Development of a rapid response biosensor for detection of Salmonella typhimurium

An integrated optic interferometer for detecting foodborne pathogens was developed. The interferometer is a planar waveguide with two thin antibody-coated channels of immunochemically selective agents that interact with antigen molecules. One channel is coated with antibody to Salmonella as a sample, and the other is coated with human immunoglobulin G as a reference channel by using reductive amination. Salmonella was introduced onto the sensing channels through the flow cell on the channels. Phase shift (pi) generated by refractive index variation, as determined by interfering the perturbed sample channel with an unperturbed reference channel and observing the fringe shift, was used for detection. Salmonella Typhimurium (heat-treated or boiled) was detected by binding to antibody against Salmonella common structural antigen immobilized on a silane-derived sensor surface at concentrations in the range of 1x10(5) to 1x10(7) CFU/ml. Salmonella (1x10(7) CFU/ml) mixed with Escherichia coli (1x10(7) CFU/ml) were readily detected without any decrease in sensitivity by the direct assay. Application of a sandwich assay with a second antibody or a gold-conjugated antibody increased the detection limit to 1x10(5) CFU/ml within a 10-min reaction time. Various methods for the immobilization of the capture antibody to the biosensor channels were compared. The greatest binding response was observed in a direct reductive amination method with a long reaction period and increased the detection limit of direct binding of Salmonella antigen to 1x10(4) CFU/ml. The biosensor was able to detect Salmonella Typhimurium in chicken carcass wash fluid originally inoculated at a level of 20 CFU/ml after 12 h of nonselective enrichment. The planar optic biosensor shows promise as a fast, sensitive, reliable, and economical means of detecting food pathogens in the future.