X-ray tomograms of E. coli cells expressing the cSeNP or GRLMR in grown in varying concentrations of SeO3 2-were acquired in a Scanco micro-CT 80 instrument. CT data is presented in Figure . Panel A shows that cells expressing the cSeNP in the indicated concentrations of SeO3 2-attenuate X-rays with an efficiency that depends on the growth condition. Attenuation of 700HU is observed for cells grown in 1mM SeO3 2-. This is significantly greater attenuation than observed for soft tissues, but less than bone. Notably, cells grown in the same concentrations of SeO3 2-but not expressing the cSeNP never attenuate at more than 100 HU. Panel B shows a voxel-parsed trace of X-ray attenuation. Here we observe that cells expressing the cSeNP frown in 2mM SeO3 2-show voxels with minor attenuation peak at 190HU and a major attenuation peak at 1100HU. Corresponding controls (not with in Se) show only an attenuation peak near 100HU, comparable to soft tissue. Panel C shows the attenuation of cSeNP expressing cells grown in concentrations of SeO3 2-ranging from 1μΜ to 10mM. Similar to panel A, attenuation increases with the concentration of SeO3 2-in which the cells were grown. Panel D shows a 9-position microcentrifuge-tube holder. The contents of each position are described in the figure legend. More white in these images corresponds to Se absorption. Panels E and F show tomograms of the samples in tubes 5 and 6. They are colored according to attenuation, with red representing attenuation near 1100HU and blue representing attenuation near 100HU. More details on this rendering are available in the SI. Overall, these tomograms show that cells expressing the cSeNP show clearly distinguishable X-ray contrast relative to cells grown in the same concentrations of SeO3 2- that do not express the cSeNP.