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Rewriting Life

A Safe and Simple Arsenic Detector

Researchers have built a bacteria-based device that sniffs out the toxic chemical.

Researchers at the University of Edinburgh, in Scotland, have genetically modified the bacteria E. coli to detect trace amounts of arsenic in drinking water. They hope that the bacteria-based technology will eventually lead to safe, precise, and easy-to-use field arsenic test kits. Chris French, professor of microbial biotechnology at Edinburgh, says that such tests could be as easy to use as home pregnancy tests and would not require a trained technician.

A student team from the University of Edinburgh has genetically engineered bacteria to detect arsenic in water. In combination with a drop of pH indicator (far right), samples turn red (middle) in the presence of arsenic and yellow in its absence.

Arsenic poisoning affects millions of people worldwide. One of the hardest hit countries is Bangladesh, where up to 35 million people get their drinking water from arsenic-contaminated wells, according to the World Health Organization. In recent years there has been a concerted effort to map the distribution of arsenic and flag contaminated wells. In some cases, scientists and aid workers have sent water samples to be tested in labs using fluorescence techniques–an expensive and time-consuming process. In other cases, they have used portable test kits. Most of these field tests, however, require training to operate and produce toxic chemicals, such as arsine gas.

“There’s definitely some drawbacks of existing field kits, and one is toxic disposal,” says Gregory Miller, an arsenic geochemist at Subsurface Technologies, an environmental remediation company in Socorro, NM. “An ideal detector should have a low detection limit, high precision, and no waste stream involved with it.”

The arsenic detector developed at Edinburgh is one of the first practical examples of a device built using the principles of synthetic biology. The aim of synthetic biology is, first, to identify the precise function of particular DNA sequences, or genetic parts. Some parts may control certain proteins. Others may act as switches for cellular processes. Then researchers mix and match to see how different parts fit together to form new genetic circuits, creating novel biological functions within living organisms, such as bacteria.

Drew Endy, assistant professor of biological engineering at MIT, likens synthetic biology to building with Lego. He has helped assemble a registry of standard genetic parts as a resource for scientists to borrow from, build on, and add to. Endy is also cofounder of iGEM, the annual International Genetically Engineered Machine competition, at which Edinburgh’s bacteria-based arsenic biosensor won the prize as best real-world application in 2006.

The Edinburgh group found that the bacterium E. coli possesses two seemingly unrelated genetic sequences that, in combination, form an effective arsenic-detection device. First, E. coli possesses a natural arsenic detoxification system that is switched on only in the presence of arsenic. The bacterium also naturally breaks down lactose to produce acid. The researchers isolated the arsenic-switch gene and attached it to the first gene involved in the breakdown of lactose.

They hypothesized that when arsenic comes in contact with this modified E. coli, it will activate the arsenic switch, whose gene in turn will activate the breakdown of lactic acid. French says that a simple color litmus test can then pick up the resulting change in pH levels, also indicating the presence of arsenic. “It works under laboratory conditions, but we haven’t tested it with real water samples,” he says.

So far, the team has worked with relatively clean lab-controlled solutions of water and inorganic arsenic mixed with modified E. coli and a pH indicator in a sealed glass tube. After a while, the water turns red or yellow, depending on the presence or absence of arsenic. A potential drawback is the test’s lag time: it takes about five hours for the biological interactions to complete.

Ultimately, the group hopes to design a cheap and simple kit of powder–freeze-dried bacteria and pH indicator–in a disposable, screw-top plastic tube. In the near future, French’s group may test its design on actual water samples from Bangladesh that contain other biological and chemical materials. For now, the researchers have added their device to Endy’s “bio-bricks” registry for other scientists who might want to build a better arsenic biosensor.

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