Engineered E. coli bacteria can now trace the outline of an image on an agar plate in a feat that shows how manipulating small organisms could lead to synthetic biological devices useful to technology and medicine.
"It looks like a pen came in and traced the outline of the image," said Jeff Tabor, a scientist at the University of California, San Francisco who helped genetically engineer the E. coli bacteria.
Tabor says getting bacteria to trace images was "significantly more complicated" than their original project, which was to create black and white photograph-like images with bacteria for the annual iGEM competition at MIT.
The complexity of this new task could pave the way to new, sophisticated chemical and environmental sensors.
Creating an image with bacteria is relatively simple. Genes that respond to the absence of light are injected into the E. coli. When they don't detect light, they produce a black pigment. If the bacteria do sense light they remain translucent. The human eye detects light in a similar manner, responding not to the light itself, but to the absence of light.
Using this technique, Tabor and his colleagues at UCSF and the University of Texas, Austen created ghostly pictures of squid and people in 2005. The images were a very high resolution, with each bacteria representing one pixel.
Tracing the edge of an image is more complicated.
Instead of responding to a physical signal — the lack of light — the bacteria also have to respond to a chemical cue from surrounding bacteria.
Using viral vectors, the scientists injected genes into the bacteria that, when triggered by a beam of light, causes light sensitive bacteria to emit a chemical that turns the surrounding, non-lightened bacteria black.
The interaction of the two signals creates a pencil-thin line through a plate of agar.
This means only cells right on the boundary of light and dark, can create the dark pigment to trace the image.
"If you are in the dark you can make the signal but can't listen to it," said Tabor, "And if you are in the light you can't make the signal but you can hear it."
The research is a good example of how a bacterial computer could operate. Each E. coli cell detects light or detects the chemical cue at the same time and processes the information simultaneously to create an edge. If the bacteria were a computer, this would be called parallel processing.
A traditional computer traces the outline of an image using serial computing, making one calculation at a time. Each pixel of the image is compared with its eight neighbors, one at a time, until the entire image is scanned. The more pixels in the image, the longer the calculations take to compute.
Every E. coli, by contrast, compares itself to its eight neighbors constantly, performing millions of calculations simultaneously. The size of the image is irrelevant; postage stamp or poster size, an outline will appear at the same time.
That doesn't mean the process is faster than a computer for most images. Due to the time E. coli requires to react to signals, it takes the bacteria about 12 hours to produce an outline.
And don't expect to drop off film and get back plate of E. coli at the local film developer, says Jim Collins, a synthetic biologist at Boston University. For one, each picture would require its own bag and storage.
For Collins the work is a great example of synthetic biology's potential.
"This is an outstanding example of advanced synthetic biology," said Collins. "They had to pull multiple systems together from multiple organisms to form a complicated network that was driven by an application."
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Chemical or environmental sensors would be more useful applications than an album of petri dishes, says Collins. Bacteria that can detect the presence and amount of light or other chemicals could be placed in mines or other contaminated areas to monitor the extent of the problem.
For the moment, however, Tabor says there are no commercial applications from the research. It is more proof of concept than anything else. The next step in their research is to engineer bacteria to reproduce color images.
Using E. coli programmed to produce three red, green and blue pigments, the scientists think they should be able to produce every color in the rainbow.