Researchers at Washington State University find urine from animals treated with antibiotics could contribute to selection for antibiotic-resistant bacteria in the environment.

Their paper titled "Urine from Treated Cattle Drives Selection for Cephalosporin Resistant Escherichia coli in Soil," is published in PLOS One, an open-access peer-reviewed online journal.

The researchers tested their hypotheses using the cephalosporins class of antibiotic, for which they note the FDA recently issued new rules for use in food animals in part because of an increasing prevalence of enteric, or intestinal bacteria that are resistant.

The typical treatment with these products, such as injections, has limited effects on enteric bacteria so the researchers tested the hypothesis that antibiotic metabolites in the urine of treated animals causes selection pressure for resistant E. coli in soil.

If this process selects for antibiotic-resistant bacteria in the environment, cattle could be shedding normal bacteria in their manure, but picking up resistant bacteria as they graze.

In the laboratory, the researchers mixed two different soil types with manure from dairy cattle, and exposed the samples to urine from antibiotic-treated cattle.

They found that the antibiotic metabolites in the urine did remain active in the soil, with the length of activity depending on temperature. Cooler temperatures lengthened the time of activity while warmer temperatures apparently helped soil microbes biodegrade the antibiotic. During the time of activity, the researchers’ analysis shows the metabolites in the urine corresponded with development of higher populations of antibiotic-resistant E. coli.

These tests were conducted in a laboratory, so further testing will be needed to determine whether the same effects would occur in a natural environment. The results suggest, the researchers say, that “engineered solutions could be developed so that this important veterinary drug can be used without perpetuating resistance in non-target enteric bacteria. This might involve bioremediation, addition of adsorption agents, or improved waste management. Such strategies might also mitigate selection from other excreted antibiotics that remain bioavailable in the environment.”

Read the full report from PLOS One.