In our current system, food from many producers is transported to centralized processing plants where it is mixed. This practice leads to an enhanced risk of widespread contamination by pathogens or chemicals due to cross-contamination and inherent harvest-to-consumption delays. To make matters worse, 20 percent of U.S. foods are imported from countries that may have drastically different food safety regulations (Kowitt 2016). Economically devastating food recalls result; approximately 47 percent are due to bacterial contamination, 6 percent are chemical7 (Kowitt 2016). Current estimates suggest that human health-related economic losses alone add up to an astronomical $55.5 billion in the United States every year. Realizing that the current food production system can not be drastically changed overnight that we cannot readily change the overall structure of our food system, what can be done? One answer is improved monitoring for food safety.
Traditional approaches for detecting biological contaminants in food and food processing equipment include biochemical analysis of microbial growth, polymerase chain reaction (PCR)-based detection of pathogen-associated nucleic acids, and enzyme-linked immunosorbent assays (ELISAs) for the antibody-based quantification of diverse analytes.8
While generally robust and sensitive, these techniques are too slow. Delays in detection lead to significant economic losses and human disease. Thus, new biotechnologies are being developed for real-time in situ detection. Common platforms include biosensors and bioreporters. Since bioreporters are more often used for the detection of chemical contamination associated with environmental pollutants, they are discussed in Chapter 5.
Biosensors are self-contained diagnostics used to detect diverse analytes (e.g., whole microbial cells or virions, proteins, (in)organic molecules) within complex samples in real time. When applied to food and water samples, they can rapidly identify compromised consumables (McGrath, Elliott, and Fodey 2012; Kabessa et al. 2016). Early detection allows for quick action; hopefully sidestepping product, financial, and human losses due to food recalls.
Biosensors are made of two components—a receptor and transducer (Leonard et al. 2003). The receptor is a solid platform onto which capture biomolecules are covalently attached. Platform materials are varied. Examples include 2D surfaces (e.g., glass, silicon, gold), 3D matrixes (e.g., porous materials, agarose gels, dextran-based hydrogels), or semi-elastic polymers such as those made of silicon (Zhang et al. 2012). Antibodies or recombinant proteins are common capture molecules. As sample materials9 flow over the receptor, the specific analyte of interest will bind, thereby changing a measurable physiochemical characteristic of the surface. Examples include a quantifiable change in fluorescence or chemiluminescence (CL), mass, temperature, or electrochemical potential (Leonard et al. 2003). The surface change is detected by the second component of the biosensor—the transducer. Transducers transform the surface signal into an electrical signal that is amplified and displayed for the user (Leonard et al. 2003).
Detection is achieved by label or label-free mechanisms. Label-based detection, involving fluorophores (e.g., Cy5 dyes) or enzyme-based CL, is conventionally used in the detection of DNA microarrays, ELISAs, and western blots. Binding of the labeled analyte increases the fluorescent or CL signal, which is detected by laser scanning. In contrast, label-free detection methods measure a physical change associated with the receptor platform when the analyte binds. Label-free methods are simpler, due to limited pre-assay sample handling. This simplicity often translates to a reduction in cost and time. Optical and electrochemical transducers10 are commonly employed in label-free detection (McGrath, Elliott, and Fodey 2012).
Optical transducers11 are one of the most promising label-free detection strategies. To understand how optical transducers work, it is helpful to be familiar with a physical phenomenon called surface plasmon resonance (SPR). When polarized light travels through a prism and hits a metal surface (such as the underside of a gold biosensor platform), it is reflected. At a particular angle of incidence, light is absorbed by the metal and converted into an oscillating wave of free electrons that propagates across the surface (i.e., SPR). As a result, the light at that particular angle is not reflected. By detecting the light that does reflect off of the surface, the angle of SPR can be deduced (i.e., it is a shadow in the band of reflected light). The angle of SPR is dependent on the mass of the reflecting material. In a biosensor equipped with an optical transducer, light is shown through a prism onto the underside of a platform (see Figure 4.1). When analyte molecules bind, a slight change in surface mass occurs. This change is indirectly detected by a change in the angle of SPR. The measurement is quantitative until the biosensor reaches capture molecule saturation.
Figure 4.1. Biosensor with an optical transducer.
Biosensor performance is influenced by numerous factors—sample preparation, capture molecule, surface material, and device size. Due to the complexity of food samples, microbial growth or immunoseparation of contaminants may be employed prior to testing (Leonard et al. 2003). High affinity molecules support the selective capture of single analytes. When whole microbial cells or virions are being detected, capture antibodies should bind antigens that are (1) surface exposed, (2) specific to
the pathogenic form12 of the organism, and (3) constitutively expressed (Leonard et al. 2003). While 3D matrices may enhance the detection of small molecules, due to the inherent increase in surface area contact between the biosensor receptor and sample, they may inhibit the capture of bulky analytes such as whole cells. Miniaturization of the biosensor device allows for reduced sample consumption and better regulation of ambient temperature (a parameter known to greatly influence analyte binding) (Leonard et al. 2003). Miniaturization also supports the development of biosensor arrays,13 platforms onto which a series of capture molecules are applied for the simultaneous detection of multiple analytes (Zhang et al. 2012). In some cases, compact disk readers have been ingeniously repurposed to “read” biosensor arrays (Zhang et al. 2012).