An ultrasound is probably most popularly recognized as a doctor’s tool to peer into the womb and take a look at a growing fetus during a woman’s pregnancy. But what if sound could be used to take a look at even smaller things – like the microorganisms in your gut? Mikhail Shapiro’s research group at the California Institute of Technology has been able to track where bacteria are located by, quite literally, listening to them.
Traditionally, scientists have used light, not sound, to locate the position of biomolecules. Molecules or even entire organisms can be engineered to “glow” by producing a protein derived from a glow-in-the-dark jellyfish that gives off light when zapped with a fluorescent laser. However, light scatters fairly easily, limiting the image contrast that can be achieved by using fluorescence. On the other hand, sound can travel great distances without being scattered (Think of dolphins, bats, and other mammals that use echolocation, or sound waves, to communicate over vast distances).
Ultrasound relies on directing sound waves towards a sample and measuring the reflected echoes. The nature of these echoes is different depending on the density of the material that the ultrasound waves hit (for instance, gas versus liquid versus solid). Shapiro and his team engineered bacteria to produce tiny gas-filled bubbles in their interiors and then injected them into mice. The location of the bacteria, which traveled to the mouse gut, could be easily tracked via ultrasound because the gas bubbles within them had a vastly different density than their surrounding liquid environment.
While currently, the gas bubbles can only be made in bacteria, the hope is that eventually, other kinds of cells will be able to make them as well, including human cells. If this becomes possible, we might be able to obtain high-contrast images of countless types of cells – circulating blood cells, migrating stem cells, and others – with nothing more than an ultrasound probe.
Popular news article: Scientists design bacteria to reflect “sonar” signals for ultrasound imaging
Original science article: Acoustic reporter genes for noninvasive imaging of microorganisms in mammalian hosts – Nature
Managing correspondent: Radhika Agarwal