Subject: Fw: Antibiotic
Resistance from Swine to Groundwater
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Science Daily
— The routine use of antibiotics in swine production
can have unintended consequences, with antibiotic resistance genes sometimes
leaking from waste lagoons into groundwater.
A
research team tracked the movement of tetracycline resistance genes from
wastewater lagoons to groundwater at two
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In
a new study, researchers at the University of Illinois report that some genes
found in hog waste lagoons are transferred – “like batons”
– from one bacterial species to another. The researchers found that this
migration across species and into new environments sometimes dilutes –
and sometimes amplifies – genes conferring antibiotic resistance.
The
new report, in the August issue of Applied and Environmental Microbiology,
tracks the passage of tetracycline resistance genes from hog waste lagoons into
groundwater wells at two
This
is the first study to take a broad sample of tetracycline resistance genes in a
landscape dominated by hog farming, said principal investigator R.I. Mackie.
And it is one of the first to survey the genes directly rather than focusing on
the organisms that host them. Mackie is a professor in the department of animal
sciences and an affiliate of the Institute for Genomic Biology.
“At
this stage, we’re not really concerned about who’s got these
genes,” Mackie said. “If the genes are there, potentially they can
get into the right organism at the right time and confer resistance to an
antibiotic that’s being used to treat disease.”
Tetracycline
is widely used in swine production. It is injected into the animals to treat or
prevent disease, and is often used as an additive in hog feed to boost the
animals’ growth. Its near-continuous use in some hog farms promotes the
evolution of tetracycline-resistant strains in the animals’ digestive
tracts and manure.
The
migration of antibiotic resistance from animal feeding operations into
groundwater has broad implications for human and ecological health. There are
roughly 238,000 animal feeding operations in the
Federal
law mandates that animal facilities develop nutrient management plans to
protect surface water and groundwater from fecal contamination. Most swine facilities
hold the effluent in large, water-filled lagoons until it can be injected into
the ground as fertilizer. Thanks to a change in the law in the late 1990s, new
lagoons must be built with liners to prevent seepage. Swine facilities in
operation prior to the new regulations are allowed to continue using unlined
lagoons, however.
Some
of these lagoons leak.
The
researchers extracted bacterial DNA from lagoons and groundwater wells at two
study sites over a period of three years. They screened these samples for seven
different tetracycline resistance genes.
They
found fluctuating levels of every one of the seven genes for which they
screened in the lagoons. They also found that these genes were migrating from
the lagoons to some of the groundwater wells.
It
should be noted that many genes that confer antibiotic resistance occur
naturally in the environment. Tetracycline is itself a bacterial product,
employed by Streptomyces bacteria long before humans discovered its usefulness.
In
order to determine the origin of the tetracycline resistance genes found in the
groundwater, the researchers conducted a genetic analysis of one gene family,
tet(W), in samples from the lagoons and from groundwater wells below
(downgradient of) and above (upgradient to) the lagoons. They found that the
variants of tet(W) genes in the upgradient, environmental control wells were
distinct from those of the lagoons, while the wells downgradient of the lagoons
contained genes consistent with both the background levels and those in the
lagoons.
“There’s
a human impact on these sites that is superimposed on a natural signal,”
said postdoctoral research assistant Anthony Yannarell, an author on the study.
One
of the two hog farms, “Site A,” was more impacted by resistance
genes from the lagoon, due to its hydrogeology. The site included two layers of
sand – at about two meters and eight meters below the surface –
through which groundwater flowed.
“Every
time we looked in the lagoon, we saw all of the genes we were looking
for,” Yannarell said. “At Site A, all the wells that were closest
to the lagoon almost always had every gene. As you got further from the lagoon
you started to see genes dropping out.”
The
resistance genes were present at much higher levels – “an order of
magnitude higher,” said the authors – in the lagoon than in the
contaminated wells. Most were diluted as they moved away from the lagoons in
the groundwater.
There
was one notable exception. A gene known as tet(C) was found at higher levels in
some of the groundwater wells at Site A than in the lagoon. Its heightened
presence was not consistent with background levels, indicating that something
in the environment was amplifying this one gene, which had originated in the
lagoon.
Perhaps
the gene had migrated to a new organism, Yannarell said, to find a host that
was more suited to conditions in the groundwater.
“What
we are seeing is that the genes can travel a lot further than the
bacteria,” Mackie said. “It’s a matter of getting the DNA
into the right organism. It’s a relay race.”
Other
authors on the study are postdoctoral research assistant S. Koike; Illinois
State Geological Survey geochemist I.G. Krapac; research assistant H.D. Oliver;
USDA Agricultural Research Service scientist and professor of crop sciences
J.C. Chee-Sanford; and visiting professor of animal sciences R.I. Aminov.
Note: This story has been adapted from a news release issued by
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