Tractor Beam Catches Bacteria

Picture of the distribution of the genetic information in an Escherichia coli bacterial cell: Physicists at Bielefeld University are the first to photograph this distribution at the highest optical resolution without anchoring the cells on a glass substrate. (Source: Bielefeld Univ.)

Picture of the distribution of the genetic information in an Escherichia coli bacterial cell: Physicists at Bielefeld University are the first to photograph this distribution at the highest optical resolution without anchoring the cells on a glass substrate. (Source: Bielefeld U.)

Up to now, if scientists wanted to study blood cells, algae, or bacteria under the microscope, they had to mount these cells on a substrate such as a glass slide. Physicists at Bielefeld and Frank­furt Univer­sities have developed a method that traps bio­logical cells with a laser beam enabling them to study them at very high reso­lutions. Using this procedure, the physicists have obtained super­reso­lution images of the DNA in single bacteria.

One of the problems facing researchers who want to examine biological cells micro­scopically is that any pre­paratory treatment will change the cells. 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. The cells are held in place by a kind of optical tractor beam, says Thomas Huser. “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.”

The Bielefeld physicists have further developed the procedure for use in super­reso­lution 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. To obtain images with such micro­scopes, researchers 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 gain three-dimen­sional images of the cells.

In their new method, the Biele­feld researchers 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,” says Robin Diekmann, a member of the Biomo­lecular Pho­tonics Research Group. “When this laser beam is directed towards a cell, forces develop within the cell that hold it within the focus of the beam,” says Diekmann. Using their new method, the Bielefeld 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, the re­searchers can study the three-dimen­sional structure of the DNA at a reso­lution of circa 0,1 micro­meters. Huser and his team want to further modify the method so that it will enable them to observe the interplay between living cells. They would then be able to study, for example, how germs pene­trate cells. (Source: Bielefeld U.)

Reference: R. Diekmann et al.: Nanoscopy of bacterial cells immobilized by holographic optical tweezers, Nat. Comms. 7, 13711 (2016); DOI: 10.1038/ncomms13711

Link: Biomolecular Photonics, Dept. of Physics, University of Bielefeld, Bielefeld, Germany

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