It's an entrenched piece of pop-science wisdom: Overuse of antibiotics in medicine is the reason bacteria evolve into antibiotic-resistant superbugs. But deep inside four-million-year-old Lechugilla cave in southern New Mexico, a population of isolated bacteria are calling that notion into serious question.

More than 1,600 feet below the earth's surface, in the 130-mile-long cave draped with lacy gypsum "chandeliers" and fingers of delicate white stalactites, mats of bacteria cling to the passage walls. Between seven and four million years ago, deep groundwater ascended and formed the cave, without any influence from surface conditions. Since Lechugilla cave was discovered in 1986, access has been restricted to researchers, maintaining its pristine condition.

Recently, scientists at McMaster University in Ontario, Canada, and the University of Akron in Ohio tested 93 different strains of bacteria harvested from Lechugilla by exposing them to a variety of antibiotics. The results were surprising: More than 60 percent of the Lechugilla strains were unfazed by several different classes of antibiotics, including synthetic drugs and some of the newest antibiotics only recently approved for clinical use. The outcome proves that antibiotic resistance doesn't spring up only when doctors unleash the drugs on disease-causing bacteria. Instead, some bacteria have natural resistance to antibiotics that far predates modern medicine.

The discovery is indeed a breakthrough: Bacteria from other regions isolated from modern antibiotics, such as the Galapagos Islands, have shown in previous studies to be incapable of combating garden-variety Amoxicillin.

The researchers hypothesize that the Lechugilla bacteria evolved this resistance in order to compete with other bacteria in the cave. Competition for the nutrients necessary to sustain life is fierce in the isolated cave, so the bacteria there evolved sophisticated ways to give themselves a leg up.

"You're living under complete starvation when you're in [Lechugilla]," says Hazel Barton, a microbiologist with the University of Akron who conducted the research. "So you can either be very good at scavenging, or you can cheat and make these chemical weapons that you would lob or spit at a neighboring bacteria, kill it, and then steal its resources."

Those chemical weapons include some compounds that may have cancer-fighting capabilities, Vanderbilt University chemist Brian Bachmann says. He's working with some of the bacteria's toxic chemical products, which have the potential to be more effective than current therapies in killing cancer cells.

"They would kill the cancer cells faster than they may kill the non-cancerous cells," Bachmann says. "That's how most cancer chemotherapy compounds work—they just kill the cancer faster than you."

Bachmann's next step will likely be to send samples to the National Cancer Institute, where they can face off against the wide array of cancer cells in the institute's collection. Many current anticancer therapies are derived from products made by the cave bacteria's distant, above-ground relatives; the possibilities for using Lechugilla bacteria are just beginning to be explored.

"We don't know why the cave organisms are producing these compounds," Bachmann says. "Presumably, they're using them to help survive in the hypercompetitive environment of the cave. That's a big mystery."

The Lechugilla bacteria don't present a threat to human health. But although the new discovery shows that human interference isn't the sole cause of antibiotic resistance in bacteria, the findings call for more medical caution, not less. Since bacteria have the innate ability to defend themselves against antibiotics, it's even more important to avoid overexposing them to the antibiotic arsenal.

But the discovery also hints at the kinds of antibiotic resistance bacteria may develop in the future, Barton says.

"We've seen a kind of resistance that hasn't developed in the clinic yet…," Barton says. "So it gives us a lead time of maybe 20 years to say, 'Well, this is coming in the future. How can we head it off right now?' That gives us the potential for developing drugs to outpace the bacteria so we can actually develop something in advance."