Biomedical Engineering Reference
In-Depth Information
Analysis of foods for the presence of both pathogenic and spoilage bacteria is a standard
practice for ensuring food safety and quality. Conventional bacterial testing methods rely
on specific microbiological media to isolate and enumerate viable bacterial cells in foods.
Generally, it consists of five steps involving preenrichment, selective enrichment, selective
plating, biochemical tests, and serological tests. These methods are sensitive, and can give
both qualitative and quantitative information on the number and the nature of the
microorganisms present in a food sample. However, this method is time consuming,
requiring 5-7 days, and relies on the bacteria's ability to multiply and form visible
colonies. Moreover, culture-medium preparation, inoculation of plates, colony counting,
and biochemical characterization make these methods very labor intensive. Especially in
the food industry, there is a need for more rapid and user-friendly methods to provide
adequate information on the possible presence of pathogens in raw materials and ready-
to-eat food products, for manufacturing process control, and for the monitoring of clean-
ing and hygienic practices.
18.3
Detection of Foodborne Pathogens Using Optical Biosensors
Biosensors use a combination of biological receptors and physical or chemical transduc-
ers, which represent a new and unique technology with great potential to meet the need
for rapid detection of low levels of biomolecules (2-5). Optical sensors measure a param-
eter of the reaction between a receptor and an analyte as a quantifiable optical signal.
Optical biosensors offer the advantages of noninvasive, nondestructive, continuous, and
simultaneous multianalyte detection (6). Optical systems do not interfere with metabo-
lism, and thus in vivo measurements are possible. In this chapter, some applications of the
following optical sensing principles in foodborne pathogen detection are discussed: (i)
SPR, (ii) resonance mirror (RM), (iii) fiber-optic biosensor, (iv) array biosensor, (v) Raman
spectroscopy, and (vi) light-addressable potentiometric sensor (LAPS).
18.3.1
Surface Plasmon Resonance
SPR is the phenomenon that occurs as a result of total internal reflection (TIR) of light
between a dielectric and a metal interface. SPR propagates along the surface and presents
itself as an electromagnetic field. Energy from the incident light is absorbed by the metal,
resulting in a decrease in reflected-light intensity. This reflectance minimum appears in the
reflected light at an acutely defined incident angle (resonance angle), which is dependent
on the refractive index of the medium close to the metal-film surface. Changes in refrac-
tive index within the evanescent field result in a shift of resonance angle, defined as an
SPR response. When biomolecules are adsorbed or they interact with already immobilized
molecules within a probed volume, defined by the size of the illuminated area and the
evanescent wave (EW) depth, an increase in surface concentration occurs and the reso-
nance angle shifts to greater values (7-9). When used to detect biomolecules, three types
of transducers are typically used in SPR (Figure 18.1). These transducers are (i) prisms
coated with a thin film of metal, usually gold or silver (~55-nm thick), typically known as
the Kretschmann configuration; (ii) waveguide coupled with a thin film of metal; and (iii)
metallized diffraction gratings where the metal thickness can be much greater (up to 150-
nm thick) (10). Several manufacturers now offer SPR sensors under the above configura-
tion including Biacore (Biacore International SA, Switzerland), Affinity Sensors
