December 6, 2012
Antibiotic-Eating Bug Unearthed
Newly Discovered Bacterium Degrades an
Antibiotic Both to Protect Itself and
It's well known how bacteria exposed to antibiotics for long periods
will find ways to resist the drugs - by quickly pumping them out of
their cells, for instance, or modifying the compounds so they're no
Now new research has uncovered another possible mechanism of
antibiotic "resistance" in soil.
paper published on Dec. 6 in the Journal of
Environmental Quality, a group of Canadian and French
scientists report on a soil bacterium that breaks down the common
veterinary antibiotic, sulfamethazine, and uses it for growth.
Certain soil bacteria are already known to live off, or "eat,"
agricultural pesticides and herbicides, says the study's leader,
Edward Topp, a soil microbiologist with Agriculture and Agri-Food
Canada in London, Ontario. In fact, the microbes' presence in farm
fields can cause these agrichemicals to fail.
But to Topp's knowledge, this is the first report of a soil
microorganism that degrades an antibiotic both to protect itself and
"I think it's kind of a game changer
in terms of how we think about our environment and antibiotic
resistance," he says.
Concerns about widespread antibiotic
resistance are what led Topp and his collaborators to set up an
experiment 14 years ago, in which they dosed soils annually with
environmentally relevant concentrations of three veterinary
Commonly fed to pigs and other
livestock, antibiotics are thought to keep animals healthier.
But they're also excreted in manure,
which is then spread once a year as fertilizer in countless North
American farm fields.
The researchers first wanted to know whether these yearly
applications were promoting higher levels of antibiotic resistance
in soil bacteria. But a few years ago, they also decided to compare
the persistence of the drugs in soil plots that had been repeatedly
dosed, versus fresh soils where antibiotics were never applied.
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, sulfamethazine - a member of
the antibiotic class called sulfonamides - disappeared up to five
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
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 determined," Topp says. "It's
actually extremely fascinating."
The work was funded by Agriculture and
Topp can be reached at 519-457-1470 e.235; firstname.lastname@example.org
Biodegradation of Veterinary Antibiotics in Agricultural Soil
- Following Long-Term
Exposure, and Isolation of a Sulfamethazine-degrading Microbacterium