win the battle against antibiotic-resistant bacteria, Ellen Silbergeld
believes that we first must determine its source.
by Melissa Hendricks
photos by Thomas Arledge
This is a world that never sleeps.
In a long, low-slung building on a Maryland poultry farm, the lights
are always on so the birds will eat around the clock. For as far
as the eye can see, a field of white undulates over the ground like
waves. Thousands of chickens are pecking, rambling, and squawking
in a discordant clamor that is overridden every few minutes by the
loud whirring of an industrial fan. A fine, powdery dust fills the
air, and an acrid odor of ammonia and manure clings
to the nostrils and coats the back of the throat.
With the endless eating beneath the fluorescent lights, the chickens
reach their optimum five-pound broiler weight when they are about
seven weeks old. When that happens, the poultry company will bus
in a crew of seven or eight chicken catchers. The men will spend
the day grabbing chickens by their feet, and stuffing them into
plastic crates for the trip to the processing plant. Bare hands
make catching easier, and many chicken catchers have the scars to
bear testament. They do not wear masks to block out the dust and
stench, and they do not wash up before taking a lunch break
there is no place to. It is a dirty job, and some of the farmers
and poultry workers who spend their days in houses like this say
they have frequent health problems bouts of diarrhea, chronic
respiratory problems. But no one has linked their work environment
to their health problems.
That is where Ellen
Silbergeld comes in. A professor of Environmental Health Sciences
at the School, Silbergeld stands outside the chicken house, against
a wall that partly buffers the wind. She is dressed in sweatshirt
and slacks, and has closely cropped hair and a smile that rarely
leaves her face. When she talks, she speaks with the emphasis and
clarity of an experienced teacher, and at this moment she is chatting
with a chicken grower named Carol Morison, enumerating the potential
health hazards of working in a chicken house. Silbergeld believes
the list is long, though one aspect of chicken growing currently
troubles her most. She is holding some of it in a small plastic
bag. The bag contains chicken feed.
Producers began mixing tetracycline, penicillin, and other antibiotics
into feed for poultry more than 50 years ago after reports showed
the drugs hastened growth in chickens. Over the years, they continued
to blend in the drugs to stave off infections and promote growth
in poultry, as well as swine and cattle. But public health and consumer
groups fear that the antibiotics used in agriculture have contributed
to the frightening emergence of antibiotic resistance among disease-causing
bacteria a problem that is growing and becoming more inimical,
as more infections show resistance to two, three, or more antibiotics.
Until recently, research and surveillance efforts to control this
problem have focused on resistance carried to consumers through
the food chain. But Silbergeld wants to understand what antibiotic-resistant
bacteria might be doing in a different place: down on the farm.
Few scientists have looked here before.
"This is a completely neglected issue," says Margaret
Mellon, director of the Food and Environment Program at the Union
of Concerned Scientists. "Very few people in the scientific
community have addressed this issue. Ellen Silbergeld is stepping
into the breach."
Silbergeld, who joined the School's Department of Environmental
Health Sciences in January, plans a systematic study aimed at understanding
how antibiotic resistance emerges and spreads through farms and
beyond, and she is visiting Maryland's Eastern Shore to begin recruiting
volunteers on the Delaware-Maryland-Virginia peninsula known as
"We hypothesized that there are two sources of antibiotic-resistant
infection: chickens and clinical use," she explains. "What
are the odds that a person is carrying a bug that came from chickens?
What are the odds that a person is carrying a bug that came from
In a study now in the planning stages, Silbergeld will use novel
molecular technology to discern which infections came from either
source. She will also use the same technology to track the flow
of resistance as it wends its way through land, water, people, and
the overall ecosystem of the community.
Silbergeld has earned a reputation as an astute scientist and tenacious
advocate who chooses her research projects carefully and is wholly
dedicated to the work she decides to pursue. Trained at Hopkins
in environmental engineering sciences (PhD '72), she has published
hundreds of papers in environmental toxicology, including research
on the biological effects of lead, dioxin, and mercury. That work
undoubtedly contributed to her winning a MacArthur Foundation Fellowship
(popularly known as the "genius award") in 1993. But in
1982, she left what she calls her "scientific cocoon"
for a decade of advocacy work with the Environmental Defense Fund.
