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Scanning electron micrograph of
methicillin-resistant Staphylococcus aureus (MRSA)
Photo Credit: NIAID |
University of Maryland-led study is
first to document environmental source of the antibiotic-resistant
bacteria in the United States
College Park, Md.--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-susceptibleStaphylococcus
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, butthe
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.
Ninety-three 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 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).
“Our findings 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.”
For more information, please contact
Kelly Blake, communications director for the School of Public Health
atkellyb@umd.edu or (301) 405-9418.
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