A new study eases concerns that irrigating crops with water released from sewage treatment plants - an increasingly common practice in arid areas of the world - fosters emergence of the antibiotic-resistant bacteria that cause thousands of serious infections each year. The research appears in ACS' journal Environmental Science and Technology.

Eddie Cytryn and colleagues explain that a large fraction of antibiotics given to people or animals pass out of the body unchanged in the urine and are transferred via sewage systems to wastewater treatment facilities.

These facilities do not completely remove common antibiotics like tetracycline, erythromycin, sulfonamide and ciprofloxacin and may actually enhance the abundance of antibiotic-resistant bacteria and antibiotic-resistance genes.

Previous studies have suggested that wastewater effluents can expand natural reservoirs of antibiotic resistance, which may contribute to clinically associated antibiotic resistance.

Arid and semi-arid areas of the world are plagued by severe water shortages, which are expected to increase as a result of growing population and global climate change.

As a result, more areas are turning to treated wastewater (TWW) to irrigate croplands. In Israel, for instance, TWW provides more than half of the water used for irrigation.

The researchers wanted to find out if long-term irrigation with treated wastewater enhances antibiotic resistance in soil microbial communities, which could potentially be transferred through agricultural produce to clinically relevant bacteria.

The authors found that levels of antibiotic-resistant bacteria and genes for antibiotic resistance in fields and orchards irrigated with freshwater and TWW were essentially identical, suggesting that antibiotic-resistant bacteria that enter soil by irrigation are not able to survive or compete in that environment.

The authors say there is "cause for cautious optimism" that irrigating with TWW is not increasing the prevalence of bacteria resistant to the antibiotics they studied.