Silbergeld is still
humbled by the power of bacteria. "They rule this planet,"
she says. "Bacteria can and will develop resistance."
As director of the toxic chemicals program at EDF, Silbergeld argued
for policy and legislative changes to protect the public against
toxins such as lead and dioxin. The job pitted her against some
formidable opponents, such as the big guns of the petroleum industry
who were fighting efforts to remove lead from gasoline. What has
given her fortitude during such moments is the teaching of her parents,
Joseph and Mary Kovner, who were activists in the labor and civil
rights movements, and her Quaker upbringing. "I have a big
belief," she declares. "If you can discern the truth,
speak truth to power."
Silbergeld was drawn to the study of antibiotic resistance for the
same reasons she chose the myriad other environmental problems that
mark her career. "The science has so many elegant questions,"
she explains. "Yet the issue doesn't have the attention it
deserves. This is the opportunity to do important science and to
do something important." She has been told that she behaves
like a magpie. "I see something bright and glittering, and
I go after it."
A relative newcomer to the study of microbes, Silbergeld is still
awed and humbled by their power. "There's the dogma of the
bacteria: they rule this planet," says Silbergeld. "Bacteria
can and will develop resistance. The antibiotics we use are all-natural
molecules. Penicillin comes from a fungus. So [in a community of]
bacteria, [some] already possess the resistance genes. They have
ancestral knowledge of our little weapons we're selling in bottles."
A seminar a few years ago at the University of Maryland launched
Silbergeld on this research trajectory. The topic was nosocomial
infections infections originating in the hospital
and the speaker alluded to the role of food-borne bacteria such
as salmonella in transmitting antibiotic resistance. Silbergeld
prodded a colleague sitting next to her for more information.
Her colleague explained that chickens are fed sub-therapeutic amounts
of antibiotics, not enough to battle a full-blown infection but
enough to make them grow faster and stave off infection, and consequently,
an ideal amount to nurture resistant bacteria. Antibiotic-resistant
bacteria in manure could then stick to a bird all the way to the
was intrigued but puzzled. Sure, a microscopic amount of manure
might remain on a chicken, but what about the rest of the waste?
As an environmental engineer, says Silbergeld, "I know waste."
And she knew that the poultry industry in Delmarva alone produces
hundreds of thousands of tons of it each year. Her environmental
engineering sensibilities suggested that parts of the antibiotic
resistance story had not yet been told.
In the meantime, Silbergeld also spoke to University of Maryland
law professors Sherrilyn Ifill and Christopher Brown, whom she had
met when Brown was filing lawsuits on behalf of lead-poisoned children.
The lawyers were helping chicken workers who had formed a group
called the Delmarva Poultry Justice Alliance to seek ways to improve
their working conditions and procure better wages.
Brown introduced Silbergeld to the workers, and Silbergeld began
talking to them about their health concerns. She knew immediately
that the workers should be part of the study she envisioned. "An
axiom of environmental health and something [Hopkins Professor]
Reds Wolman taught me is if you're worried about environmental exposure,
go look at occupational exposure."
Silbergeld also began visiting poultry farms, talking to chicken
workers and growers, and simply "hanging out." A grower
with whom she has worked closely is Carol Morison, who is also the
executive director of the Delmarva Poultry Justice Alliance.
Morison jokes that she became a farmer the day she said "I
do" to her husband, Frank, who has been in the business for
all of his 43 years. "It's a good life but you can't make a
living on it," she asserts. Both she and her husband have full-time
jobs outside of the farm.
Speaking to Silbergeld behind the chicken house, Morison points
to a feed bin. Decades ago, Delmarva had many independent chicken
farmers, she says. "Now, farmers don't own the chickens or
the feed. We contract with Perdue [or another company]. The company
formulates the feed and dumps it in the feed bin." The feed
ticket lists some of the ingredients, but the precise formula is
a mystery. "Perdue doesn't tell us what the mixture is,"
says Morison. "It's supposedly a trade secret."
