Engineered bacteria reflect 'sonar' signals for ultrasound imaging

January 09, 2018 // By Rich Pell
Scientists at the California Institute of Technology (Pasadena, CA) have engineered bacteria to reflect sound waves and thereby reveal their location in the body.

Similar to how submarines reflect sonar to reveal their locations, such engineered bacterial cells promise the ability to be injected therapeutically into a patient's body — e.g., as probiotics to help treat diseases of the gut or as targeted tumor treatments — and then located with the use of ultrasound machines, which generate images that reveal the locations of the microbes. The images would let doctors know if the treatments made it to the right place in the body and were working properly.

"We are engineering the bacterial cells so they can bounce sound waves back to us and let us know their location the way a ship or submarine scatters sonar when another ship is looking for it," says Mikhail Shapiro, assistant professor of chemical engineering, Schlinger Scholar, and Heritage Medical Research Institute Investigator. "We want to be able to ask the bacteria, 'Where are you and how are you doing?' The first step is to learn to visualize and locate the cells, and the next step is to communicate with them."

Currently, visualizing bacterial cells as well as communicating with them — i.e., gathering information on what's happening in the body and giving the bacteria instructions about what to do next — is not yet possible. Imaging techniques that rely on light — such as taking pictures of cells tagged with a "reporter gene" that codes for green fluorescent protein — only work in tissue samples removed from the body, as light can't penetrate into deeper tissues of the body, like the gut, where the bacterial cells would reside.

Ultrasound techniques, however, can offer a solution because sound waves can travel deeper into bodies. Shapiro says the idea occurred to him when he learned about gas-filled protein structures in water-dwelling bacteria that help regulate the organisms' buoyancy.

The structures - called gas vesicles - could, he hypothesized, bounce back sound waves in ways that make them distinguishable from

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