Shape-shifting microbes: Optically trapped

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  • Published: Oct 15, 2012
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Shape-shifting microbes: Optically trapped

Grabbed by the lasers

Researchers in Germany have developed an optical trap for bacteria. The system allows them to grab hold of the microbes and scan them with a laser. Rohrbach et al/Nature Photonics 

Researchers in Germany have developed an optical trap for fast-moving bacteria. The system allows them to grab hold of the microbes and scan them with a laser.

Alexander Rohrbach and Matthias Koch in the Department of Microsystems Engineering (IMTEK) of the University of Freiburg created a tube with light that can confine even the most agile of unicellular organisms, this takes the concept of optical tweezers two a second dimension. Previously, optical traps have been used to hold atoms, mesoscopic objects such as beads and organelles, cells and other objects up to a tenth of a millimetre in size but with only one point trap at the object's edges. The new technique also now allows them to manipulate the microbe's orientation and scan its shape. A fast moving, focused laser beam lets them exert an equally distributed force over the constantly changing form of the entire helical bacterium. Measurements of deflected light also facilitated high-speed three-dimensional imaging. The team reports more details in the journal Nature Photonics.

Pictured: A fast-moving laser beam holds Spiroplasma in place and records its movements in detail. The conventional optical micrograph in the background shows it was previously only possible to reveal a bare outline.

The team looked at Spiroplasma melliferum, this corkscrew-shaped bacterium is just 200 nanometres in diameter and lacks a stiff cell wall. It moves by spiralling and kinking through the medium in which it finds itself. The lack of inflexible cell wall allows it to rapidly change shape as it does so. Conventional optical microscopy is limited in imaging such microbes because of their rapid movements and small size. "You can image them [with specialist techniques] but because of the limited depth of field bacteria rapidly swim out of focus or have to be squeezed between two cover slips," Rohrbach tols SpectroscopyNOW. "Our trap can hold them in the imaging plane and, at the same time, record images with a very high temporal resolution and spatial precision."

The team points out that the properties of scattered laser light allow them to record the microbe's shape deformations space-resolved at 800 Hz by exploiting local phase differences in coherently scattered trapping light. "By localizing each slope of the bacterium we generate high-contrast, super-resolution movies in three dimensions, without any object staining," they explain. "This approach will help in investigating the nanomechanics of single wall-less bacteria while reacting to external stimuli on a broad temporal bandwidth."

"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. The tool could thus become a practical tool for basic research into bacterial infectious diseases. "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," he adds.

The team hopes to study the behaviour and cellular mechanics of other bacteria that similarly lack a stiff cell wall. Such species are difficult to treat with antibiotics as many of those pharmaceuticals target receptors and enzymes in the cell wall of other microbes or else inhibit the biomolecular construction of the cell wall itself.

Bringing coherence to infection studies

"Our coherent scanning technique results in three-dimensional optical images with a resolution that is clearly beyond conventional optical resolution in all three directions, revealing much more contrast than incoherent point-scanning techniques based on fluorescence principles," the team concludes. The work could lead to a better understanding of the spread of many forms of bacterial infection as well as offering new hope of developing treatments for such infections.

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Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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