Ultrasound can now track bacteria in the body like sonar detects submarines.
For the first time, researchers have genetically modified microbes to form gas-filled pouches that scatter sound waves to produce ultrasound signals. When these bacteria are placed inside an animal, an ultrasound detector can pick up those signals and reveal the microbes’ location, much like sonar waves bouncing off ships at sea, explains study coauthor Mikhail Shapiro, a chemical engineer at Caltech.
This technique, described in the Jan. 4 Nature, could help researchers more closely monitor microbes used to seek and destroy tumors or treat gut diseases (SN: 11/1/14, p. 18).
Repurposing ultrasound, a common tissue-imaging method, to map microbes creates “a tool that nobody thought was even conceivable,” says Olivier Couture, a medical biophysicist at the French National Center for Scientific Research in Paris, who wasn’t involved in the work.
Until now, researchers have tracked disease-fighting bacteria in the body by genetically engineering them to glow green in ultraviolet images. But that light provides only blurry views of microbes in deeper tissue — if it can be seen at all. With ultrasound, “we can go centimeters deep and still see things with a spatial precision on the order of a hundred micrometers,” Shapiro says.
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Shapiro and his colleagues engineered a strain of E. coli used to treat gut infection to form gas compartments, and injected these bacteria into mice’s bellies. Unlike glowing bacteria — which could only be pinpointed to somewhere in a mouse’s abdomen — ultrasound images located the gas-filled microbes in the colon. The researchers also used their ultrasound technique in mice to image Salmonella bacteria, which could be used to deliver cancer-killing drugs to tumor cells.
Bacteria that produce ultrasound signals can also be designed to help diagnose illnesses, Shapiro says. For instance, a patient could swallow bacteria engineered to create gas pockets wherever the microbes sense inflammation. A doctor could then use ultrasound to search for inflamed tissue, rather than performing a more invasive procedure like a colonoscopy.