A team led by researchers at the University of Maryland School of Public Health has found that the "superbug" methicillin-resistant Staphylococcus aureus (MRSA) is prevalent at several U.S. wastewater treatment plants (WWTPs). 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.
"MRSA infections acquired outside of hospital settings - known as community-acquired MRSA or CA-MRSA- are on the rise and can be just as severe as hospital-acquired MRSA. However, we still do not fully understand the potential environmental sources of MRSA or how people in the community come in contact with this microorganism," says Amy R. Sapkota, assistant professor in the Maryland Institute for Applied Environmental Health and research study leader. "This was the first study to investigate U.S. wastewater as a potential environmental reservoir of MRSA."
Because infected people can shed MRSA from their noses and skin and through their feces, wastewater treatment plants are a likely reservoir for the bacteria. Swedish researchers have previously identified the presence of MRSA in WWTPs in Sweden, and this new UMD-led study confirms the presence of MRSA in U.S. facilities. The study was published in the November issue of the journal Environmental Health Perspectives.
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 WWTPs. These plants were chosen, in part, 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 WWTPs, with MRSA in half of all samples and MSSA in 55 percent. MRSA was present in 83 percent of the influent- the raw sewage - at all plants, but the percentage of MRSA- and MSSA-positive samples decreased as treatment progressed. Only one WWTP 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.