Environmental Science
Second Runner-up: Antibiotic resistance genes
“Antibiotic Resistance Genes as Emerging Contaminants: Studies in Northern Colorado” by Amy Pruden, Ruoting Pei, Heather Storteboom, and Kenneth Carlson, Colorado State University, 2006, 40 (23), 7445–7450.
In 2002, when environmental microbiologist Amy Pruden joined the faculty at Colorado State University, researchers were already worried about the upward trend of concentrations of antibiotics in the environment. Yet, no one really understood the environmental impact of these drugs. “The big question that a lot of people are raising is: what are the actual impacts and should we care?” says Pruden. To find the answer, Pruden teamed up with her colleague Kenneth Carlson, who had documented unnaturally high levels of antibiotics in nearby rivers and sediments. Together they embarked on the project that led to their ES&T paper.
Antibiotic resistance is a growing problem for human health, Pruden points out, and both the Centers for Disease Control and Prevention and the World Health Organization are concerned. Although the problem is largely attributed to the overprescription of antibiotics, a handful of studies have linked overuse of antibiotics in agricultural settings to resistance in human infections. Pruden knew that investigating the environmental impacts of antibiotics meant going to the source: antibiotic resistance genes (ARGs) in microbes themselves.
Excessive agricultural and urban use had been shown to cause the antibiotics tetracycline (tet) and sulphonamide (sul) to accumulate in the sediments of the Cache la Poudre River. So, Pruden and Carlson decided to look at the levels of several tet- and sul-resistance genes in river sediments as well as in nearby dairy lagoon water, irrigation ditch water, a wastewater recycling plant, and two drinking-water treatment plants. (Dairy lagoons are large ponds for storing waste from dairy farms; wastewater from the lagoons is eventually used for irrigation.)
Using a quantitative DNA amplification technique, the team measured ARG levels and found significantly higher amounts of the genes in all sites impacted by agriculture and urban activity than the ARG levels at a pristine site that received less effluent from urban or agricultural activities. In addition, they found a consistent pattern in the ARG levels—the highest levels were found in dairy lagoons, followed by irrigation ditch water and river sediments impacted by agriculture and urban activity. This suggested a potential route by which the antibiotics and ARGs spread in the environment: from dairy farms to irrigation ditches to rivers. Samples from the wastewater and drinking-water plants also contained high amounts of two kinds of ARGs, tet(W) and tet(O).
“We think of contaminants as they get broken down into something worse,” says Pruden. “This is kind of analogous: the presence of antibiotics induces the presence of these genes, which are themselves contaminants.” Pedro Alvarez of Rice University, an expert in ARGs, says the novelty of the paper lies in the idea that genes may be contaminants. “They had the vision to recognize that genes, genetic elements, and genetic vectors represent an emerging class of environmental pollutants that we are going to have to deal with,” he says.
More work still needs to be done, Pruden emphasizes. For example, just how these contaminants lead to antibiotic-resistant infections in humans is still unknown. And future work might uncover ways to tackle the problem “right where it is created,” says Pruden, “rather than at the hospital level [by] trying to create more antibiotics.”
