Researchers create a tiny tractor beam that totes bacteria around

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One of the problems with imaging living biological cells is that they don’t want to be held still. Or, more accurately, they don’t want to be held to a surface like a microscope slide. Prepping and fixing the cells changes them irrevocably, altering whatever a scientist was trying to observe. Red blood cells are like this: try to stick them fast to a cover slip and their form changes, killing them before anyone gets quality images out of the scope. Lots of bacteria are like this, too. But what if they didn’t have to be stuck against a surface at all? What if they could be held quite still without a physical touch?

“The principle underlying this laser beam is similar to the concept to be found in the television series Star Trek,” says Dr. Thomas Huser, head of the research group.

Continuing to build on the same underlying concepts as the optical tweezers from earlier this fall, physicists from the University of Bielefeld have further developed that procedure for use in superresolution fluorescence microscopy. The idea is that this new method can get fluorescence images of living cells with resolution we could heretofore only get with electron microscopy, which you can only use on nonliving cells. Consequently, even living cells that resent being affixed to a plate can now be imaged with higher resolution.

Distribution of the genetic material inside these rod-shaped E. coli bacteria. Image: Diekmann, Huser et al, 2016

To obtain images with such microscopes, researchers add fluorescent probes to the cells they wish to study, which then light up when a laser beam is directed at them. The experimental setup then uses sensors to record this fluorescence, so that researchers can even get three-dimensional images of the cells.

In their new method, the Bielefeld researchers use a second laser beam as a single-cell optical trap — in other words, a tiny tractor beam — so that the cells float under the microscope and can be moved at will. “When this laser beam is directed towards a cell, forces develop within the cell that hold it within the focus of the beam,” says Robin Diekmann, coauthor. What forces? Depending on the power of the laser, the tractor beam has a quality called “stiffness,” against which cells experience a restoring force.

Using their new method, the physicists succeeded in holding and rotating first a polystyrene bead and then individual bacterial cells in midair, on a plane just a few micrometers above the slide, in such a way that they could take images of the cells from several sides and focus the image throughout the whole depth of the cells. With the ability to rotate the cells in 3-space, the researchers can study the three-dimensional structure of the cells’ genetic material, at micron resolutions.

Abbreviations: AOTF: acousto-optical tunable filter, FT: focusing telescope, TM: translatable mirror, DCM: dichroic mirror, NA: numerical aperture, TIR: total internal reflection, ND: neutral density filter wheel, PMF: polarization-maintaining fiber, 4f-T: 4f-telescope, M: mirror, BP: band-pass filter, SP: short-pass filter. Image and description: Diekmann, Huser et al, 2016

Diekmann and colleagues’ next steps will be to deploy the combination of fluorescence microscopy and optical tweezers to study cells through different parts of their life cycles, including watching bacteria and other cells as they’re infected by other pathogens. No word on how long it’ll be before we have a tractor beam that can scale up for starships.



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