Silbergeld recently completed a study in which she examined whether
chicken workers, with all their exposure to the birds and manure,
were being colonized by bacteria (including resistant microbes)
that frequently grow in chickens. She also wanted to know whether
other members of the community were becoming infected, perhaps through
contact with poultry workers or some other pathway. With funding
from the U.S. Food and Drug Administration, she and her colleagues
enrolled 34 chicken workers and community residents, and screened
their stool samples for bacteria.
researchers found that 41 percent (7/17) of chicken catchers were
colonized with Campylobacter jejuni, a bacterium rarely found in
healthy people but found in many chickens and other farm animals.
They also identified campylobacter in 63 percent of poultry processing
plant workers. Further, and perhaps most surprising, all nine of
the community members who lived near but did not work in the poultry
industry tested positive for the bacterium. Silbergeld calls these
data "very worrisome."
The scientists are still analyzing the data, and do not yet know
whether the campylobacter are antibiotic resistant. However, many
of the volunteers did harbor antibiotic-resistant enterococci, a
common source of nosocomial infections such as blood infections
or urinary tract infections. Silbergeld says it's conceivable that
a person infected with enterococci may not experience any symptoms
and unwittingly act as a vector, passing infection to others. She
reported these findings in March at the International Conference
on Emerging Infectious Diseases in Atlanta, sponsored by the Centers
for Disease Control and Prevention.
Is campylobacter making the workers ill? That is hard to tell since
many of the poultry workers lack health insurance and do not go
to a doctor or hospital when they are sick. But informally, Silbergeld
has found that workers frequently have gastrointestinal symptoms
that would be consistent with campylobacteriosis.
Silbergeld is now embarked upon a research project that builds and
expands upon this survey. It is the most ambitious study that she,
and perhaps any scientist, has undertaken to understand the ecological
flow of antibiotic resistance.
With agricultural microbiologist Sam Joseph and agronomist Lewis
Carr, both from the University of Maryland, Silbergeld will first
define a region of several hundred square miles in the farm country
around Salisbury and Pocomoke City, Maryland. She will then recruit
volunteers in this geographic area, and determine whether antibiotic-resistant
infections among the group originated at a local hospital or a farm.
The researchers are focusing on enterococcus, which has shown increasing
antibiotic resistance. The key to the study is a gene chip, similar
to the technology scientists recently used to search for the source
of the anthrax used in the fall terrorism attacks. Shyam
Biswal, PhD, assistant professor, Environmental Health Sciences,
is designing the microchip for this study. The tool enables researchers
to trace the path of a bacterium through its evolutionary genetics
the constant genetic mutations that occur as bacteria divide,
multiply, and move through the environment.
by announcements from major poultry companies that they
are reducing antibiotics in their feed, Silbergeld wants
to see the data.
The gene chip will contain all of the genetic regions for antibiotic
resistance in enterococcus that vary from one strain of the bacterium
to the next. These variable regions include both the single nucleotide
polymorphisms (SNPs) and the longer sequences called variable number
tandem repeats (VNTRs).
Using the chip, Silbergeld and her colleagues will determine the
genetic resistance signatures of enterococci from farms as well
as those from local hospitals (possibly by screening sewage). They
will then use it to analyze enterococci resistance genes isolated
from volunteers' stool samples. By comparing genetic signatures,
the researchers will deduce which cases of genetic resistance originated
in the hospital and which on the farm.
Further, the scientists plan to use the technology to test bacterial
genes obtained from dirt, dust, and groundwater around the farm
to map the flow of resistance. Silbergeld entertains many scenarios
for a potential pathway. In one, for example, an antibiotic-resistant
bacterium would colonize a chicken, be excreted in the bird's feces,
get spread on a farm field with the manure, then seep into the groundwater
and maybe even into wells.
Hearing this, Morison nods knowingly and points to the fields surrounding
the chicken house. Land could not get any flatter or lower than
this. Many farmers have shallow wells, she notes.
