Urine can be used as a fertiliser without fear it will fuel the spread of antibiotic resistance, researchers have revealed – although they urge caution against using fresh bodily waste to water crops.
Urine is rich in nitrogen and phosphorus and has been used for generations to help plants grow. But the presence of urinary tract infections in donors means the fluid can also contain DNA from bacteria – including genes for antibiotic resistance.
One concern is that even if the microbes themselves are killed either as a result of storage conditions or pasteurisation, this DNA could be taken up by other bacteria in the environment and end up causing antibiotic-resistant infections in humans.
The spread of antibiotic resistance is an issue experts have said is as great a threat to humanity as the climate crisis.
But now experts say that, for stored urine at least, bacterial DNA is not passed on to other microbes.
“I think this is an important step in demonstrating that we have methods where we can reduce the risks that the things in urine pose,” said Dr Krista Wigginton, a co-author of the research from the University of Michigan.
Writing in the journal Environmental Science and Technology, Wigginton and colleagues report how they investigated the issue by focusing on rings of DNA found within certain microbes, including bacteria, and which can be passed between them. Such DNA plasmids have previously been found in urine.
The team collected more than 100 litres of urine from male and female donors across Vermont and stored it for between 12 and 16 months – enough time for a substance within the urine, known as urea, to break down to ammonia.
The team introduced plasmid DNA that contained genes for resistance towards two antibiotics, tetracycline and ampicillin, into the aged urine and carried out a number of tests to see whether the common soil bacterium Acinetobacter baylyi would take up the DNA and hence gain antibiotic resistance.
The results reveal no difference in the uptake of either antibiotic resistance gene by Acinetobacter baylyi, whether aged urine immediately spiked with plasmid DNA was used, aged urine incubated with the plasmid DNA for 10 hours or plasmid DNA in water.
However, as the time the plasmid DNA spent in the aged urine increased to 24 hours, a 99% drop in the uptake of the antibiotic resistant genes by the soil bacteria was seen. That, the team said, suggested the contents of the urine affected the plasmid DNA over time, making it less able to be taken up – possibly by severing the plasmid.
“We found that after a pretty short time in this stored urine … the DNA had lost its ability to transform these environmental bacteria,” said Wigginton. But, she noted: “When we first excrete the DNA, the bacteria can take it up.”
The team carried out further tests on filtered, and filtered and heat treated, aged urine, finding these measures removed components of the urine that are important in reducing the ability for the plasmid DNA to be transferred to the soil bacteria. The team say that suggests either bacteria within the urine itself, or enzymes in the aged urine, are responsible for decreasing the ability for the plasmid DNA to be shared with the soil bacteria.
The team conclude that, provided urine is aged – as is the case in most systems designed to collect urine for fertiliser – it is unlikely to spread antibiotic resistance if used on crops.
The study has limitations, including that it only looked at plasmid DNA and only examined the spread to one type of bacteria.
But Wigginton said the idea of using human urine for fertiliser was a growing field: an estimated 330 tonnes of nitrogen and 20 tonnes of phosphorus a day could be retrieved should 10% of the US population collect their urine.
“Humans have been collecting urine and using it for fertiliser for a long, long time, but then in the west that really stopped with the invention of sewage system,” she said. “We are just trying now to figure out with this infrastructure system that we have, how do we pull back and think differently about what goes into this sewage system and capture some of those valuable products before [they] get mixed and diluted with everything else?”