Urine reuse as fertilizer is not likely to transfer antibiotic resistance

Urine is a goldmine of useful substances that can be captured and converted into products such as fertilizer. However, going "green" with urine carries some potential risks. For instance, DNA released from antibiotic-resistant bacteria in urine could transfer resistance to other organisms at the site where the fertilizer is used. Now, research published in ACS' Environmental Science & Technology (ES&T) shows this risk is likely to be minimal.

Upcycling urine isn't new: Farmers have collected and fertilized crops with it for millennia. The Rich Earth Institute in Brattleboro, Vermont, operates the only contemporary community-scale system in the U.S. for capturing urine and processing it into fertilizer, according to the authors of this ES&T study. In addition, some researchers have installed urine collection toilets and waterless urinals at various locations, but these are mainly for research or demonstration purposes. If the practice took hold with 10% of the U.S. population, it could save millions of gallons of flushing water and recover about 300 tons of nitrogen and 18 tons of phosphorus per day, the authors calculate. However, urine contains bacteria, including strains resistant to antibiotics. Previous studies have reported finding antibiotic-resistant DNA in urine, but it has been unclear whether that DNA could move into microbes in the environment if the urine is applied to soil. So, Krista Wigginton and colleagues conducted experiments to see if upcycling urine could spread that resistance.

In their study, the researchers used "aged" urine that had been stored in a sealed container for several months. This traditional practice increases ammonia concentration, raises pH and alters the microbial makeup of the liquid. After incubating DNA containing resistance genes for tetracycline and ampicillin in the urine, the team found that the genetic material rapidly lost 99% of its ability to confer resistance on a soil bacteria. In sum, urine-derived fertilizer poses a low risk of spreading resistance from extracellular DNA in the environment, the team says.

American Chemical Society