"But there could be other scenarios," says Silbergeld.
"The airborne route could be the sleeper." The study includes
plans to test the air in chicken houses.
web of resistance is likely to be intricate, says Silbergeld. She
notes that resistance genes can cross species. "That's why
bacteria are so evil and cool. That's the hot scientific aspect
Part of the challenge in solving the antibiotic resistance quandary
is that no one knows how much antibiotics are used in American agriculture.
The Food and Drug Administration does not require companies that
manufacture antibiotics for agriculture to report these amounts.
Last year, however, the Union of Concerned Scientists published
an estimate showing that U.S. livestock producers use 24.6 million
pounds of antibiotics every year for non-therapeutic purposes, which
UCS says is 70 percent of the total used in the country.
The American Medical Association, the World Health Organization,
and many other health and scientific groups are seeking restrictions
or bans on the use of medically important antibiotics in agriculture.
Sweden and Denmark have banned the use of antibiotics for growth
promotion in agriculture. The European Union now has a similar prohibition.
Industry sees things differently. The UCS estimate is "way
off base," says Ron Phillips, a public information officer
with the Animal Health Institute, a trade group representing animal
pharmaceutical manufacturers. According to an AHI survey, only 20.5
million pounds of antibiotics go to animal use, and most of it for
treating and preventing disease. Further, AHI asserts that agricultural
use of antibiotics accounts for less than 5 percent of antibiotic
resistance in humans.
Nevertheless, Perdue and two other poultry companies, Tyson and
Foster Farms, say they have been reducing the amount of antibiotics
in their feed, according to a story in the Feb. 10 New York Times.
Perdue and Tyson say they are using only antibiotics that are not
similar to those used in human medicine. Foster Farms has withdrawn
all antibiotics except those used for treating sick birds.
Richard Lobb, a spokesman for the National Chicken Council, explains:
Companies have been able to decrease antibiotic use without sacrificing
animal survival, he says, due to improved growing conditions. "It's
related to improved breeding of animals, better husbandry, bio-security,"
companies, he says, now ask farmers to wear protective suits and
booties or to use a footbath before entering a chicken house to
protect the flock.
Out by the chicken house, Morison says she is dubious. Pointing
toward the building, she says that without antibiotics in the feed
"those birds would be dying." They are simply too densely
packed to avoid the spread of infection.
Silbergeld maintains her scientific reserve. Rather than rely on
company statements, she wants data. She plans to analyze the feed
samples she's gathered for their antibiotic content. Tucking the
bag safely in a pocket, she declares, "Just prove it."
But she is buoyed by the news story. It suggests that poultry companies
are starting to pay attention to public health and citizens' groups,
says Silbergeld. "We certainly welcome a very productive and
Many questions remain, however. "We don't really know to what
extent the agricultural use is part of the big picture of drug-resistant
infections," adds Silbergeld. "Ten percent? Is it only
through food? Are there other ways of transferring resistance? Are
there reservoirs of resistance?"
Despite these questions, the data are there to justify taking measures
to protect public health, says Silbergeld. She believes the FDA,
for example, should not license any further dual-use antibiotics
- those prescribed for both people and animals. In a New York Times
op-ed this past November, Silbergeld called on Bayer to withdraw
its agricultural drug Baytril, a fluoroquinolone that is chemically
similar to Ciprofloxacin, the antibiotic widely used last fall in
treating those exposed to anthrax.
It would be naïve to expect the United States to immediately
halt agricultural antibiotic use, she adds. Only relatively small
countries have taken such a step, and they've had to climb a steep
learning curve. In the vastly larger U.S. agricultural system, reducing
the industry's dependence on antibiotics would take a lot more thinking
and planning. "It would be as big as taking lead out of gasoline,"
remarks Silbergeld. But tough questions should not stymie efforts
to try, says this seasoned advocate.
What drives her to find the answers, says Silbergeld, are her visits
with the poultry workers. She has promised to return to Delmarva
this spring to explain the results of her study at an open meeting.
She will do the same when she completes her current study. This
promise, she says, keeps her focused. "We've got something
to live up to."