The routine use of antibiotics in swine production can have unintended consequences, with antibiotic resistance genes sometimes leaking from waste lagoons into groundwater.
In a new study, researchers at the University of Illinois report that some genes found in hog waste lagoons are transferred like batons from one bacterial species to another. The researchers found that this migration across species and into new environments sometimes dilutes and sometimes amplifies genes conferring antibiotic resistance.
The new report, in the August issue of Applied and Environmental Microbiology, tracks the passage of tetracycline resistance genes from hog waste lagoons into groundwater wells at two Illinois swine facilities.
This is the first study to take a broad sample of tetracycline resistance genes in a landscape dominated by hog farming, said principal investigator R.I. Mackie. And it is one of the first to survey the genes directly rather than focusing on the organisms that host them. Mackie is a professor in the department of animal sciences and an affiliate of the Institute for Genomic Biology.
"At this stage, we're not really concerned about who's got these genes," Mackie said. "If the genes are there, potentially they can get into the right organism at the right time and confer resistance to an antibiotic that's being used to treat disease."
Tetracycline is widely used in swine production. It is injected into the animals to treat or prevent disease, and is often used as an additive in hog feed to boost the animals growth. Its near-continuous use in some hog farms promotes the evolution of tetracycline-resistant strains in the animals digestive tracts and manure.
The migration of antibiotic resistance from animal feeding operations into groundwater has broad implications for human and ecological health. There are roughly 238,000 animal feeding operations in the U.S., which collectively generate about 500 million tons of manure per year. Groundwater comprises about 40 percent of the public water supply, and more than 97 percent of the drinking water used in rural areas.
Federal law mandates that animal facilities develop nutrient management plans to protect surface water and groundwater from fecal contamination. Most swine facilities hold the effluent in large, water-filled lagoons until it can be injected into the ground as fertilizer. Thanks to a change in the law in the late 1990s, new lagoons must be built with liners to prevent seepage. Swine facilities in operation prior to the new regulations are allowed to continue using unlined lagoons, however.
Some of these lagoons leak.
The researchers extracted bacterial DNA from lagoons and groundwater wells at two study sites over a period of three years. They screened these samples for seven different tetracycline resistance genes.
They found fluctuating levels of every one of the seven genes for which they screened in the lagoons. They also found that these genes were migrating from the lagoons to some of the groundwater wells.
It should be noted that many genes that confer antibiotic resistance occur naturally in the environment. Tetracycline is itself a bacterial product, employed by Streptomyces bacteria long before humans discovered its usefulness.