Published online 18 March 2010 | Nature | doi:10.1038/news.2010.134

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It's a wrap for bacteria

Atomically thin carbon sheets offer bacteria a protective shell in electron microscopes.

bacteria wrapped in grapheneBacteria wrapped in graphene sheets could be easier to image.V. Berry

A team of scientists has wrapped bacteria in one-atom thick sheets of carbon known as graphene. The carbon cloak could one day help researchers to image tiny cells at higher resolution than is currently possible, according to preliminary results presented on Monday at the meeting of the American Physical Society in Portland, Oregon.

Most images of cells and organisms are taken using light, but light can only achieve so much. A fundamental 'diffraction limit' means that images made with light cannot incorporate details that are less than a few hundred nanometres across, much larger than many features scientists want to study. One way to achieve a better result is with electrons — by shining a beam of the charged particles through a sample, scientists can achieve atomic-resolution images of materials.

But this technique, known as transmission electron microscopy, comes at a price. To be imaged, samples must be placed in a vacuum, where they are exposed to fast-moving electrons. This is problematic for living specimens. "Vacuum and electrons are very hazardous to bacteria," says Christian Kisielowski, an electron microscopist at the Lawrence Berkeley National Laboratory in California. The former can cause them to dry out and die, whereas the latter rips apart fragile hydrogen bonds that help to hold together molecules inside the microbes.

Electron protection

Graphene may offer some protection. Vikas Berry, a chemical engineer at Kansas State University in Manhattan, presented unpublished results in a 15 March session. He and his co-workers began with graphene flakes, to which they applied a common lectin protein that would cause the flakes to bond to certain kinds of bacteria known as Gram-positive bacteria. The group then put the treated graphene into a beaker filled with two types of Gram-positive bacteria: Bacillus cereus and Bacillus subtilis. Within seconds, the tiny sheets wrapped themselves around the organisms. "It's really like swaddling a baby," Berry says.

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The researchers then put their shrink-wrapped bacteria under the transmission electron microscope. Preliminary results indicated that the bacteria were doing well, especially when compared with unprotected controls. Because graphene is an electrical conductor, the electrons could penetrate the shell, but Berry believes it also offered protection. Graphene is an excellent thermal conductor, and Berry suggests that it might be shunting blistering heat away from the bacteria. What's more, it seems to seal the bacteria off from the corrosive environment of the vacuum. There's even a possibility that live bacteria could be imaged, he says. "If you can keep bacteria alive in the vacuum, that would be really awesome."

"It would be a breakthrough," agrees Kisielowski. But he is quick to add that Berry and his co-workers still have a long way to go before they can prove that the graphene is offering protection. Even if the bacteria seem to be doing well, there's still a chance that the blistering electron beam is boiling off hydrogen. More systematic studies using graphene and special hydrogen-rich test crystals will be needed to show that samples really are protected, he says.

Berry admits to seeing some evidence of small amounts of hydrogen leaking from the bacteria, but he still believes they fare far better than they otherwise would. He now plans to increase the energy of his microscope's electron beam to see how far that protection extends. 

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