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 explained.

"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 in pigs.

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 soil.

Her findings so far confirm other results that have identified loamy soils as those that can be safely applied with fertilizer.

"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 sufficiently.

"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 degradation."

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 systems."

"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.