McMaster University researchers have demonstrated the ability of a biosensing platform to enable visual detection of Escherichia coli contamination in a variety of food matrices, including milk, ready-to-eat (RTE) foods, produce, and leafy greens. The platform uses a deoxyribozyme (DNAzyme)-crosslinked hydrogel that produces a visible color change when E. coli is present, enabling equipment-free, point-of-use detection.

A pre-publication version of the study was made available by Nature’s npj Science of Food.

The researchers described the sensor as a low-cost system designed to support onsite monitoring for foodborne pathogens along the food supply chain. The hydrogel contains a DNAzyme-substrate complex that responds specifically to E. coli. When the bacteria are present, proteins released during bacteriophage-induced cell lysis trigger cleavage of the DNA crosslinks within the gel. This process causes the hydrogel to degrade and release entrapped gold nanoparticles, producing a visible color change that can be interpreted with the naked eye.

Detection Demonstrated in Multiple Food Types

The researchers tested the sensor’s functionality across several food products known to be associated with E. coli contamination.

In milk samples spiked with approximately 10⁵ colony-forming units per milliliter (CFU/mL) of E. coli, the sensor successfully detected the pathogen after an 18-hour incubation at 37 °C. Additional testing indicated the system could detect contamination at concentrations corresponding to approximately 10² CFU/mL in diluted milk samples.

The platform also identified E. coli contamination in liquid samples extracted from RTE foods, including rotisserie chicken purge and packaged hard-boiled eggs. In these tests, samples spiked with 10⁵ CFU/mL produced visible detection signals following dilution to address viscosity issues.

Similarly, the sensor successfully detected E. coli in liquid extracted from produce, including baby-cut carrots.

Performance in Leafy Greens Samples

Because leafy greens are frequently implicated in foodborne illness outbreaks, the researchers evaluated the platform’s compatibility with lettuce and mixed salad greens using several sample collection approaches.

The sensor detected E. coli in water droplets collected from the surface of iceberg lettuce and in wash water from a rinsed salad mix containing romaine lettuce, kale, and spinach. In wash-water experiments, contamination levels of both 10³ and 10⁵ CFU/mL were successfully identified.

To better simulate real-world contamination scenarios, the researchers also directly inoculated leafy greens and then extracted liquid samples using a stomaching method, which is more faithful to real-world sample collection. The hydrogel sensor detected E. coli in these samples at recovered concentrations of approximately 10⁴ and 10⁶ CFU/mL.

Importantly, the system detected E. coli even in the presence of other microorganisms present in the food samples. Previous testing of the DNAzyme component showed specificity for E. coli compared with other bacterial species, including Listeria monocytogenes, Pseudomonas aeruginosa, and methicillin-resistant Staphylococcus aureus (MRSA).

Potential Applications and Limitations

The authors said the results demonstrate the versatility of the sensing platform across multiple food matrices, suggesting potential use for onsite pathogen detection along the food production and value chain.

However, the researchers noted several limitations that would need to be addressed for practical implementation. The current detection time of approximately 18 hours is comparable to conventional culture-based methods but slower than polymerase chain reaction (PCR) assays and some newer biosensor technologies. The platform’s sensitivity may also require improvement for certain applications.

Future work could explore adapting the hydrogel system for detecting other foodborne pathogens. The researchers noted that DNAzymes targeting bacteria such as Salmonella Typhimurium have already been developed and could potentially be incorporated into similar biosensing platforms.

The authors also suggested that integrating the technology into food packaging for continuous contamination monitoring could be another possible application.