They did this experiment, Topp explains, because of previous work indicating that pesticides often break down more quickly in soils with a long history of exposure, indicating that pesticide-degrading microbes have been selected for over time.
Still, it came as a surprise when they saw antibiotics also degrading much faster in long-term, treated plots than in fresh, control soils, he says. In particular, sulfamethazinea member of the antibiotic class called sulfonamidesdisappeared up to five times faster.
The researchers subsequently cultured from the treated plots a new strain of Microbacterium, an actinomycete that uses sulfamethazine as a nitrogen and carbon source. Extremely common in soil, actinomycete bacteria are known to degrade a wide range of organic compounds. And now at least two other sulfanomide-degrading Microbacterium strains have been reported, Topp says: one from soil and another from a sewage treatment plant.
Taken together, the findings suggest that the capability to break down sulfanomides could be widespread. And if it's indeed true that "the microbiology in the environment is learning to break these drugs down more rapidly when exposed to them, this would effectively reduce the amount of time that the environment is exposed to these drugs and therefore possibly attenuate the impacts," Topp says.
Not that negative impacts aren't still occurring, he cautions. In particular, long-term exposure to antibiotics puts significant pressure on soil bacteria to evolve resistance, which they typically do by giving and receiving genes that let them detoxify drugs, or keep the compounds out of their cells.
What the new research suggests, though, is that soil bacteria could be swapping genes for breaking down antibiotics at the same time.
"My guess is that's probably what's happening, but it remains to be det
|Contact: Madeline Fisher|
American Society of Agronomy