MRSA Found in US Wastewater Treatment Plants

A team of researchers led at the University of Maryland School of Public Health has found that the superbug MRSA is prevalent in several wastewater treatment plants across the U.S.

Methicillin-resistant Staphylococcus aureus (MRSA) is well known for causing difficult-to-treat and potentially fatal bacterial infections in hospital patients, but since the late 1990s it has also been infecting otherwise healthy people in community settings. Infected people can shed MRSA from their noses and skin and through their feces, so wastewater treatment plants are a likely reservoir for the bacteria. Swedish researchers have previously identified the presence of MRSA in wastewater treatment plants in Sweden, and this new UMD-led study confirms the presence of MRSA in U.S. facilities.

The research team, including University of Maryland School of Public Health and University of Nebraska Medical Center researchers, collected wastewater samples throughout the treatment process at two Mid-Atlantic and two Midwestern wastewater treatment plants. These plants were chosen because treated effluent discharged from these plants is reused as "reclaimed wastewater" in spray irrigation activities. The researchers were interested in whether MRSA remained in the effluent.

They found that MRSA, as well as a related pathogen, methicillin-susceptible Staphylococcus aureus (MSSA), were present at all four wastewater treatment plants, with MRSA in half of all samples and MSSA in 55 percent. MRSA was present in 83 percent of the influent, but the percentage of MRSA and MSSA-positive samples decreased as treatment progressed. Only one wastewater treatment plant had the bacteria in the treated water leaving the plant, and this was at a plant that does not regularly use chlorination, a tertiary step in wastewater treatment.

93 percent of the MRSA strains that were isolated from the wastewater and 29 percent of MSSA strains were resistant to two or more classes of antibiotics, including several that the U.S. Food and Drug Administration has specifically approved for treating MRSA infections. At two wastewater treatment plants, MRSA strains showed resistance to more antibiotics and greater prevalence of a gene associated with virulence at subsequent treatment stages, until tertiary chlorination treatment appeared to eliminate all MRSA. This suggests that while wastewater treatment plants effectively reduce MRSA and MSSA from influent to effluent, they may select for increased antibiotic resistance and virulence, particularly at those facilities that do not employ tertiary/chlorination treatment.

“Our finding raise potential public health concerns for wastewater treatment plant workers and individuals exposed to reclaimed wastewater, “says Rachel Rosenberg Goldstein, environmental health doctoral student in the School of Public Health and the study’s first author. “Because of increasing use of reclaimed wastewater, further research is needed to evaluate the risk of exposure to antibiotic-resistant bacteria in treated wastewater.”

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