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Light tube grabs, scans tiniest bacterium


FREIBURG, Germany – A light tube that can grab, orient and record the movements of tiny, agile unicellular organisms may soon help scientists better understand bacterial infectious diseases.

The new light-scanning optical trap developed by scientists at the Department of Microsystems Engineering at the University of Freiburg traps and scans tiny elongated bacteria with the help of a quickly moving focused laser beam. The beam exerts an equally distributed force over the entire bacterium, which constantly changes its complex form. Simultaneously, the team was able to record the trapped bacterium’s movements in high-speed three-dimensional images by measuring minuscule deflections of the light particles.

With new optical tweezers, a quickly moving focused laser beam holds a tiny, spiral-shaped bacterium in place and records its movements in detail. In the background is an image taken with conventional microscopy in which it is possible to make out only the bare outline of the optically trapped bacterium. Courtesy of the University of Freiburg.

The physicists investigated spiral-shaped bacteria called spiroplasmas that are only 200 nm in diameter. Since these bacteria have no cell walls, they can rapidly change their shape and move. Optical tweezers can grab bacteria, but only at one point, and they cannot manipulate their orientation. Conventional optical microscopes also cannot sufficiently image these bacteria because of their small size and rapid movements. Using the new optical trap, the team was able to hold and orient the spiroplasma over its entire length as well as capture a 3-D image.

Light that hits the bacterium is deflected, then overlaps the nondeflected light and is amplified, enabling the generation of 3-D images that have not only high contrast, but also increased resolution. This makes it possible to acquire up to 1000 3-D images per second and record the bacterium’s rapid movements in detail.

“The movements of the bacteria are connected with extremely small changes in energy that are usually almost impossible to measure,” said team member Dr. Alexander Rohrbach, a member of the university’s Cluster of Excellence BIOSS, the Center for Biological Signalling Studies. “Interesting from a biological point of view are the signals that the bacterium sends out when it changes shape, because they provide clues about the molecular processes going on inside of it – for instance, as a reaction to stress states the bacterium has been subjected to.”

Next, the scientists plan to use this method to study the behavior and cellular mechanics of other cell wall-less bacteria that are difficult to treat with antibiotics. The study appeared in Nature Photonics (doi: 10.1038/nphoton.2012.232).

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