Water discharged into lakes and rivers from municipal sewage treatment plants may contain significant concentrations of specific genes that make bacteria antibiotic-resistant, claims a new study.

Researchers at the University of Minnesota (Minneapolis, USA) studied the impact of tertiary-treated municipal wastewater on the quantity of several antibiotic resistance determinants in Lake Superior in the Duluth (MN, USA) area. The researchers collecting surface water and sediment samples from 13 locations in Duluth-Superior Harbor, the St. Louis River, and Lake Superior. Quantitative Polymerase Chain Reaction (qPCR) was used to target three different genes encoding resistance to tetracycline (tet(A), tet(X), and tet(W)), the gene encoding the integrase of class 1 integrons (intI1), total bacterial abundance (16S rRNA genes), and total and human fecal contamination levels (16S rRNA genes specific to the genus Bacteroides).

The results showed that the quantities of tet(A), tet(X), tet(W), intI1, total Bacteroides, and human-specific Bacteroides were typically 20-fold higher in the tertiary-treated wastewater than in nearby surface water samples. In contrast, the quantities of these genes in the St. Louis River and Lake Superior were typically below detection. Analysis of sequences of tet(W) gene fragments from four different samples collected throughout the study site supported the conclusion that tertiary-treated municipal wastewater is a point source of resistance genes into Duluth-Superior Harbor. The study was published in the November 15, 2011, issue of Environmental Science & Technology.

“Current wastewater treatment removes a very large fraction of the antibiotic resistance genes,” said lead author associate professor Timothy LaPara, PhD, of the department of civil engineering. “But this study shows that wastewater treatment operations need to be carefully considered and more fully studied as an important factor in the global ecology of antibiotic resistance.”

The ever-increasing presence of antibiotic-resistant bacteria has raised substantial concern on the future effectiveness of antibiotics. While antibiotic use in agriculture has been heavily scrutinized, the roles of sewage and municipal wastewater treatment have received little attention as a reservoir of resistance. Antibiotic-resistant bacteria develop in the gastrointestinal (GI) tracts of people taking antibiotics; these bacteria are then shed during defecation. Antibiotic resistance can then be transferred between organisms via lateral gene transfer, becoming part of the ongoing major health problem of antibiotic resistant pathogens.

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