July 2012, Vol. 24, No.7

Research Notes

Bacteria resistant to multiple types of antibiotics find their way into Lake Geneva

Antibiotic-resistant bacteria are increasing in the waters of Lake Geneva near Lausanne, Switzerland. Researchers from the Swiss Federal Institute of Aquatic Science and Technology (Eawag; Dübendorf) investigated to determine if the bacteria are entering the lake through a wastewater treatment plant discharge.

The Lausanne wastewater treatment plant treats about 90,000 m3/d (24 mgd). The flow comes from the region’s 214,000 inhabitants, a number of smaller health-care centers, and the University Hospital of Canton Vaud (Lausanne), a major health-care facility.

Researchers found that the hospital’s wastewater contained high levels of bacteria that are highly resistant, or multiresistant, to multiple antibiotics, according to an Eawag press release. They also discovered that while the treatment plant eliminates more than 75% of the bacteria, the proportion of especially resistant bacteria is greater in treated wastewater, according to the release. This occurs because bacteria from the human body encounter other bacteria adapted to other environments and transfer mobile genetic elements because of high bacterial cell densities, said Helmut Bürgmann, an Eawag microbiologist who contributed to the study.

While it is neither unusual for bacteria to take up antibiotic-resistant genes nor for this to be a hazard in itself, a problem occurs when the levels of multiresistant genes are elevated in Lake Geneva’s water and sediment, particularly close to the wastewater discharge location, Bürgmann said in the news release. This could increase risk that antibiotic-resistant genes in bacteria will be transferred to pathogens, either in the lake or in humans, if mobile genetic elements for antibiotic resistance migrate to drinking water, the news release says.

But Nadine Czekalski, another study researcher, explained in the news release that there is no need to panic. She explained that the drinking water is withdrawn 3 km away from the effluent discharge area, multiresistant genes were not detected near the withdrawal area, and the drinking water is treated before distribution.

Both Bürgmann and Czekalski agree on the need for caution and support the Swiss federal government’s plans to introduce an additional process that would largely inactivate microconstituents and antibiotic-resistant microorganisms at selected treatment plants, the news release says. The researchers also recommend additional separate treatment of hospital wastewater.

Wood removes nitrates from runoff

Wood chips may solve the nitrate pollution problem caused by stormwater runoff from agricultural fields, according to a U.S. Agricultural Research Service (ARS) study.

Nitrates that leach from agricultural fields make their way into local waterways and contribute to oxygen-deficient dead zones along coasts. But microorganisms living in wood can denitrify nitrates into nitrogen gas or nitrous oxide, according to an ARS news release. These gases then diffuse into the atmosphere.

ARS microbiologist Tom Moorman and other researchers at the ARS National Laboratory for Agriculture and the Environment in Ames, Iowa, installed perforated plastic drainage pipes 1.2 m (4 ft) below the soil surface, the news release says. The researchers dug trenches on either side of the pipes, filled them with wood chips, and buried them. Then the crop fields were planted with a corn and soybean rotation for a 9-year study, the news release says.

The research team found that the buried wood chip walls act as bioreactors, consistently removing nitrates. From 2001 to 2008, the annual nitrate loss in plots with conventional drainage averaged 54.5 kg/ha (48.6 lb/ac); losses in plots with denitrification walls dropped to 24.4 kg/ha (21.8 lb/ac), the news release says. Also, the wood-chip walls became more effective over time until they began decomposing — the average denitrification potential increased from thirty-onefold in 2003 to four-thousandfold in 2004, the news release says.

About 50% of the wood, which was buried 889 to 991 mm (35 to 39 in.) underground, decomposed after 5 years. About 75% decomposed after 9 years. This information helps researchers calculate the functional life expectancy of a denitrification wall, the news release says.

The researchers published their findings in the journal Ecological Engineering (Vol. 36, November 2010, pp. 1567–1574).


© 2012 Water Environment Federation. All rights reserved.