Press release: An X-ray view of the density distribution in biological cells

Nr. 259/2009 - 10.12.2009

Advancement of lensless X-ray microscopy provides new insights

(pug) 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 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 this week, 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. The short wavelength applied 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.

The new method applied here was developed last year within the team led by Franz Pfeiffer from the Technical University of Munich, a co-author of the present study, and 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”, says Klaus Giewekemeyer, first-author of the study.

Original article:
Klaus Giewekemeyer K. Giewekemeyer, P. Thibault, S. Kalbfleisch, A. Beerlink, C. M. Kewish, M. Dierolf, F. Pfeiffer, T. Salditt et al. Quantitative biological imaging by ptychographic x-ray diffraction microscopy. Proceedings of the National Academy of Sciences of the United States of America 2009 (PNAS early online edition), doi: 10.1073/pnas.0905846107.