The bacteria bots wouldn't be able to make it in larger blood vessels on their own, however. The current would be too strong for them to swim against. So the researchers envisage using a larger, magnetically steerable microvehicle to carry the bots close to a tumor. "The vehicle will be a type of polymer, or possibly another type of material," says Martel. "We have a way to release the bacteria while the vehicle stays there and dissolves."

Martel's vehicle contains magnetic nanoparticles and can be moved at about 200 microns per second. He says that he and his team correct the microvehicle's course approximately 30 times a second. While they have developed the microvehicle and bacterial microbots independently, they are now working to combine the two technologies. "We think in two years we'll be able to do that," says Martel.

"This work is promising but, as with any transformative idea, there are a lot of challenges that need to be addressed," says Bahareh Behkam, an assistant professor at Virgina Polytechnic Institute, who has also used bacteria to propel microbeads. She suggests that it could be difficult to maintain normal blood flow and to retrieve the magnetic particles from the body after the procedure is complete.

Some researchers also question whether the body's immune system would attack the bacteria before they could reach a tumor, but Martel defends the approach. "We are very confident from our preliminary tests that this [scenario] will not be an issue," he says. Because the immune system has not encountered these bacteria before, he says, it would not have time to wipe out the microbots before they reach their target.