A recent study published in Frontiers in Microbiology has found that secondary-treated wastewater used for irrigating lettuce can transfer antibiotic-resistant bacteria and genes to the crops, while tertiary treatment significantly reduces this risk.
The research was conducted using a controlled experimental setup. Lettuce plants were divided into three groups, each irrigated with either potable tap water, secondary-treated wastewater, or tertiary-treated wastewater. Each group included 936 plantlets and the experiment was replicated to ensure reliable results.
Researchers measured the presence of Escherichia coli (E. coli) and extended-spectrum beta-lactamase-producing E. coli (ESBL-E. coli), which are indicators of antimicrobial resistance. They also quantified several key antibiotic resistance genes (ARGs), including blaCTX–M–1, blaTEM, sul1, and tetA.
Results showed that both E. coli and ESBL-E. coli were undetectable in potable water and tertiary-treated wastewater samples but were present at much higher levels in secondary-treated wastewater. Similarly, ARGs were most abundant in secondary-treated wastewater, less so in tertiary-treated water, and lowest in potable water.
In lettuce grown with secondary-treated wastewater, E. coli was detected in 94% of plants compared to 33% when either tertiary-treated or potable water was used for irrigation. ESBL-E. coli was found in 61% of the secondary wastewater group but not detected in the other two groups.
The study noted that some ARGs were already present at baseline levels in seedlings before any irrigation took place, suggesting contamination may occur independently of irrigation practices.
According to the researchers: "Biological treatment, that is, secondary treatment, is insufficient to eliminate detectable fecal bacteria and ARB, with residual bacteria levels several orders of magnitude higher than those in potable or tertiary-treated water."
They further stated: "Tertiary treated water appeared to pose a comparably low risk to potable water for irrigation with respect to antimicrobial resistance transmission in this controlled study, and should not be assumed to be equivalent under field conditions."
Despite these findings from controlled experiments showing limited transfer of ARGs from treated wastewater into crops—only about 6% from secondary- and about 4% from tertiary-treated water—the authors caution that real-world factors such as rainfall and environmental microbial contamination could influence outcomes differently.
The study highlights the importance of advanced (tertiary) treatment processes if treated wastewater is used for irrigating fresh produce intended for raw consumption.
Future research will need to address how variables like soil interactions and seasonal changes affect antimicrobial resistance gene transmission through agricultural use of recycled water.
