Direct detection of Escherichia coli O157:H7 in unpasteurized apple juice with an evanescent wave biosensor

An evanescent wave biosensor was used to detect Escherichia coli O157:H7 in unpasteurized apple juice. Light is launched from a 635 nm laser diode into silica or polystyrene optical waveguides, generating an evanescent field which extends from the waveguide surface. Fluorescent molecules within the evanescent field are excited resulting in an emission signal that the biosensor then detects and quantifies. A sandwich immunoassay was performed on the waveguides using cyanine 5 dye-labeled anti-E. coli O157:H7 antibodies for generation of the specific fluorescent signal. The lower limit of detection was between 6.0 x 10(2) and 6.0 x 10(4) CFU/mL with silica waveguides and between 3.2 x 10(2) and 3.2 x 10(4) CFU/mL using polystyrene waveguides. One-hundred percent correct identification of true positive samples occurred at 6.0 x 10(4) and 3.2 x 10(4) CFU/mL for silica and polystyrene waveguides, respectively. Signals from a variety of non-E. coli O157 bacteria, including closely related enterotoxigenic strains of E. coli at concentrations of similar to10(6) CFU/mL, were below the limits of detection. Assays were conducted in near real-time with results obtained within 15 min of sample processing.