Cereus bacteria go for the gold.

For centuries prospectors have relied on plants, dogs and even bees to home in on mineral deposits. Now a group of researchers at the U.S. Geological Survey (USGS) in Denver reports that a sporeforming bacterium, Bacillus cereus, has a particular liking for topsoils overlying deposits of gold, copper and other ores that may be buried several hundreds of feet deep. While microorganisms have been used in oil exploration, this is the first indication that a bacterium might be a useful guidepost for mining, say the scientists.

Geomicrobiologists John Watterson and Nancy Parduhn presented their findings in Denver last week at the USGS-sponsored McKelvey Forum on Mineral Resources. Watterson outlined research done over the last two and a half years at a big copper deposit in Montana in which the count of B. cereus can run up to 100,000 times that of surrounding soils. Parduhn, following up on these studies, described recent soil surveys near gold deposits in California, Colorado and Nevada. She too discovered that B. cereus bacteria living over mineralized bedrock outnumbered their counterparts in unmineralized terrain.

Except for a few brazen plants and microorganisms, B. cereus is uniquely adapted to these kinds of deposit areas. Most other bacteria wouldn't dare enter Watterson's copper deposit, for example, because they would be liked either by the copper or by the penicillin and other antibiotics produced by metal-tolerant fungi that live in the soil. But B. cereus, the researchers believe, has learned to survive by stealing a water molecule from each penicillin molecule, leaving a gap in the penicillin that traps a copper molecule before either can hurt the bacterium. In this way, the bacteria continually detoxify the copper molecules migrating away from the ore site. (Penicillin is commonly used to treat copper toxicity in humans.) Watterson has found that the penicillin resistance of B. cereus increases the closer one gets to the deposit. The bacteria are also unusual becausethey feed on the metal-tolerant fungi, unlike other members of their genus.

Parduhn and Watterson suspect that B. cereus may have devised other survival mechanisms at different mineral sites and that it may respond differently to different minerals, but more study is needed. It is possible, says Parduhn, that the microbe is really keyed to an element associated with gold, such as arsenic, and not so much to gold itself. The researchers also think that B. cereus is not the only microorganism that prefers metal-rich areas. "We may well find there are others," says Watterson. "We're just looking at the tip of the iceberg."

Because each B. cereus seems to be genetically coded to survive in its particular soil, Parduhn believes the microbes may provide a much more sensitive way of searching for mineral deposits than conventional geochemical techniques. The researchers have yet to do extensive testing of their microbe method in areas previously untested for minerla deposits. But Parduhn expects that in a couple of years they will be out of the research phase and into an exploration mode.