Durable strains provide clues about why some are resistant to antibiotics.
No place on Earth demonstrates the resilience or inventiveness of life quite like Lechuguilla Cave, whose subterranean tunnels stretch for 130 miles through Carlsbad Caverns National Park in New Mexico.
Deep in the cave, deprived of sunlight and mostly starved of life-giving water, a lush garden of bacteria grows. Untouched by humans for all of their 4 million years, these strains of bacteria thrive on the harsh minerals of the geological formations to which they cling and fend off other life forms that would prey on them.
New research suggests it could tell us volumes about the medicines doctors rely upon to combat infection and why, increasingly, they are failing.
Scientists who collected 93 strains of bacteria from the forbidding depths found that all were resistant to at least one of modern antibiotic used to fight bacterial infections and some were resistant to at least 14. In addition, virtually all of the 26 antibiotics tested proved useless in killing at least one of the strains of bacteria collected.
That these life forms evolved in ways that appear to anticipate medicines attests to bacteria's remarkable powers of survival. It also suggests that the rise in antibiotic-resistant diseases isn't due entirely to the runaway use of these drugs; rather, bacteria are programmed to endure.
"It's awe-inspiring," said Gerard Wright, a microbiologist from the Institute for Infectious Disease Research at McMaster University in Canada and senior author of the study, which was published online by the journal PLoS ONE. "It gives you real respect for the genetic diversity and the ability of these organisms to evade toxic molecules."
Scientists have long believed that the ability of disease-causing bacteria to outwit antibiotic medicines was a man-made phenomenon, said Eileen Choffnes, director of the Institute of Medicine's forum on microbial threats. The growing use of antibiotics was thought to have spurred adaptations that made many of these bacterial pathogens less vulnerable to modern drugs. But this research demonstrates that antibiotic resistance emerged millions of years before those medicines were used, she said.
Researchers have harvested bacteria on the Earth's surface that was thousands of years old. "This pushes it way back," said Dr. Brad Spellberg, an infectious disease researcher at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center.
For the study, about 500 strains were brought to the surface by Hazel Barton, a spelunker and microbiologist at the University of Akron in Ohio. She burrowed into areas of the cave where the National Park Service could ensure that no more than six humans had ever been near.
Then, 93 strains that survived and were chosen for evaluation were subjected to 26 antimicrobial agents, ranging from natural products such as vancomycin to synthetic agents such as ciprofloxacin and linezolid. In one group, 70 percent of the samples were resistant to three to four classes of antibiotics, on average.
Tetracycline antibiotics were effective against all of the samples. But sulfamethoxazole, trimethoprim and fosfomycin were not. Three ancient strains of bacteria proved resistant to daptomycin, the newest class of antibiotic approved.
Barton said that the discovery that ancient bacteria were immune to so many modern medicines was "a eureka moment." "As a scientist, that's what you live for," she said. "It's the discovery of the unexpected that keeps you going back for more."