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MRSA Superbug Found In US Wastewater Plants

Scientists have found the superbug MRSA (methicillin-resistant Staphylococcus aureus) at four US wastewater treatment plants. In the past, the drug-resistant bacterium, which causes potentially fatal, difficult to treat infections, was mostly found in people in hospital settings, but is now increasingly found in healthy people in the wider community.

The team, led by researchers from the University of Maryland School of Public Health, report their findings in the November issue of the journal Environmental Health Perspectives.

Study leader Amy R Sapkota, assistant professor in the Maryland Institute for Applied Environmental Health, says in a statement:

"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."

But, she explains, how MRSA arises in the wider community and how healthy people become infected that way is still somewhat of a mystery.

We do know that people infected with the superbug shed it from their noses, their skin, and in their feces, so wastewater would be a logical place to look.

Researchers in Sweden have already reported finding MRSA in wastewater treatment plants in Sweden, but this study is thought to be the first to investigate wastewater as a potential reservoir of MRSA in the US. The Study For the study, the researchers collected samples of wastewater at different stages of the treatment process at two Mid-Atlantic and two Midwestern treatment plants.

One of the reasons the researchers picked these four plants, is because their effluent, after removal of solids and certain impurities, is used as reclaimed wastewater, for instance in landscape irrigation. They wanted to find out if MRSA was present in the effluent.

When they analyzed the samples, they found MRSA, plus another related pathogen, MSSA (methicillin-susceptible Staphylococcus aureus) in all four plants.

MRSA was present in half of the samples, and MSSA was present in 55% of them.

When they looked at the samples taken at different stages of the process, they found MRSA was present in 83% of the influent (the raw, as yet untreated sewage), at all four plants, but this percentage went down as treatment progressed.

93% of the MRSA strains and 29% of MSSA strains they found were resistant to two or more classes of antibiotics, including several approved specifically for treating MRSA infection.

Two of the plants had MRSA strains that were resistant to more antibiotics and more likely to possess a gene that made them more virulent, in amounts that increased with treatment stage, until the strains were completely eliminated through tertiary chlorination.

Only one of the four wastewater treatment plants had MRSA present in the treated water leaving the plant. The researchers noted this was also the only plant in the four that did not regularly chlorinate the water. Implications The team suggests these findings show that while wastewater treatment plants do effectively reduce the amount of MRSA and MSSA present as the treatment progresses from influent to effluent, they may actually be selecting for increased antibiotic resistance and virulence, particularly if there is no tertiary chlorination.

First author Rachel Rosenberg Goldstein, environmental health doctoral student in the University of Maryland School of Public Health, says these findings should make us concerned about the health of workers at wastewater treatment plants, and people exposed to reclaimed wastewater.

"Because of increasing use of reclaimed wastewater, further research is needed to evaluate the risk of exposure to antibiotic-resistant bacteria in treated wastewater," she urges.

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