Escherichia coli O157:H7 (E.coli O157:H7) is a common pathogen that causes millions of foodborne diseases worldwide every year and can be fatal even at low concentrations. Thus, it is important to determine the contamination in real food on its early state. However, several food matrices such as fats and proteins found in sausage can interfere with the separation and early diagnosis of food poisoning bacteria. Herein, we report a sensitive and rapid colorimetric immunoassay of E.coli O157:H7 in sausage by employing magnetophoresis and nanozyme-based color development, that uses dextrin-coated iron oxide nanoparticle whose dispersity was greatly enhanced by (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) and functionalizing its surface with a specific antibodies (immuno-Dex@IONPs). Nylon-flocked swabbing was employed to recover the bacteria present on the surface of the sausage through electrostatic interaction, followed by twirling to viscous solution whose viscosity was optimized with hyaluronic acid (HA). Upon addition of immuno-Dex@IONPs, viscous HA solution become the magnetophoresis medium to separate bacteria@immuno-Dex@IONPs complexes from free immuno-Dex@IONPs. A large complex of bacteria@immuno-Dex@IONPs that were formed in the presence of target bacteria were shown to have significantly enhanced mobility upon the application of external magnetic field. The concentration of target bacteria was determined by the intensity of color generated by the peroxidase-like activity of IONPs and substrates, hydrogen peroxide (H2O2) and 3,3`,5,5`- tetramethybenzidine (TMB). The limit of detection (LOD) for E. coli O157:H7 was determined to be as low as single digit level in sausage sample. The great sensitivity of this system for detecting E.coli O157:H7 is due to the high capture efficiency of nylon flocked swab from the complex food matrices and the specific signal amplification through the catalytic activity of Dex@IONPs.

Colorimetric detection of Escherichia coli O157:H7 by using magnetophoresis and iron oxide nanocatalyst

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