Physicists in Germany have developed a novel technique for trapping biological cells with a laser beam. Using this technique, the researchers obtained super-resolution images of chromosomal DNA within E. coli cells.

“One of the problems facing biologists who want to examine biological cells microscopically is that any preparatory treatment will change the cells,” the researchers said.

“Many bacteria prefer to be able to swim freely in solution. Blood cells are similar: they are continuously in rapid flow, and do not remain on surfaces. Indeed, if they adhere to a surface, this changes their structure and they die.”

“Our new method enables us to take cells that cannot be anchored on surfaces and then use an optical trap to study them at a very high resolution,” added Professor Thomas Huser, head of the Biomolecular Photonics Research Group in the Faculty of Physics at the University of Bielefeld and corresponding author of an article about the research that was published in the journal Nature Communications on Dec. 13, 2016.

“The cells are held in place by a kind of optical tractor beam. The principle underlying this laser beam is similar to the concept to be found in the television series Star Trek.”

Prof. Huser added: “what’s special is that the samples are not only immobilized without a substrate but can also be turned and rotated. The laser beam functions as an extended hand for making microscopically small adjustments.”

He and co-authors have further developed the procedure for use in super-resolution fluorescence microscopy.

“This is considered to be a key technology in biology and biomedicine because it delivers the first way to study biological processes in living cells at a high scale — something that was previously only possible with electron microscopy,” they said.

To obtain images with such microscopes, biologists add fluorescent probes to the cells they wish to study, and these will then light up when a laser beam is directed towards them. A sensor can then be used to record this fluorescent radiation so that researchers can even obtain 3D images of the cells.

In their novel method, Prof. Huser and his colleagues use a second laser beam as an optical trap so that the cells float under the microscope and can be moved at will.

“The laser beam is very intensive but invisible to the naked eye because it uses infrared light,” said lead author Dr. Robin Diekmann, also from the University of Bielefeld.

“When this laser beam is directed towards a cell, forces develop within the cell that hold it within the focus of the beam.”

Using the new method, the physicists have succeeded in holding and rotating bacterial cells in such a way that they can obtain images of the cells from several sides.

Thanks to the rotation, they can study the three-dimensional structure of DNA at a resolution of circa 0.0001 mm.

The team wants to further modify the method so that it will enable biologists to observe the interplay between living cells.

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R. Diekmann et al. 2016. Nanoscopy of bacterial cells immobilized by holographic optical tweezers. Nat. Commun. 7, 13711; doi: 10.1038/ncomms13711