Detection of Salmonella typhimurium Grown Directly on Tomato Surface Using Phage-Based Magnetoelastic Biosensors

A phage-based magnetoelastic (ME) biosensor method is being developed for on-site pathogen detection in fresh fruits and vegetables. Salmonella typhimurium was directly grown with minimal nutrients on tomato surfaces in order to mimic natural environmental conditions. S. typhimurium was inoculated on the surface of fresh tomatoes, and ME biosensors were used to detect the bacteria. The populations of S. typhimurium after 24-h incubation time at 37A degrees C and 100% relative humidity were 6.1 and 7.8 logCFU/cm(2) after starting with initial inoculations of 3.0 and 5.0 logCFU/cm(2), respectively. After evident growth, measurement sensors with E2 phage immobilized on their surfaces and control sensors devoid of E2 phage were placed on the inoculated tomato surfaces and the resonant frequency shifts were measured. As the population of S. typhimurium increased, the resonant frequency shifts of the measurement sensors significantly increased (P < 0.01), exhibiting 6,680 A +/- 665 and 9,384 A +/- 457 Hz for the populations of 6.1 and 7.8 logCFU/cm(2), respectively. SEM images confirmed that the measurement sensor resonant frequency shifts were due to S. typhimurium binding with E2 phage. This study demonstrated that the ME biosensor method could detect S. typhimurium grown directly on tomato surfaces with limited sample preparation procedures. Therefore, the ME biosensor could be applied as a cost and time effective, relatively simple, practically suitable on-site detection method for S. typhimurium in fresh produce.