The world's most potent antibiotics are just fodder for most soil microbes, which use these drugs to fortify themselves, according to a study by researchers at Harvard University, Boston, Massachusetts.
A study of soil microbes taken from 11 sites found bacteria that could resist antibiotics 50 times stronger than the standard for bacterial resistance.
The researchers say that while the "ultra-bugs" don't normally cause disease, the bacteria might pass drug resistance onto their deadly kin.
While hunting for soil bacteria that can turn plant waste to biofuels, a team of microbiologists led by George Church of Harvard University, Boston, Massachusetts, decided to grow soil samples in pure antibiotics as a control.
"We expected not to find a lot of bacteria that could eat antibiotics for breakfast. We were kind of surprised," New Scientist magazine quoted Church, as saying.
For the study, the team collected more dirt from farms, forests and parks around the northeast United States and Minnesota. All the soil samples contained bacteria that can live on antibiotics, and many subsisted on multiple drugs.
The researchers found that not only could the soil bacteria survive on older antibiotics that many bacteria have developed resistance to, such as penicillin, but they could digest modern-day silver bullets as well, including ciprofloxacin.
Moreover, many of the bacteria were found to be resistant to the bulk of antibiotics, though they often could not grow without alternative food sources.
"They are resistant to virtually all antibiotics," said microbiologist Morten Sommer, also at Harvard.
The team found that among 75 strains they tested, half were resistant to clinical doses of 17 of 18 antibiotics.
Sommer said that the trait is particularly worrisome. The bacteria were not known to attack humans, but some were close relatives, such as members of the Burkholderia cepacia complex, a group of bacteria that infect people with cystic fibrosis, and Serratia marcescens, which can cause blood infections in people with compromised immune systems.
Church said the finding underscores the extent to which bacteria have developed resistance to antibiotics, a process that started almost as soon as penicillin was introduced in the 1940s. Overuse and misuse of antibiotics have since fuelled the rise of drug-resistant superbugs.
"This is yet another way of looking at resistance," Church said of the study.
He said the microbes he found may be using a new way to disarm the antibiotics, but it may take some time to figure that out.
The study is published in the journal Science.