By LEE BOWMAN
Scripps Howard News Service
January 20, 2006
A new study suggests that the average shovel full of soil has bacteria with the potential to make the most potent antibiotic drugs worthless. But the microbes also may be key to finding new ways to avoid the growing problem of antibiotic-resistant germs.
Soil bacteria, and particularly strains called actinomycetes, living in close contact with many other bacteria, develop their own natural antibiotics to survive.
However, the other bacteria become resistant to those antibiotics in ways that are identical to resistance patterns found in bacteria that infect humans, according to scientists at McMaster University in Hamilton, Ontario.
"By evolving in an environment of antibiotic production, incredibly resilient bacteria must develop diverse ways to survive or resist the toxic antimicrobial compounds produced by their neighbors," said Gerry Wright, chairman of biochemistry and biomedical sciences at McMaster's school of medicine. He is senior author of the study published Friday in the journal Science.
"Their coping mechanisms may be able to give us a glimpse into the future of clinical resistance to antibiotics," Wright added, "and also guide the development of therapies to counteract this resistance."
After the discovery of penicillin in 1928 and streptomycin in 1943, pharmaceutical companies have been sifting through dirt to find new antimicrobial agents produced by bacteria. Approximately two-thirds of all known antibiotics are produced by actinomycetes.
But when Wright, doctoral candidate Vanessa D'Costa and colleagues screened 480 strains of soil bacteria isolated from urban, agricultural and forested land for resistance to 21 commonly prescribed antibiotics, they were stunned at the germs' ability to shrug off antimicrobials.
Not only were the bacteria resistant to an average of seven or eight antibiotics, but each strain was resistant to more than one drug.
The study "illuminates the dark side of the antibiotic paradigm," said Rockefeller University microbiologist Alexander Tomasz in a commentary also published in Science. "Microbes that synthesize the sophisticated chemicals that have been key to humankind's success in controlling bacterial disease also possess equally sophisticated mechanisms to protect themselves against their own toxic products."
The bacteria studied showed resistance to all major classes of antibiotics, regardless of whether the compounds had been produced naturally, were semi-synthetic or made up completely of synthetic chemicals.
Significantly, the researchers found that the way several bacteria were resistant to vancomycin, one of the most commonly prescribed medicines for drug-resistant staph infections, was identical to the resistance pathway found in hospitals and clinics.
"The link between clinical and soil-associated resistance to vancomycin illustrates the value of studying resistance in the soil to rationally anticipate future clinical resistance," Wright said.
"It suggests that the soil serves as an under-recognized source of resistance, resistance that has the potential to reach clinics," he added, although the connection between what happens with bacteria in dirt versus bacteria that colonize humans is not well understood.
The researchers also found bacteria that produced enzymes capable of breaking down or rendering inactive two drugs that were only recently approved by the U.S. Food and Drug Administration.
The drugs, telithromycin and tigecycline, have not been widely used in patients.
Wright said: "Studying enzymes that inactivate antibiotics can serve as a foundation for the development of new combination therapies for resistant bacterial strains. Studying antibiotic resistance from an evolutionary perspective is one way that researchers are attempting to stay one step ahead of resistant bacteria."
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