Dramatically Enhanced Immunochromatographic Assay Using Cascade Signal Amplification for Ultrasensitive Detection of <i>Escherichia coli</i> O157:H7 in Milk

Abstract

The conventional colloidal gold immunochromatographic assay (AuNP-ICA) cannot meet the requirements for the rapid and sensitive detection of <i>Escherichia coli</i> (<i>E. coli</i>) O157:H7 because of its poor sensitivity. Herein, a novel two-step cascade signal amplification strategy that integrates in situ gold growth and nanozyme-mediated catalytic deposition was proposed to enhance the detection sensitivity of conventional AuNP-ICA dramatically. The enhanced strip displayed ultrahigh sensitivity in <i>E. coli</i> O157:H7 detection and had a detection limit of 1.25 × 10¹ CFU/mL, which is approximately 400-fold lower than that of traditional AuNP-ICA (5 × 10³ CFU/mL). The amplified strip had no background signal in the T-line zone in the absence of <i>E. coli</i> O157:H7 even after one round of cascade signal amplification. The enhanced strip demonstrated excellent selectivity against <i>E. coli</i> O157:H7 with a negligible cross-reaction to nine other common pathogens. Intra-assays and interassays showed that the improved strip has acceptable accuracy and precision for determining <i>E. coli</i> O157:H7. The average recoveries in a real milk sample ranged from 87.33 to 112.15%, and the coefficients of variation were less than 10%. The enhanced strip also showed great potential in detecting a single <i>E. coli</i> O157:H7 cell in a 75 μL solution.