The use of biosensors is an exciting field in applied microbiology. The basic idea is simple, but the actual operation is quite complex and involves much instrumentation. Basically, a biosensor is a molecule or a group of molecules of biological origin attached to a signal recognition material. When an analyte comes in contact with the biosensor, the interaction will initiate a recognition signal which can be reported in an instrument.

Many types of biosensors have been developed, such as enzymes (a great variety of enzymes have been used), antibodies (polyclonal and monoclonal), nucleic acids, cellular materials, and others. Whole cells may also be used as biosensors. Analytes detected include toxins (staphylococcal enterotoxins, tetrodotoxins, saxitoxin, botulinum toxin, and others), specific pathogens (salmonella, staphylococcus, Escherichia coli O157:H7, etc.), carbohydrates (fructose, lactose, galactose, etc.), insecticides and herbicides, ATP, antibiotics (e.g., penicillins), and others. The recognition signals used include electrochemical (potentiometry, voltage changes, conductance and impedance, light addressable, etc.), optical (such as UV, bioluminescence and chemilumin-escence, fluorescence, laser scattering, reflection and refraction of light, surface plasmon resonance, and polarized light), and miscellaneous transducers (such as piezoelectric crystals, thermistors, acoustic waves, and quartz crystals).

An example of a simple enzyme biosensor is the sensor for glucose. The reaction involves the oxidation of glucose (the analyte) by glucose oxidase (the biosensor) yielding the end products, gluconic acid and hydrogen peroxide. The reaction is reported by a Clark oxygen electrode which monitors the decrease in oxygen concentration amperometrically. The range of measurement is from 1 to 30 mM with a response time of 1 to 1.5 minutes and a recovery time of 30 seconds. The lifetime of the unit is several months. Some of the advantages of enzyme biosensors are their strong binding to the analyte, high selectivity and sensitivity, and rapid reaction time. Some of the disadvantages are expense, loss of activity when enzymes are immobilized on a transducer, and loss of activity due to deactivation. Other enzymes used include galactosidase, glucoamlyase, acetylcholinesterase, invertase, and lactate oxidase. Excellent review articles and books on biosensors are presented by Eggins [20], Cunningham [21], Goldschmidt [22], and others.

Recently much attention has been directed to the field of "biochips" and "microchips" development to detect a great variety of molecules including foodborne pathogens. Due to advancements in miniaturization technology, as many as 50,000 individual spots (e.g., DNA microarrays), with each spot containing millions of copies of a specific DNA probe, can be immobilized on a specialized microscope slide. Fluorescent labeled targets can be hydridized to these spots and be detected. An excellent article by Deyholos et al. [23] described the application of microarrays to discover genes associated with a particular biological process such as the response of a plant (arabidopsis) to NaCl stress and detailed analysis of a specific biological pathway such as one-carbon metabolism in maize.

Biochips can also be designed to detect all kinds of foodborne pathogens by imprinting a variety of antibodies or DNA molecules against specific pathogens on the chip for the simultaneous detection of pathogens such as salmonella, listeria, Escherichia coli, and Staphylococcus aureus on the same chip. According to Heron writing in 2000 [24], biochips are an exceedingly important technology in life sciences, and at that time the market value was estimated to be as high as $5 billion by the middle of the present decade. This technology is especially important in the rapidly developing field of proteomics which requires massive amount of data to generate valuable information.

Certainly, the development of these biochips and microarray chips is impressive for obtaining a large amount of information for biological sciences. As for foodborne pathogen detection, there are several important issues to consider. These biochips are designed to detect minute quantities of target molecule. The target molecules must be free from contaminants before being applied to the biochips. In food microbiology, the minimum requirement for pathogen detection is 1 viable target cell in 25 g of a food such as ground beef. A biochip will not be able to seek out such a cell from the food matrix without extensive cell amplification (either by growth or PCR) or sample preparation by filtration, separation, absorption, centrifugation, etc., as described in this chapter. Any food particle in the sample will easily clog the channels used in biochips. These preparations will not allow the biochips to provide "real-time" detection of pathogens in foods.

Another concern is viability of the pathogens to be detected by biochips. Monitoring the presences of some target molecule will only demonstrate the presence or absence of the target pathogen and will not show the viability of the pathogen in question. Some form of culture enrichment to ensure growth is still needed in order to obtain meaningful results. It is conceivable that the biomass of microbes can be monitored by biochips but instantaneous detection of specific pathogens such as salmonella, listeria, and campylobacter in a food matrix during food processing operations is still not possible. The potential of biochip and microarrays for food pathogen detection is great, but at present much more research is needed to make this technology a reality in applied food microbiology.

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