CIDRAP-Hospital wastewater boosts growth of resistant bacteria, study finds

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Hospital wastewater boosts growth of resistant bacteria, study finds

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Hospital wastewater boosts growth of resistant bacteria, study finds​

Chris Dall, MA

Today at 10:18 a.m.
Antimicrobial Stewardship


Hospital wastewater has long been viewed as a potential driver of antibiotic resistance in the environment. New research out of Sweden bolsters this argument.

In a study published late last month in Nature Communications, a team of researchers from the University of Gothenburg collected wastewater from 10 hospitals and eight municipal treatment plants in Sweden’s Vastra Gotaland region. In a series of experiments, they found that the water from the hospital sewers promoted the growth of antibiotic-resistant bacteria, while the water from the municipal treatment plants had no effect.

Senior study author Joakim Larsson, PhD, who directs the Centre for Antibiotic Resistance Research at the University of Gothenburg, said the findings provide direct evidence linking antibiotics excreted by hospital patients to observed environmental effects and highlight hospital sewers as a more likely site for resistance evolution than previously considered. And the implications stretch beyond Sweden.

“The findings suggest that similar resistance selection occurs in the sewers of most hospitals worldwide,” Larsson said in a university press release.

Hospital vs municipal wastewater​

The findings add to the research that Larsson and his collaborators have conducted on antibiotics in hospital wastewater, which is also known to harbor resistant bacteria excreted by patients.

They note that while several studies have reported higher rates of resistant bacteria in hospital wastewater, that observation alone “is an indication but not strong evidence of resistance selection by HWW [hospital wastewater], as one can reasonably assume that patients, on average, carry more resistant faecal bacteria than the general population.”

More than one in three Swedish hospital patients are receiving antibiotics at any given time, according to the study. The question is whether the presence of antibiotic residues and resistant bacteria from patients is creating selection pressure for the emergence of resistant bacteria in hospital sewers.

In a previous study, Larsson and his colleagues found that filtered wastewater from one large Swedish hospital strongly selected for multidrug-resistant strains of Escherichia coli. The aim of the current study was to see whether they might find similar results with wastewater from other Swedish hospitals, and to compare antibiotic resistance selection in hospital wastewater with corresponding municipal wastewater treatment plants.

The findings suggest that similar resistance selection occurs in the sewers of most hospitals worldwide.

In the first experiment, the researchers collected samples from the hospitals and the treatments plants, filtered them to remove all naturally occurring bacteria, then exposed E coli isolates to the water samples. Significant selection for E coli strains that were resistant to the antibiotics ciprofloxacin and cefotaxime was observed with all the hospital sewer samples and none of the municipal wastewater samples.

In the second experiment, the researchers placed plastic plates in the sewer pipes of the hospital and municipal sites for 30 days, then analyzed the slimy microbial communities, known as biofilms, that grew on the plates.

They found significantly higher resistance to ciprofloxacin and cefotaxime in the hospital biofilms compared with the hospital wastewater, which the authors say provides direct evidence of resistance selection occurring naturally in the hospital sewer environment. At the municipal wastewater sites, the resistance levels to the antibiotics were similar in the biofilms and the wastewater.

Targeted measures​

Finally, Larsson and his colleagues found that when they engineered a strain of E coli to be resistant to fluoroquinolones and beta-lactam antibiotics, the strain was able to grow well in the hospital wastewater samples. This and other evidence from the experiments indicated that ciprofloxacin, a commonly used fluoroquinolone in hospitals—and to a lesser extent cefotaxime (a beta-lactam)—was a key driver of resistant bacteria in the hospital wastewater.

Since ciprofloxacin is one of the most widely used antibiotics worldwide, the authors say it’s “highly plausible” that wastewater from hospitals around the world are also selecting for resistance.

Larsson said knowing which bacteria are promoting resistance in the environment, and where it’s occurring, can help narrow down the targets for interventions to reduce selection pressure. Other studies have found that the high level of antibiotic residues in wastewater from antibiotic manufacturing facilities also promotes the emergence and growth of resistant bacteria.

“Knowing where resistant bacteria are promoted—and where they are less likely to be—is crucial when deciding how resources and efforts should be allocated to limit risks for antibiotic resistance development,” he said.