A bacterium locked away from the outside world for over four million years has been found to be resistant to most antibiotics – including those considered our "last resort". The bacterium, Paenibacillus, was discovered 1,000ft underground in the Lechuguilla Cave in New Mexico.

The cave – the deepest in the US and one of the longest in the world – is designated a UNESCO World Heritage Site. Because of its extreme fragility, the cave has been closed off since its discovery in 1986, bar a few scientific research expeditions.

Because there has been such little human activity, microbial life like bacteria has been preserved, allowing researchers to study how microbes have evolved – without external influencing factors.

Scientists from McMaster University and The University of Akron, Ohio, analysed Paenibacillus to better understand its antibiotic resistant properties. Writing in Nature Communications, they said: "In a previous study, we discovered that multi-drug resistance is common in bacteria isolated from Lechuguilla Cave, an underground ecosystem that has been isolated from the surface for over four million years.

"Here we use whole-genome sequencing, functional genomics and biochemical assays to reveal the intrinsic resistome of Paenibacillus, a cave bacterial isolate that is resistant to most clinically used antibiotics."

The team found the bacterium is resistant to 18 different antibiotics, including the last resort drug daptomycin. Its method of defence was found to be identical to bacteria found in soils – mutations that protect individual bacterial cells from the effects of antibiotics develop over time. It is these individuals that reproduce, eventually rendering the antibiotics useless.

We are currently poised to run out of effective antibiotics. This would plunge the world into a medical dark age. Basic operations would be extremely risky and cancer treatments would become as– if not more – deadly than the disease.

Understanding how resistance emerges is key to combatting problems of the near future. The discovery that Paenibacillus is resistant to most antibiotics – despite being locked away since before the dawn of man – means resistance genes have existed for millions of years, long before antibiotics were first used to treat diseases.

The findings showed five pathways of resistance of particular concern. Identifying them means researchers are better able to create drugs that develop resistance through these mechanism.

Study author Gerry Wright said: "The diversity of antibiotic resistance and its prevalence in microbes across the globe should be humbling to everyone who uses these life-saving drugs. It reflects the fact that we must understand that antibiotic use and resistance go hand in hand."