[Paleopsych] Bacteria act as glue in nanomachines

[Steve Hovland]

Prachi Patel Predd
Electrodes snare microbes in key sites on silicon wafers.





Click here to see video of live bacteria trapped by electric currents.

? The Hamers Group   Media box

Electric currents are being used to move bacteria around silicon chips and 
trap them at specific locations. The technique could help to assemble 
nanomachines from miniature parts, and to create a new generation of 
biological sensors.

Nanodevices are typically built by connecting tiny components. But such a 
delicate task is not easy. So, many researchers are exploring ways to fix 
components in place using the binding properties of biological molecules, 
notably DNA.

Robert Hamers and his colleagues from University of Wisconsin-Madison 
propose using entire microbes instead. The cells have surface proteins that 
attach to certain biological molecules. Once the cells are placed at 
specific sites on a silicon wafer, nanoparticles tagged with these 
molecules can bind to the cells in those locations. This is easier than 
dragging the nanoparticles themselves to the right spot, because their high 
density makes them harder to move through fluid media than the less dense 
living cells.

The technique gives one a way to fix components such as quantum dots or 
carbon nanowires at very precise locations, explains Paul Cremer, a 
bioanalytical chemist at Texas A&M University in College Station. "That's 
potentially very exciting," he says.

Golden rods

The researchers use Bacillus mycoides, rod-shaped bacteria that are about 5 
micrometres long. They pass a solution containing the cells over a silicon 
wafer with gold electrodes on its surface. The charge on the electrodes 
captures the bacteria, which flow along the electrodes' edges like luggage 
on a conveyor belt.

The electrodes have tiny gaps between them. When a bacterium reaches a gap, 
it is trapped there by the electric field. It can be released by reducing 
the field between the electrodes, or permanently immobilized by increasing 
the voltage enough to break its cell wall.

 I think of it like catch-and-release fishing.

Robert Hamers
University of Wisconsin-Madison



Cells have been manipulated using electric currents before but it is 
typically done using larger cells, which are moved around as they are 
observed under a microscope. Hamers' work is unique because the locations 
of the bacteria are detected electrically.

When a cell bridges the gap between two electrodes, it acts like a wire and 
increases the current, signalling the bacterium's presence. Hamers 
presented the work on 17 March at a meeting of the American Chemical 
Society in San Diego.

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"I think of it like catch-and-release fishing," he says. "You can collect 
the cell, measure it and then if you want you can release the field and let 
it go again."

He believes that electrical detection will allow the method to be used on 
organisms that are too small to be seen with an optical microscope. It 
should also help the automation of nanoscale assembly. "You don't want to 
have to visually inspect every electrode to see what's happening," he says. 
"You could have a computer detect it electrically."

As well as providing the glue for miniature devices, the system could also 
be used to detect harmful biological agents such as anthrax spores or 
certain strains of Escherichia coli bacteria. The electrodes on the chip 
could be coated with biomolecules designed to bind to particular pathogens 
and hold them in place, and other pathogens would flow away when the 
electrode voltage was reduced below a certain threshold.