Agriculture Reference
In-Depth Information
metal layer (SPR) (Tubb and others 1997). Thus, the main difference between ATR
and SPR is that the former measures the changes of EW at the interphase directly and
the latter measures the induced changes in the resonant excitation of the free electrons
of the metal layer providing the anchoring sites for the specifi c receptor. Both ATR
(Geng and others 2006) and SPR (Fratamico 1998) have been applied to measure food
pathogens.
Acoustic wave biosensors are based on the decrease of oscillating frequency of
bioreceptor-coated piezoelectric crystals upon the binding of target analyte. The
change in frequency is governed by the ratio of the mass of analyte and the piezoelec-
tric crystal (Griffi ths and Hall 1993). Recently, this type of approach has been applied
to measure
E
. coli O157:H7 (Campbell and Mutharasan 2007). The sensitivity of this
type of sensor is superior. However, the fabrication and treatment of the crystal require
considerable technical training and expertise (Invitski and others 1999).
The electrochemical sensor involves the use of a receptor-coated electrode that
expresses a change in electro-properties upon the binding of target analyte. The best
known electrochemical devise for measuring specifi c analyte is the glass pH-electrode
that expresses potential change upon the binding of protons on the glass surface. In
the medical fi eld, the most widely used electrochemical sensor is the glucose monitor-
ing sensor (D'Costa and others 1986). This approach has also been applied to measure
food pathogens using alkaline phosphatase labeled antibodies to link to the bacteria
that were captured by the antibodies immobilized on the electrode (Gehring and others
1996). The enzyme then was used to convert phenolic phosphate to phenolic com-
pound that could be characterized by its specifi c redox potential. The magnitude of
the redox current could relate to the number of pathogen captured.
Thermometric biosensors exploit the fundamental property of biological reactions,
i.e. absorption or evolution of heat (Spink and Wadsö 1976). This is refl ected as
a change in the temperature within the reaction medium. Its exploitation in biosen-
sors led to the development of thermometric devices (Mosbach and Danielsson 1974).
These predominantly measure the changes in temperature of the circulating fl uid
following the reaction of a suitable substrate with the immobilized enzyme mole-
cules. The most basic version of such a device is a thermometer, routinely used
for measurement of body or ambient temperature. Based on similar principles,
in thermometric devices the heat is measured using sensitive thermistors. Such a
device is popularly referred to as an enzyme thermistor, ET (Danielsson and Mosbach
1988). Several instruments were designed in the past 2 decades and they combined
the principles of calorimetry, enzyme catalysis, immobilization on suitable matrices,
and fl ow injection analysis for small metabolite detections. Because of its low sen-
sitivity, the application of thermal sensor for pathogen detection has not yet been
attempted.
The detection of microorganisms by DNA amplifi cation has been extensively
applied. Using polymerase chain reaction (PCR) target nucleic segments of defi ned
length and sequence are amplifi ed by repetitive cycles of strand denaturation, anneal-
ing, and extension of oligonucleotide primers by the thermostable DNA polymerase,
Thermus aquaticus
(Taq) DNA polymerase (Bsat and others 1994). PCR has distinct
advantages over culturing and other methods for the detection of microbial pathogens
and offers the advantages of specifi city, sensitivity, rapidity, accuracy, and capacity to
