Using the engineering approach of synthetic biology, Chris Anderson has set out to program bacteria to selectively kill cancer cells. He is combining DNA sequences from different types of bacteria and inserting them into the bacterium E. coli to create an organism that can evade the immune system, home in on tumors, and trick cancer cells into letting it inside, where it releases a toxin.

Anderson has built and tested all the biological parts for the cancer-­killing bug and is now working on putting them together. "All of these things exist as little genetic programs," he says. He also expects to be able to engineer bacteria for other medical purposes, because "everything is designed in a modular way, so the parts can be used for a totally different application that shares some of the same problems." For example, the genetic parts he has developed could be used to deliver medicine to an HIV-infected immune cell.

Credit: Tami Tolpa

1) Engineered bacteria are injected into the bloodstream; polysaccharide molecules on their surfaces allow them to evade the immune system

2) When they detect the low-oxygen environment of a tumor, the bacteria produce invasin, a protein that allows them to infiltrate the cancer cells

3) The invasin binds to the cancer cells, prompting the cells to engulf the bacteria

4) The cancer cell bursts the bacterium, releasing a toxic enzyme that kills the cell

--Emily Singer