Direct detection of Salmonella Typhimurium on fresh spinach leaves using phage-based magnetoelastic biosensors

This paper presents an investigation into the use of phage-based magnetoelastic (ME) biosensors for the direct detection of Salmonella Typhimurium on fresh spinach leaves. The biosensors used in this investigation are composed of a freestanding, strip-shaped ME resonator (2 mm × 0.4 mm × 15 μm) immobilized with a landscape phage that possesses high binding affinity for S. Typhimurium. These biosensors were, due to their wireless, freestanding nature, directly placed on wet leaf surfaces pre-spiked with various concentrations of S. Typhimurium. The change in the resonant frequency of the biosensors caused by the selective binding of S. Typhimurium was, then, measured wirelessly and compared with that of control sensors (without phage). The presented methodology requires no pre-test sample preparation (i.e., washing of leaf surfaces and collection of Salmonella-containing liquid samples, followed by purification and concentration), and hence, a total test time of 45 min was realized. The limit of detection was determined from dose response plots to be 1.24 × 10³ cells/mm² and 1.29 × 10³ cells/mm² for the adaxial and abaxial surfaces of spinach leaves, respectively. In addition, the topography of leaf surfaces was characterized in terms of mean roughness and flatness along with their associated periodicity, all of which are anticipated to affect the physical contact between the placed biosensors and Salmonella cells on the leaf surface. The results indicate that the use of sufficiently small biosensors with a proper number may be the key to improving the limit of detection.