"These screening assays are alternatives to biological assays
used by regulatory agencies," Schneider says. "The fluorescence
assay is very rapid and detects the drugs when they are at or above
the tolerance level. TRL methods require more cleanup, but have greater
Chemist Steven Lehotay, along with a previously visiting scientist,
Michelangelo Anastassiades, developed the QuEChERS method, which stands
for "quick, easy, cheap, effective, rugged, and safe" and
is pronounced "catchers." The streamlined approach makes it
easier and less expensive for analytical chemists to examine fruits
and vegetables for pesticide residues. Lehotay says the method reduces
procedural stepsand that lessens the chance for a mistake. A single,
easy-to-clean Teflon tube is the only item to be washed and reused,
eliminating all the glassware used in conventional methods. Furthermore,
less than 10 mL of solvent waste is generatedmuch less than the
75-450 mL generated by other methods.
"Several monitoring laboratories, including a few in FDA, are
evaluating QuEChERS for use in routine monitoring and other approaches
designed to safeguard the food supply," Lehotay says. He and colleagues
are now working to adapt the concept to analyze meats for veterinary
Chemist Keith Fagerquist, along with Lehotay and chemist Alan Lightfield,
developed a method to measure and confirm beta-lactam antibiotics in
pork and cattle kidney tissue. The method-which uses liquid chromatography/tandem
mass spectrometryis fast and looks for multiple residues in tissue
samples, where antibiotics tend to concentrate the most.
Methods Detect Pathogens and Toxins in Food
Antibodies are protein molecules that bind to antigenssuch as
bacteriaand remove them from the body. Researchers can use antibodies
to isolate pathogens or chemicals in food products as well.
Andrew Gehring, another chemist in the unit, is working with Tu on
a procedure that combines immunomagnetic capture with TRF to simultaneously
detect Escherichia coli and Salmonella in ground beef,
ground turkey, alfalfa sprouts, and seeds. The procedure uses magnetic
beads that are coated with pathogen-specific antibodies. The antibodies
bind to the bacteria, and the magnetism pulls them out of complex mixtures
of food. Once extracted, the bacteria can be more easily detected.
Gehring is also developing a luminescence-based method coupled with
an ELISA (enzyme-linked immunosorbent assay) to detect and confirm E.
coli O157:H7. An ELISA is a sensitive laboratory test that uses
antibodies and enzymes to detect and measure specific antigens in samples.
Gehring's test can be completed in 8 hours and can detect 1-10 bacteria
per gram of ground meat. USDA's Food Safety Inspection Service (FSIS)
would ultimately like to be able to detect 1 bacterium in 25 grams of
Chemist Marjorie Medina developed a biosensor immunoassay using surface
plasmon resonance (SPR) to detect Staphylococcus aureus enterotoxin
A (SEA) and B (SEB)-toxins that cause gastroenteritisin foods
such as ham, milk, and eggs. Conventional heating and processing kills
the bacterium but not its toxins. Bacteria produce toxins under stressful
conditions, such as when they are too crowded or denied food or when
they're fighting back against antibiotics.
"SPR uses light reflected off thin metal films," Medina explains.
"Toxin molecules in the sample bind to the sensor surface, and
the refractive index at the surface changes. The time it takes for a
response from the interaction provides a measure of how much toxin,
if any, is actually present in the food sample."
Medina says that FSIS is interested in an alternative to the conventional
method to detect enterotoxins in whole eggs. Her semi-automated method
has several advantages over other methods and may detect multiple bacterial
toxins in a single food sample.
Medina also developed a latex particle agglutination assay for detection
of SEA and SEB that causes the toxins to clump together. The method
takes advantage of an antibody's ability to bind to a unique antigen
in pathogen cells. The assay is simpler to use than other methods and
can detect as little as 10 parts per billion of toxin per gram of sample.By
Agricultural Research Service Information Staff.
This research is part of Food Safety (Animal and Plant Products),
an ARS National Program (#108) described on the World Wide Web at www.nps.ars.usda.gov.
Shu-I Tu is in the USDA-ARS
Microbial Biophysics and Residue Chemistry Research Unit, Eastern
Regional Research Center, 600 East Mermaid Ln., Wyndmoor, PA 19038-8598;
phone (215) 233-6466, fax (215) 233-6581.
"New Detection Methods Improve Food Safety" was published
in the January
2005 issue of Agricultural Research magazine.