Immunoassays constitute another practical option for pathogen detection. Rather than analyzing nucleic acids, this type of assay is directed at identification of specific proteins or toxins. This methodology is based on the manufacture of antibodies specific for an organism or toxin. The most common format resembles that of an enzyme-linked immunosorbent assay (ELISA), in which one antibody is used in a capture capacity, and a second "signal" antibody is employed to bind in a sandwich format. A common immunoassay format links the signal antibody with an optical indicator that allows measurement of the toxin concentration. A number of immunoassays have been developed to target water-borne and other pathogenic microorganisms (Shelton and Karns, 2001; McBride et al., 2003). The throughput of immunological methods can be increased when combined with techniques such as flow cytometry, which in some cases can screen millions of cells or particles per hour for specific binding events.

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