Enterohemorrhagic Escherichia coli Detection by Aptamer-Functionalized Stokes-Shifted Quantum Dots

Escherichia coli O157:H7 is a prolific serotype of the Shiga toxin-producing E. coli strain, known for its enterohemorrhagic pathogenicity and transmission in contaminated food and water. The O157:H7 serotype, evolving from the nonpathogenic sorbitol-fermenting O55:H7 strain, is widely implicated as the causative organism in numerous food poisoning outbreaks reported globally. The public health implications of the spread of this pathogen necessitate its detection through field-deployable sensing systems, reproducible at scale. Accordingly, this study describes the development of a simple aptasensing setup based on magnetically separated Quantum Dots (QDs) with typical Stokes-shifted luminescence, decorated with an E. coli-specific aptamer sequence. The QD was covalently functionalized with the aptamer (Apt) by simple carbodiimide chemistry, producing a QD-Apt conjugate, and incubated with a cDNA-MB conjugate (produced by covalently functionalizing magnetic beads (MB) with a ssDNA sequence partially complementary to the aptamer), yielding the QD-Apt@cDNA-MB complex. Displacement of the cDNA-MB conjugate from the complex by cells of the pathogen, with which the QD-Apt binds with higher affinity, led to the loss of luminescence intensity in the magnetically separated supernatant containing the remaining QD-Apt@cDNA-MB construct (alongside the displaced cDNA-MB fragments), constituting the signal transduction mechanism of the E. coli detection system. The luminescent biosensor achieved a low detection limit (∼56 CFU · mL^-1), high sensitivity, and specificity for the target pathogen, against other common food pathogens. The performance of the luminescent biosensor, in the absence of prior enrichment or sample concentration steps, renders it promising for large-scale environmental monitoring applications, toward the control of E. coli O157:H7 contamination.