurs with urinary tract infections, Dr. O’Donnell says BCG causes the white blood cells to release a tumor-killing compound called TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) that is effective in destroying cancer cells but spares normal cells. Chemotherapy agents, such as Valstar® (valrubicin), Mutamycin® (mitomycin)‚ Adriamycin® (doxorubicin) or Thioplex® (thiotepa), may also be used for intravesical therapy.
