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Reducing levels of antibiotics and antibiotic resistance genes

Antibiotic resistance is a growing
human health concern. Researchers around the globe have found
antibiotics and other pharmaceuticals to be present in surface waters
and sediments, municipal wastewater, animal manure lagoons and
underlying groundwater. In a recent article in the November/December
2007 issue of Journal of Environmental Quality, researchers at Colorado
State University (CSU) describe a study to find out if animal waste
contributes to the spread of antibiotics and antibiotic resistance
genes (ARG), and if they can be reduced by appropriate manure
management practices.
Antibiotic resistance is a growing human health concern. Researchers around the globe have found antibiotics and other pharmaceuticals to be present in surface waters and sediments, municipal wastewater, animal manure lagoons and underlying groundwater. In a recent article in the November/December 2007 issue of Journal of Environmental Quality, researchers at Colorado State University (CSU) describe a study to find out if animal waste contributes to the spread of antibiotics and antibiotic resistance genes (ARG), and if they can be reduced by appropriate manure management practices.

In the study, funded by the U.S. Department of Agriculture’s Agricultural Experiment Station at CSU and the National Science Foundation (NSF), researchers investigated the effects of manure management on the levels of antibiotics and ARG in manures. The study was conducted at two scales. In the pilot scale experiment, horse manure was spiked with the antibiotics chlortetracycline, tylosin and monensin and compared to horse manure that was not spiked with antibiotics to determine the response of ARG in unacclimated manures. In the large scale experiment, dairy manure and beef feed lot manure, which were already acclimated to antibiotics, were monitored over time.

The manures were subjected to high intensity management (HIM: amending with leaves and alfalfa, watering and turning) and low intensity management (LIM: no amending, watering and turning) for six months. During this time, the levels of antibiotics were monitored using high performance liquid chromatography (HPLC) and tandem mass spectrometry (MS/MS). In addition, two types of ARG that confer resistance to tetracycline – tet(W) and tet(O) – were monitored using quantitative polymerase chain reaction (QPCR).

In the pilot study, chlortetracycline, tylosin and monensin all dissipated more rapidly in the HIM manure than in the LIM manure. In the large scale study, feed lot manure initially had higher concentrations of the several tetracycline antibiotics than the dairy manure. After four months of treatment, tet(W) and tet(O) decreased
significantly in dairy manure, but two more months of treatment were necessary for similar reductions of ARG in the feed lot manures.

The results showed that HIM was more effective than LIM at increasing the rate of antibiotic dissipation, but it was not a significant factor in reducing the levels of ARG. The length of treatment time was the main factor in reducing the levels of both antibiotics and ARG. For manures with initially high levels of antibiotics, treatment times of at least six months may be necessary for a significant reduction
in levels of antibiotics and ARG. The results also provided evidence that ARG may be present for extended time periods even after antibiotics have fully dissipated.

Scientists at Colorado State University are continuing research in this area by examining full scale local on-farm waste management practices. Together, this research will lead to a better understanding of possible ARG mitigation strategies so that best management practices can be developed to reduce the effects that animal waste may have on the spread of ARG. An abstract of the report can be viewed at: http://jeq.scijournals.org/cgi/content/abstract/36/6/1695.