Scientists design bacteria to reflect sonar signals for ultrasound imaging

Researchers at Caltech for the first time, created bacterial cells with the ability to reflect sound waves, reminiscent of how submarines reflect sonar to reveal their locations. 

The ultimate goal is to be able to inject therapeutic bacteria into a patient's body-for example, as probiotics to help treat diseases of the gut or as targeted tumor treatments - and then use ultrasound machines to hit the engineered bacteria with sound waves to generate images that reveal the locations of the microbes.

"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."

The team's goal was to transfer the genes for making gas vesicles from the water-dwelling bacteria into a different type of bacteria-Escherichia coli, which is commonly used in microbial therapeutics, such as probiotics.

One of the challenges the team hit involved the transfer of the genetic machinery for gas vesicles into E. coli. They first tried to transfer gas-vesicle genes isolated from a water-dwelling bacterium called Anabaena flos-aquae, but this didn't work-the E. coli failed to make the vesicles. They tried again using gas-vesicle genes from a closer relative of E. coli, a bacterium called Bacillus megaterium. This didn't succeed either, because the resulting gas vesicles were too small to efficiently scatter sound waves. Finally, the team tried a mix of genes from both species - and it worked. The E. coli made gas vesicles on their own.