Multiscale photoacoustic tomography using reversibly switchable bacterial phytochrome as a near-infrared photochromic probe

Combining a reversibly photoswitchable bacteriophytochrome with photoacoustic imaging allows for exceptionally sensitive tomographic imaging deep in living mice, as well as super-resolution photoacoustic microscopy. Photoacoustic tomography (PAT) of genetically encoded probes allows for imaging of targeted biological processes deep in tissues with high spatial resolution; however, high background signals from blood can limit the achievable detection sensitivity. Here we describe a reversibly switchable nonfluorescent bacterial phytochrome for use in multiscale photoacoustic imaging, BphP1, with the most red-shifted absorption among genetically encoded probes. BphP1 binds a heme-derived biliverdin chromophore and is reversibly photoconvertible between red and near-infrared light-absorption states. We combined single-wavelength PAT with efficient BphP1 photoswitching, which enabled differential imaging with substantially decreased background signals, enhanced detection sensitivity, increased penetration depth and improved spatial resolution. We monitored tumor growth and metastasis with ∼100-μm resolution at depths approaching 10 mm using photoacoustic computed tomography, and we imaged individual cancer cells with a suboptical-diffraction resolution of ∼140 nm using photoacoustic microscopy. This technology is promising for biomedical studies at several scales.