Soil microbes are reservoir for antibiotic resistance.

Bacteria that live in dirt are surprisingly resistant to antibiotics, even those drugs they presumably have never encountered before, according to new research.

The majority of medical antibiotics originally came from soil bacteria, which produce chemicals to kill off other microbial species that compete for the same resources. To one-up the competition, many of these bacteria have evolved ways to detoxify antibiotics that neighboring species secrete.

Few of these soil bacteria are species that make people sick. However, notes Gerald Wright of McMaster University in Hamilton, Ontario, scientists had never examined the extent of antibiotic resistance in soil bacteria. Such knowledge might give researchers clues to how infection-causing microbes develop antibiotic resistance.

Wright and his colleagues first collected 480 strains of soil bacteria from a variety of locations, including urban, agricultural, and forest areas. They then treated cultures of the microbes with high concentrations of 21 commonly prescribed antibiotics. These included drugs that have been used for decades, such as penicillin and erythromycin, as well as drugs that were only recently invented and marketed, such as telithromycin and tigecycline.

Each of the bacteria was resistant to at least seven of the antibiotics. A couple of strains were resistant to 15 of the drugs, the researchers report in the Jan. 20 Science.

Wright notes that he and his team don't yet know whether soil bacteria can transfer their antibiotic-resisting capabilities to infectious bacteria. However, some of the strains that his team studied seem to have novel ways to fight the drugs with which they were treated. "If you can see how they get around these drugs, you might be able to identify what mechanisms could eventually evolve in [infectious] bacteria," he adds.--C.B.