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Salmonella enteritidis change shape as they grow. This scanning electron
micrograph shows a mixture of small cells with filaments and very large cells
that lack filaments. Small cells arise only during certain growth stages and
efficiently contaminate eggs when the time is right. Click the image for
more information about it.
Tracking Antimicrobial Resistant Organisms
Durham March 8, 2005
Antibiotics have been used for years to fight bacterial infections,
but some bacteria are developing resistance to these antimicrobial drugs.
Agricultural Research Service (ARS)
scientists in Athens, Ga., are tracking antimicrobial resistance and seeking
ways to minimize it.
Fedorka-Cray, research leader of the agency's
Epidemiology and Antimicrobial Resistance Research Unit at Athens, leads a
team that is testing for antimicrobial resistance in food-borne microbes.
In these studies, bacterial samples are taken from sick farm animals,
healthy farm animals and animal slaughter facilities. The lab's scientists then
isolate, test and characterize more than 17,000 bacterial samples a year.
Patterns of resistance are difficult to discern because bacteria don't
react predictably and uniformly to antibiotic treatment. For instance, there
are many different types of Campylobacter, but each responds differently to
Another potentially harmful bacterium, Salmonella, has more
than 2,400 different types, and each one appears to develop resistance to
antibiotics at a different rate. Of all Salmonella types tested from
1997 to 2003, the rate of single-drug resistance has remained relatively stable
at 9.5 percent of the samples. However, the number of Salmonella types
that are resistant to more than five drugs rose from 11 percent to 20 percent.
Those that are resistant to more than 10 drugs rose from a scant 0.8 percent to
almost 6 percent.
Fedorka-Cray's research group has developed the nation's largest
descriptive database of resistant populations of bacteria recovered from
animals over time. The data will be used to determine the probability that
resistance will occur or be maintained if antibiotics are used. Changes in
antibiotic use in food-animal production are being made in response to the
development of resistance to the drugs.
more about the research in the March 2005 issue of Agricultural
ARS is the U.S. Department of
Agriculture's chief scientific research agency.