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X-ray Determines Cell Density

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GÖTTINGEN, Germany, Dec. 31, 2009 – Researchers usually have to destroy their samples to determine the densities and volumes of the different components which make up a biological cell. Larger subunits of the cell, consisting of many different biomolecules, are often destroyed before the mass and volumes can be determined.

A team of researchers led by Tim Salditt of the University of Göttingen together with colleagues from the Technical University of Munich (TUM) and the Swiss Paul-Scherrer-Institute has now demonstrated a way to study the density distribution of a biological cell without destroying it.

In a study published in the Proceedings of the National Academy of Science, the research teams led by Klaus Giewekemeyer and Tim Salditt report on an experiment at the Swiss Light Source, where they illuminated bacterial cells with an intense X-ray beam and determined the projected density distribution from the diffracted light.

Deinococcus radiodurans in x-ray-light. Blue colors indicate areas with low density, red areas mark highest density. These are the regions carrying the compressed DNA-molecules. (Image: Tim Salditt, University of Goettingen)

The applied short wavelength enables the study of extended samples without considerable attenuation of the beam. Furthermore, for most biological materials local density differences can be determined particularly well by the method, and almost independently of the chemical composition.

The physicists studied the bacterium Deinococcus Radiodurans, an abundant coccoid with a remarkable adaptability to hostile environments, and most notably to strong ionizing radiation. The protozoa can survive a radiation dose a thousand times stronger than that lethal for any other known living species. How the bacterium can handle the efficient repair of radiation damage might be connected with the special packing of the genetic material within the bacterium. The researchers were able to visualize the genetic material in the cells in a contrast based on the different delay times imposed on the X-ray waves when traversing a sample of varying density.

This imaging technique, called ptychography, was first introduced in the 1970s for electron diffraction. It consists in measuring full far-field diffraction patterns as a small illumination is scanned on a sample. While its use in electron microscopy is still limited, ptychography has gained tremendous popularity in the X-ray imaging community in the last few years, thanks to the development by Franz Pfeiffer, now chair of the biomedical physics group at TUM, and his team.

The new method has now been demonstrated for the first time successfully on a weakly scattering biological sample. The advantages granted by the novel method are the easy access to quantitative density information and the scalability of the image sharpness with the intensity of the incoming X-ray beam.

As a next step the researchers plan to extend the method to three dimensions by illuminating the sample from many different directions.

“This could, for example, help us to gain new insights into the packing of genetic material in a bacterial cell”, said Giewekemeyer.

This research is supported by the German Research Foundation (DFG), the Helmholtz Society and the German Ministry of Education and Research.

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Dec 2009
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