E. coli Cell Nucleoid Organization. Soft X-ray tomographic (SXT) imaging shows the organization of cell nucleoids (yellow) within cells (clear or blue background) of two different strains of E. coli (top, wild type strain MG1655; bottom, Hua38 mutant strain SK3842). Various cell morphologies found to be representative of the three growth stages are shown at left (lag phase), middle (exponential phase), and right (stationary phase). [From Hammel, M. H., et al. 2016.]
Soft X-ray tomography (SXT) is a non-invasive, 3D imaging technique that can measure volumes, surfaces, interfaces, membranes, and organelle connectivity within intact cells. SXT data are collected on a transmission soft x-ray microscope, called XM-2, located at the Advanced Light Source. In SXT, the specimen is illuminated with photons at energies from within the “water window,” a region of the electromagnetic spectrum in which water is transparent to soft x-rays (i.e., between the K-absorption edge of carbon at 282 eV and oxygen at 533 eV). As the illumination passes through the specimen, it is attenuated according to the type and concentration of molecules in the specimen. Photons within the water window are more strongly absorbed by carbon-containing and nitrogen-containing organic materials than they are by water by a full order of magnitude. Consequently, subtle differences in biochemical composition produce measurable contrast in soft x-ray images. SXT data currently have spatial resolutions of 35 or 50 nm, depending on the type of information required and the type of specimen being imaged. Consequently, SXT visualizes structures ranging from molecular machines to entire cells.
Algal Cell Morphology. Soft X-ray tomography provides a reconstructed cell with segmented nucleus (purple), chloroplast (green), mitochondria (red), lipids (yellow), and starch granules within the chloroplast (blue). (A) A representative orthoslice of the reconstructed cell. (B) Three-dimensional view. (C) Chloroplast and nucleus. (D) Fully segmented cell. [From Roth, M. S., et al. 2017.]
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Hammel, M. H., et al. 2016. "HU multimerization shift controls nucleoid compaction," Science Advances 2(7), e1600650. DOI: 10.1126/sciadv.1600650.
Roth, M. S., et al. 2017. "Chromosome-level genome assembly and transcriptome of the green alga Chromochloris zofingiensis illuminates astaxanthin production," PNAS 114(21), E4296-4305. DOI: 10.1073/pnas.1619928114.
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