Nature Nanotech. 9, 311–316 (2014)

Gas-filled microbubbles are used at present in contrast-enhanced ultrasound imaging for biomedical applications; however, these structures tend to be unstable at sizes below 1 μm. For molecular imaging, nanoscale contrast agents are required but few such systems have been developed. Now, Mikhail Shapiro and colleagues report the ability of gas-filled nanostructures formed from microorganisms to show stable ultrasound contrast both in vitro and in vivo. The gas vesicles, formed from bacteria or archaea, have specific, genetically encoded protein shells and allow gas in the surrounding media to freely diffuse in and out of them, but do not allow water to permeate inside. The absence of a pressure build-up between the outside and inside of the vesicles allows them to remain stable. The nanostructures are either cylindrical or bioconical in shape and have widths between 45–250 nm and lengths between 100–600 nm. The critical pressure at which the gas vesicles collapse varies with the microorganism from which they are formed. This can be exploited to cause 'serial collapse' of multiple vesicles, enabling the different vesicles to be distinguished in situ.