Agriculture Reference
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failure and hemolytic anemia. From 0 to 15% of hemorrhagic colitis victims may
develop HUS, which can lead to the permanent loss of kidney function. In the elderly,
HUS, plus two other symptoms, fever and neurological symptoms, constitutes throm-
botic thrombocytopenic purpura (TTP). The mortality rate of this condition in the
elderly can be as high as 50%. Thus, there is a need of rapid and sensitive methods
to detect this pathogen in foods.
Traditional bacterial pathogen detection methods can take several days to confi rm
a positive sample. In the case of Campylobacter , culturing and plating takes 14-16
days for a positive result (Brooks and others 2004). Different selective media are used
for the detection of particular bacteria species. They can contain inhibitors (in order
to stop or delay the growth of nontargeted strains) or particular substrates that only
the targeted bacteria can degrade or that confers a particular color to the growing
colonies, such as rainbow agar for Salmonella detection. (Fratamico 2003). Although
these methods are inexpensive they are very time consuming. To assure the safety of
our foods and a rapid response to help those affl icted, clearly more rapid detection
methods are needed.
Biosensors for Pathogen Detection
Efforts to develop alternative food pathogen detection methods in general include the
development of biosensor technologies for pathogen detection. Biosensors use bio-
logical receptor compounds (e.g., antibody, enzyme, nucleic acid, etc.) and the trans-
duction of the molecular interaction through changes in physical and/or physicochemical
properties in real time to detect the presence of the entity specifi c to the bioreceptor
(Leonard and others 2003) Principally, there are four types of biosensors that measure
signal transduction through changes in optical, mass, electrochemical, and thermal
properties (Goepel 1991; Seyhi 1994; Goepel and Heiduschka 1995). Some of the
general principles and applications of biosensors are briefl y described as follows.
Optical biosensors based on the evanescent wave (EW) use the technique of attenu-
ated total refl ection (ATR) spectroscopy and surface plasmon resonance (SPR) to
measure real-time interaction between biomolecules. The basis of ATR is the refl ection
of light inside the core of a waveguide when the angle of incidence is greater than the
critical angle. Waveguides can be slab guides, planar integrated optics, or optical
fi bers. Light waves are propagated along waveguides by the law of total internal
refl ection (TIR). Even though the light is totally internally refl ected, the intensity does
not abruptly fall to zero at the interface, resulting in generation of evanescent wave
(EW), which penetrates exponentially into the medium of lower refractive index
(Squillante 1998). The wavelength of light, ratio of the refractive indices, and angle
of the light at the interface determine the penetration depth (Anderson and others
1993), which are typically 50 to 1000 nm; thus the EW is able to interact with many
monolayers at the surface of waveguides (Lave and others 1991). Reactions occurring
very close to the interface perturb the evanescent wave, and the changes in signals
can be related to the amount of binding between the target and immobilized ligand at
the interface.
When metal surfaces are used to immobilize the receptors, the change in EW
induced by the surface binding may also change the plasmon resonance of the surface
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