Bacteria lurking in soil in the 1960s and 70s resist an antibiotic that didn’t exist until decades later. Three strains of what amount to future-predicting bacteria showed extreme resistance to six common antibiotics, including ciprofloxacin, which was first sold in 1989.

“You can pretty safely say that there is no way these bacteria have seen them before,” says Cristiane San Miguel, a microbiologist at Rutgers University in New Brunswick, New Jersey, US. She presented the findings this week at the American Society for Microbiology‘s annual meeting in Boston, US.

One strain of soil bacteria was even able to fend off a dose of ciprofloxacin that would be lethal to humans.

Dirt seems to be a rich source of antibiotic-resistant bacteria, which probably developed such defences as part of the evolutionary arms race that has been going on for billions of years between soil-dwelling microbes.


Many antibiotics drugs come from naturally occurring molecules produced by soil bacteria and fungi, though some drugs, such as Cipro (the brand name of ciprofloxacin), have been developed in the lab.

Bacteria to the future

To determine whether resistance to new drugs can be found in soil, San Miguel and her colleague Robert Tate turned to a company that stocks thousands of strains of frozen bacteria.

Her team revived three strains: two of them opportunistic pathogens called Klebsiella pneuomoniae that were isolated from dirt in 1973 and 1974, then frozen; the third, a bug called Alcaligenes, last tasted agar in 1963.

All the strains flourished when San Miguel exposed them to a range of antibiotics, many still used to battle infections.

Perplexingly, all the bacteria fended off a lethal dose of rifampicin, an antibiotic introduced in 1967, and Cipro, a 19-year-old drug that resembles nothing seen in nature. “I was certainly expecting the Cipro to have an impact and it did not,” she says.

Ancient genes

Next San Miguel plans to determine the genes responsible for the resistance.

Gerry Wright, a microbiologist at McMaster University in Hamilton, Ontario, Canada, says soil bacteria are probably a trove of antibiotic resistance that finds its way to human pathogens.

“The origins of many of the antibiotic resistance genes that are floating around in the clinic are out in the environment and have probably been out there for thousands and millions of years,” he says.

Bacteria needn’t be exposed to a specific antibiotic to develop resistance, and he suggests that natural variation or prior exposure to undiscovered Cipro-like molecules could explain the bacteria’s retroactive resistance.

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