In a new study, researchers at the University of Illinois report that some genes found in hog waste lagoons are transferred 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. 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. During the study, 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. In order to determine the origin of the tetracycline resistance genes found in the groundwater, the researchers conducted a genetic analysis of one gene family, tet(W), in samples from the lagoons and from groundwater wells below (downgradient of) and above (upgradient to) the lagoons. They found that the variants of tet(W) genes in the upgradient, environmental control wells were distinct from those of the lagoons, while the wells downgradient of the lagoons contained genes consistent with both the background levels and those in the lagoons. The resistance genes were present at much higher levels in the lagoon than in the contaminated wells. Most were diluted as they moved away from the lagoons in the groundwater. There was one notable exception. A gene known as tet(C) was found at higher levels in some of the groundwater wells at Site A than in the lagoon. Its heightened presence was not consistent with background levels, indicating that something in the environment was amplifying this one gene, which had originated in the lagoon. Perhaps the gene had migrated to a new organism, Yannarell suggests, to find a host that was more suited to conditions in the groundwater. “What we are seeing is that the genes can travel a lot further than the bacteria,” Dr. Mackie says. “It’s a matter of getting the DNA into the right organism. It’s a relay race.”