Radiation-resistant bacteria that can dispose of heavy metals are being developed to help clean up soil and water contaminated by toxic radioactive waste. Researchers have constructed a version of the highly radiation-resistant bacterium Deinococcus radiodurans that converts the toxic mercury in such wastes to a less toxic form.

The United States has a major pollution problem with waste from its nuclear weapons manufacturing program. From 1945 until the 1980s, around three million cubic metres of radioactive waste were disposed of by burying it in the ground. Leakage of the buried waste at the 3,000 disposal sites has contaminated surrounding soil and groundwater with radioactive uranium-235, heavy metals like mercury and toxic organic solvents such as toluene.

This lethal mix now affects 75 million cubic meters of soil and two billion liters of groundwater, and the cost of cleaning it up using purely physico-chemical technology is estimated at around $265 billion. So the search is on for less expensive ways of detoxifying the contaminated ground. One of these could be bioremediation. This is the use of living organisms, usually bacteria or plants, which can break down the pollutants to use as nutrients, or convert them into some less harmful form.

The problem with radioactive waste, however, is that most living organisms are highly sensitive to radiation, which kills cells and damages DNA. Enter Deinococcus, the most radiation-resistant organism known to man. This astounding bacterium grows happily in levels of radioactivity of 60 grays (Gy) per hour -- about ten times the lethal dose for a human -- that is well above those in the waste. It is also quite tolerant of organic solvents.

But Deinococcus does not like mercury, and this, radioactivity apart, is one of the most problematic of the pollutants. In the radioactive waste, mercury occurs in a highly toxic ionic form, 'Hg(II)'. Researchers have genetically engineered a suite of mercury-handling genes into Deinococcus, which enable it to convert the Hg(II) into less toxic elemental mercury (Hg), which is volatile and so can disperse, and is also chemically almost inert.

The genes come from the biotechnologist's friend, the bacterium Escherichia coli, which perhaps surprisingly for a bacterium that lives in the hospitable environment of the human gut can detoxify ionic mercury. The Bethesda team found that they could combine the mercury-detoxifying genes with specialized toluene-degrading genes taken from the harmless soil bacterium Pseudomonas putida.

The end result is a strain of Deinococcus that converts mercury, breaks down toluene to use as a source of carbon and energy, and does all this while thriving at radiation levels that no other organism can resist.

Nature Biotechnology 18, 85 - 90 (2000)