Antibiotic-resistant bacteria cut off from the outside world for more than four million years have been found in a deep cave.
The discovery is surprising because drug resistance is widely believed to be the result of too much treatment.
However, the resistant bugs from Lechuguilla Cave in New Mexico, in the US, have had no contact with humans.
They are thought to have picked up their resistance from natural anti-bacterial chemicals in the environment.
“Our study shows that antibiotic resistance is hard-wired into bacteria. It could be billions of years old, but we have only been trying to understand it for the last 70 years,” said Dr Gerry Wright, from McMaster University in Canada, who has analysed the microbes.
“This has important clinical implications. It suggests that there are far more antibiotics in the environment that could be found and used to treat currently untreatable infections.”
The research is published in the online journal Public Library of Science ONE.
Lechuguilla Cave, which is 1,604ft deep, is one of the largest and deepest unspoiled cave systems in the world. Access to the cave is limited to a handful of cavers and researchers each year.