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  • Gene test reveals arsenic-releasing microbes in water


Scientists have developed a test to detect a bacterial gene that can trigger the release of arsenic into drinking water — a problem that threatens the health of tens of millions of people in South Asia.

"This is the first test ever developed to detect this activity in the environment," says Dianne Newman, whose team published their research today (15 October) in Science.

The test is based on a gene — called arrA that enables bacteria called Shewanella to acquire energy in a way that ultimately leads to the release of arsenic into water from the surrounding earth. In doing so, the bacteria convert a harmless form of arsenic into a toxic one, which is then released into the environment (see 'Microbes to blame' for arsenic threat to millions).

In Bangladesh, about a quarter of a million people will die from cancer because of drinking arsenic-contaminated water, according to figures from the World Health Organisation.

Newman's team created a 'primer', a chemical designed to match and bind to the arrA gene. Using this primer in a technique called polymerase chain reaction (PCR), which creates many copies of small sections of genetic material, the researchers were able to see whether 13 species of bacteria that also cause the release of arsenic also had the same gene. Twelve of them did.

Although the bacteria were not closely related, and had been taken from a range of locations — including an Australian gold mine and lakes in the United States — the arrA gene was remarkably similar in each species.

According to Newman, the same bacteria are probably in Bangladesh "although we don't know because we don't have access to those waters and sediments".

"Based on everything we've seen, however, these bacteria are quite ubiquitous, so I wouldn't be at all surprised if they were there," says Newman. "In fact, I'd be surprised if they weren't there."

The test could be used to identify areas where arsenic-releasing bacteria are present or active. It will allow researchers to study the conditions that regulate whether the gene is active, enabling them to predict when the arsenic might move from earth to water.

"My hope is that our test will be useful to people in developing countries," Newman told SciDev.Net. "If other people wanted to use this method, all they'd have to do would be to perform PCR on their sample."

"The entire process, including the extraction of the genetic material, can be done during a single day's work," says Davin Malasarn, lead author of the paper in Science.

Just as some bacteria can cause the release of the toxic form of arsenic, another species — found in Australia — performs the reverse reaction. Applying this knowledge could lead to the use of bacteria to clean up contaminated water, a process known as bioremediation.

Joanne Santini from Australia's La Trobe University, who is a co-author of today's Science paper, told SciDev.Net that the researchers are hoping to sequence the entire genome — all of the genetic material — of each type of microbe.

"This information will allow us to determine what roles these organisms play in the cycling of arsenic in the environment, and how we could use these organisms for bioremediation," says Santini.

According to Jon Lloyd of the University of Manchester, who has researched the role of bacteria in arsenic contamination in Bangladesh, the development of a molecular tool to monitor the release of arsenic due to bacterial activity is "a very useful step towards clarifying the mechanism of arsenic mobilisation in contaminated aquifers".

"This work potentially gives us a robust biological approach to augment the geochemical and mineralogical techniques that the scientific community has traditionally relied on to help identify the causes of this catastrophic problem," Lloyd told SciDev.Net.

"If this study plays a role in finally determining whether microbes are the cause of arsenic poisoning worldwide, we will be one step closer to identifying ways of controlling this environmental disaster," he says.

Although Bangladesh is the country most badly affected by arsenic in drinking water, arsenic contamination is also a problem elsewhere in South Asia, and in South-East Asia.

Last week, India's Centre for Science and Ecology published a report in Down to Earth magazine claiming to show evidence of high concentrations of arsenic in groundwater in the Indian state of Uttar Pradesh. The state authorities are disputing CSE's claim, and have threatened legal action unless the claim is retracted.

Link to full paper by Malasarn et al in Science

Link to editorial in Down to Earth magazine

Science 306, 455 (2004)

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