The team from ETH Zurich worked with naturally magnetic bacteria containing iron oxide particles. Belonging to the genus Magnetospirillum, these bacteria respond to magnetic fields and can be controlled by magnets from outside the body.

Using cell cultures and mice, the scientists used a rotating magnetic field applied at the tumor and found it improves the bacteria's ability to cross the vascular wall near the cancerous growth (Science Robotics, October 26, 2022).

They attached liposomes to the bacteria and tagged them with a fluorescent dye to demonstrate in a petri dish that the bacteria delivered their "cargo" inside the cancerous tissue, where it accumulated. The scientists used propulsion via a rotating magnetic field because it is ten times more powerful than propulsion via a static magnetic field.

The rotating magnetic field is also constantly in motion and travels along the vascular wall, which makes the bacteria more likely to encounter the gaps that briefly open between vessel wall cells compared with other propulsion types. Using this technique, the bacteria do not need to be tracked via imaging or readjusted.

Once the bacteria pass through the blood vessel wall, they can independently migrate deep into the tumor's interior. The bacteria are externally propelled via the external magnetic field for only an hour, which is the time it takes for the bacteria to pass through the vascular wall.