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Bacterium in a Laser Trap

Freiburg Researchers Develop Light Tube that Can Grab and Scan Even the Tiniest of Unicellular Organisms

Freiburg, Oct 04, 2012

Bacterium in a Laser Trap

With the optical tweezers, a laser beam holds a bacterium in place and records its movements in detail. In the background is an image taken with a conventional microscope in which it is only possible to make out the bare outline of the bacterium.

Scientists from the Department of Microsystems Engineering (IMTEK) of the University of Freiburg have constructed an innovative new optical trap that can grab and scan tiny elongated bacteria with the help of a laser. The physicists Prof. Dr. Alexander Rohrbach und Matthias Koch created a kind of light tube that traps the agile unicellular organisms. Optical tweezers could previously only be used to grab bacteria at one point, not to manipulate their orientation. The Freiburg researchers have now succeeded in using a quickly moving, focused laser beam to exert an equally distributed force over the entire bacterium, which constantly changes its complex form. At the same time, they were able to record the movements of the trapped bacterium in high-speed three-dimensional images by measuring miniscule deflections of the light particles. The team reports their findings in the current online edition of Nature Photonics.

The scientists investigated so-called spiroplasmas in their study. These spiral-shaped bacteria have a diameter of only 200 nanometers, roughly equivalent to the thickness of 1,000 atoms. Since they do not have a cell wall, they can rapidly change their shape and thereby move. It is not possible to image these bacteria sufficiently well through a conventional optical microscope due to their small size and their rapid movements. Using the newly developed light-scanning optical trap, however, the biophysicists were able to hold and orient the bacterium over its entire length.

One important attribute of laser light is that interfering light particles can increase or decrease their brightness. When light hits the bacterium and is deflected from it, this light superposes with the non-deflected light and thus is amplified, enabling the generation of three-dimensional images that do not only have a high contrast but also an increased resolution. In this way, it is possible to acquire up to 1,000 three-dimensional images per second and record the rapid movements of the bacterium in detail in a movie. “This is fascinating in terms of physics, because the movements of the bacteria are connected with extremely small changes in energy that are usually almost impossible to measure,” says Rohrbach, a member of the Cluster of Excellence BIOSS, the Centre for Biological Signalling Studies of the University of Freiburg. This makes the discovery into a practical tool for fundamental research. “Interesting from a biological point of view are the signals that the bacterium sends out when it changes its 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.”

In the future, the Freiburg scientists plan to use this method to study the behavior and cellular mechanics of other bacteria that do not have a cell wall and that are therefore difficult to treat with antibiotics. These studies could thus help scientists to reach a better understanding of bacterial infectious diseases.

Original Publication:
M. Koch, A. Rohrbach. Object-adapted optical trapping and shape-tracking of energy-switching helical bacteria. Nature Photonics, 2012.
Publication at


Prof. Dr. Alexander Rohrbach
Department of Microsystems Engineering (IMTEK)
University of Freiburg
Phone: 0761/203-7536

Click here for a printable version (pdf) of the press release.