To find out where a submarine is, you send out a sonar signal which that sub reflects back. It may some day be similarly possible to locate therapeutic bacteria within a patient's body, as scientists at the California Institute of Technology have created the first bacterial cells that are capable of reflecting sound waves.

Assistant professor Mikhail Shapiro first got the idea about six years ago, when he heard about aquatic bacteria (Anabaena flos-aquae) that regulate their buoyancy using gas-filled protein structures known as vesicles. He wondered if those vesicles could reflect sound waves in a fashion that made them distinguishable from other cell types.

It turns out that they could, as evidenced by experiments in which ultrasound was used to locate the bacteria within the guts of mice.

His team then set about transferring the vesicle-making genes from Anabaena flos-aquae into Escherichia coli bacteria, harmless strains of which are commonly used in therapeutic applications such as probiotics. After a couple of challenges were overcome, the scientists eventually succeeded in engineering E. coli that produced their own vesicles.

As was the case with the Anabaena flos-aquae, tests showed that ultrasound could be used to image and locate the altered E. coli bacteria within mouse guts.

Although the technology should soon be available to other scientists conducting animal research, it may be some time before it's developed to the point that it can be used in humans. Once it is, it could help doctors to confirm that therapeutic bacteria have successfully reached their destination within the body – such as the gut or tumor sites – and that they're active.

"We want to be able to ask the bacteria, 'Where are you and how are you doing?'" says Shapiro. "The first step is to learn to visualize and locate the cells, and the next step is to communicate with them."

A paper on the research was published this week in the journal Nature.