Research News
Scanning electron microscope image of EHEC (short for: Enterohemorrhagic Escherichia coli). Photo: Manfred Rohde/HZI
Ticket to a Strange World
Since the outbreak of EHEC in the north of Germany, much could be learned about the pathogenic agent: That it is a dangerous variant of our intestinal bacterium Escherichia coli; that it excretes a cytotoxin and that in addition to bloody diarrhea it can cause life-threatening kidney failure. Now we also know what EHEC looks like - thanks to the impressive photographs by Dr Manfred Rohde, which went to the media. At the Helmholz Centre for Infection Research (HZI) Rohde operates a field emission scanning electron microscope enabling us to cast a glance at the world of bacteria. "The device can manage an 800,000-fold magnification", says Rohde. "Yet as regards biological specimens only a maximum of 200,000-fold makes sense."
Well-focused photographs can be obtained only after a painstaking specimen preparation. After Manfred Rohde has chemically secured the bacteria structures with formaldehyde and glutaraldehyde, he needs to high-vacuum dry the specimen for microscopy. For this, he transfers it step by step into acetone. However, the simple evaporation of acetone creates surface tension and can destroy biological structures. In order to avoid this, the specimen dries in liquid carbon dioxide. At the critical point of balance between liquid and gaseous carbon dioxide no surface tensions occur. Finally, Rohde overlays the specimen with a wafer-thin layer of gold. It now is conductive and ready for microscopy.
An electron beam is emitted from a wolfram crystal with zirconium, scans the specimen surface and knocks secondary electrons from out of the gold layer. These are captured by two detectors in different positions. On the basis of the signals from both detectors, a computer then instantly calculates an image resembling 3D. "The higher a given point of the specimen is, the more electrons it emits and the area appears lighter in colour", explains Rohde. Incidentally, his images are in high demand not only in our country. "We also work for groups in India, New Zealand, Australia and China. Yet the special feature is not our microscope but our preparation technique."
