Scientists who collected 93 strains of bacteria from the forbidding depths of Lechuguilla found that all were resistant to at least one of the antibiotics that modern medicine uses to fight bacterial infections and some were resistant to at least 14. In addition, virtually all of the 26 antibiotics tested as part of the study 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, try as you might to kill them, bacteria are programmed to endure.

The findings make it clear that humans will always have to contend with the problem of antibiotic resistance, no matter what steps are taken to prevent it, said Dr. Brad Spellberg, an infectious disease researcher at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center: "There's never going to be a point where we can say, 'OK, we're up, we're ahead, they're done.' "

Researchers have harvested bacteria on the Earth's surface that was thousands of years old and reported last year that some of them were resistant to antibiotics. That might have been due to exposure to natural antibiotic threats in their environment.

"This pushes it way back," Spellberg said.

Barton's samples were kept alive in lab dishes that approximated their nutrient-poor origins. The 93 strains that survived and were chosen for evaluation were subjected to 26 antimicrobial agents, ranging from natural products such as vancomycin to completely synthetic agents such as ciprofloxacin and linezolid.

In one group of bacterial strains — the "gram-positive" strains — 70% of the samples were resistant to between three and four classes of antibiotics, on average. The same was true of 65% of the "gram-negative" strains.

Tetracycline antibiotics were effective against all of the bacterial samples. But sulfamethoxazole, trimethoprim and fosfomycin were not. Three ancient strains of bacteria in the Streptomyces genus proved resistant to daptomycin, the newest class of antibiotic approved for clinical use.

Spellberg said the findings underscore the need for measures like the one taken Wednesday by the FDA. In 2010, the agency said that nearly 29 million pounds of antibiotic agents were fed to the nation's livestock each year. That practice accelerates bacteria's adaptation to the drugs, he said.