‘Superbug’ found in US wastewater-reuse plants

Researchers at the University of Maryland’s School of Public Health (UMSPH) in the USA reported on 5 November 2012 that the “superbug” methicillin-resistant Staphylococcus aureus (MRSA) is prevalent at several US wastewater-reuse plants (WWTPs).

However, MRSA was found in the final effluent at one only of the four WWTPs studied, and this was at a site that did not use chlorination.

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. Swedish researchers have previously identified the presence of MRSA in WWTPs in Sweden.

The research team, including UMSPH and University of Nebraska Medical Center researchers, collected wastewater samples throughout the treatment process at four 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.

“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,” says Amy R Sapkota, assistant professor in the Maryland Institute for Applied Environmental Health and research study leader. “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.”

“Our findings raise potential public health concerns for WWTP workers and individuals exposed to reclaimed wastewater,” says Rachel Rosenberg Goldstein, environmental health doctoral student in the UMSPH 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.”

This was the first study to investigate US wastewater as a potential environmental reservoir of MRSA. It 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%. MRSA was present in 83% of the influent at all plants.

As treatment progressed through the plants, however, the percentage of MRSA- and MSSA-positive samples decreased. 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.

At two WWTPs, 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 WWTPs 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 treatment (via chlorination).

The paper Methicillin-Resistant Staphylococcus aureus (MRSA) Detected at Four US Wastewater Treatment Plants was published in the November 2012 issue of Environmental Health Perspectives.