X marks the subcellular spot

Margaret W. Bushee, margaret.bushee@photonics.com

Using x-ray techniques to image live cells on the nanoscale has long been problematic. The advantage of x-ray microscopy is that, unlike conventional electron scanning, it penetrates below the surface to yield images with high spatial and temporal resolution. However, x-rays also can be toxic to fragile biological samples.

The limitations of x-ray microscopy have been overcome in large part thanks to biophysicists Dr. Franz Pfeiffer, Dr. Pierre Thibault and Martin Dierolf, all of Technical University Munich in Germany. The Swiss Light Source, a dedicated high-power microscope at Paul Scherrer Institute, has been instrumental in their accomplishments.

Most recently, the researchers published images of the nucleoid structure, or genetic material, of the bacteria and polyextremophile Deinococcus radiodurans, which is characterized by an unusual tolerance to ionizing radiation. Capturing a clear image of this subcellular structure is an important step for scientists in the quest to study disease and survival. The study was published online in the November 2009 issue of the Proceedings of the National Academy of Sciences.

The rendering of these images was made possible by the Pfeiffer team’s further refinement of a technique introduced in the 1970s called ptychography, which involves recording full far-field diffraction patterns while a sample is raster-scanned (line by line) through the focal spot of the beam, one minuscule section at a time. An algorithm designed by the team uses this data to create an image of the sample in its entirety.

A year prior to its recent breakthrough, the team demonstrated the superresolution capability of ptychography in conjunction with x-rays by combining two independent techniques – coherent diffractive imaging and scanning transmission x-ray microscopy – to image a buried gold nanostructure. Hard, or high-energy, x-rays were used during this “lensless microscopy” technique. The results were published in the July 18, 2008, issue of Science.

Now that the investigators have researched the resolution, reproducibility and reliability of their technique, they are working on three-dimensional imaging of biological cells. X-ray imaging’s capacity to visualize what is beneath the surface could bring new insights to biotechnology and evolutionary biology.