User-friendly, signal-enhanced planar spiral coil-based magnetoelastic biosensor combined with humidity-resistant phages for simultaneous detection of Salmonella Typhimurium and Escherichia coli O157:H7 on fresh produce

A user-friendly, signal-enhanced magnetoelastic (ME) biosensor was newly constructed, consisting of three sensors, a movable pencil-type planar spiral coil, and a set of hand-held signal amplifiers. It was combined with humidity-resistant, wild-type phages instead of humidity-susceptible E2 phage to simultaneously detect Salmonella Typhimurium (ST) and Escherichia coli O157:H7 (EC) on the surface of an apple. The set of hand-held signal amplifiers, four times smaller than those in the planar spiral coil-based ME biosensor, was constructed with an optimal DC magnetic field strength of 76 G. These optimized hand-held signal amplifiers enhanced the signal amplitude of the sensor by up to 1.5 times, enabling the distinction of individual peaks from three sensors. Both phage-immobilized sensors detected each target specifically under ambient humidity without the need for a bulky humidifier. The optimized exposure time for phage binding with each target on fresh produce was 16 min, almost twice as fast as the rectangular solenoid coil-based ME biosensor. The signal-enhanced ME biosensor detected its target bacteria from a mixture of ST and EC inoculated on an apple simultaneously with an enhanced detection limit of 1.7 ± 0.4 log CFU/25 mm² for ST and 1.6 ± 0.3 log CFU/25 mm² for EC. Notably, the detection limit and sensitivity did not show significant differences between each single analyte and bacterial mixture, indicating no interference with another coexisting pathogens. Our findings highlight the signal-enhanced ME biosensor as a rapid, efficient, specific, and user-friendly method for the simultaneous detection of fresh produce-borne pathogens.