Immunomagnetic separation and size-based detection of Escherichia coli O157 at the meniscus of a membrane strip

We developed a facile method for the detection of pathogenic bacteria using gold-coatedmagnetic nanoparticle clusters (Au@MNCs) and porous nitrocellulose strips. Au@MNCs were synthesized and functionalized with half-fragments of Escherichia coli O157 antibodies. After the nanoparticles were used to capture E. coli O157 in milk and dispersed in a buffer solution, one end of a test strip was dipped into the solution. Due to the size difference between the E. coli-Au@MNCcomplexes (approximately 1 mm) and free Au@MNCs (approximately 180 nm), only E. coli-Au@MNC complexes accumulated at the meniscus of the test strip and induced a color change. The color intensity of the meniscus was proportional to the E. coli concentration, and the detection limit for E. coli in milk was 103 CFU mL(-1) by the naked eye. The presence of E. coli-Au@MNC complexes at the meniscus was confirmed using a real-time PCR assay. The developed method was highly selective for E. coli when compared with Salmonella typhimurium, Listeria monocytogenes, and Staphylococcus aureus.