BUFFALO, N.Y. -- Besides producing the raw material that ends up
as the roast beef or ham on your dinner table, livestock farms also
are big producers of manure. Farmers get rid of manure in an
environmentally responsible way, by turning it into fertilizer for
their fields or those of other farmers.
But deep in those piles of dung lie not just beneficial, organic
matter, but the residues of antibiotics used to promote growth in
livestock and to treat their diseases.
How much of these antibiotics ends up in the environment, and
thus could potentially alter microbial ecosystems in humans,
animals and the environment is the focus of research being
conducted by Diana Aga, Ph.D., assistant professor of chemistry in
the University at Buffalo College of Arts and Sciences.
"A lot of research is done to study how antibiotics used in
human medicine result in the development of resistance in
microorganisms," explained Aga, "but how about microbial resistance
due to exposure to antibiotics in the environment?"
She explained that people may be infected by resistant pathogens
in the environment through direct contact or by indirect means,
such as through the food supply.
Aga is one of a handful of scientists in the world looking at
the question from a unique vantage point, taking into consideration
the complete journey made by animal antibiotics and their
metabolites from the barnyard to the crop field and, possibly, to
supplies of drinking water.
While other researchers, particularly those at government
agencies such as the U.S. Geological Survey, examine the ultimate
destination of antibiotics, such as levels found in rivers and
groundwater, those studies do not distinguish between antibiotics
excreted by animals or humans, Aga explained.
"Direct evidence that links antibiotic use in animal production
and resistance in bacteria that infect humans is lacking," said
Aga. "We are only now beginning to do the studies that will be able
to address that issue."
She noted that government agencies, such as the USDA and the
FDA, are being pressured by environmentalists to ban the use of
antibiotics as growth promoters in animals.
"But there are a lot of economic issues to consider before
taking that step," explained Aga, who noted that in Switzerland,
Denmark and Sweden such bans of antibiotics as growth promoters are
already in place.
When drugs are administered to animals, whether it's to treat
diseases or for growth promotion, as much as 50 per cent or more is
not metabolized and is excreted by the animal intact, Aga
"So when manure is used to fertilize fields, you're now exposing
the microorganisms in the soil to low levels of these drugs,
creating the perfect conditions for selectively proliferating
resistant bacteria," she said.
"In our studies of swine and cattle manure, we found between 5
ppb (parts per billion) and 20,000 ppb of tetracycline, which is
really high," said Aga.
Tetracycline, which is prescribed to combat a broad range of
bacterial infections in humans, also is used as a growth promoter
Aga noted that levels of antibiotics in animals vary depending
on the stage of life.
"For example, when a pig is almost ready for slaughter, the use
of antibiotics is curtailed to ensure that the meat is not
contaminated with antibiotics," she said.
Aga is framing her findings in terms of how farmers can minimize
the potential for the development of resistant bugs in fertilized
Her findings so far confirm other results that have identified
loamy soils as those that can be safely applied with
"The sandier soils are not good candidates for fertilizing with
manure that may be contaminated with antibiotics because the
antibiotics could leach easily before they can break down
"But that's not the case with loamier soils," she said, "In
fact, after two weeks, we have seen as much as 50 per cent
According to Aga, if antibiotics degrade quickly in the field,
they will not likely pose a problem. Like pesticides and other
environmental pollutants, degradation is slowed down in colder,
less sunny environments, she added.
"Our work is focused on understanding the fate, transport and
ecotoxicological impacts of antibiotic residues in the
environment," she said. "We hope to offer fundamental knowledge
that could be used as a basis for developing management practices
and policies that could prevent contamination of soil and aquatic
"Manure is a very good source of organic fertilizer. We don't
want to overreact," said Aga, whose parents run a small poultry
farm in the Philippines.
Aga is organizing an international symposium, "Ecotoxicity and
Environmental Chemistry of Antibiotics," for the annual meeting of
the Society of Environmental Toxicology and Chemistry to be held in
November in Austin, Texas. The symposium will bring together
scientists from different countries to share results in the field
and build collaborations.