MIT magnetic beads help detection of pathogens in food and water

29 Aug 2023

Dynabeads already used for molecule capture can also produce Raman signal for optical analysis.

Tests for contamination in water, blood or food could become more rapid thanks to a Raman-based optical analysis method developed at MIT.

The new approach uses Dynabeads, artificial spherical particles with particular magnetic properties made from a magnetic iron core surrounded by a polymer shell, developed for use in cell isolation and molecular separation.

In their normal usage, coating the Dynabeads with antibodies can allow them to bond with target molecules or specific pathogens, after which the beads can be separated from the target media, used for further culturing, or examined using fluorescence staining; a time consuming process.

A project at MIT, including the lab of Loza Tadesse which is studying optical techniques for point-of-care diagnostic devices and Rohi Karnik's group working on microfluidics, has now spotted that a faster route to analysis is possible since the Dynabeads can also act as strong Raman reporters.

As described in a paper submitted to the Journal of Raman Spectroscopy (pdf), using Dynabeads in a Raman-based analysis procedure could form the basis for a portable device for quickly detecting a range of bacterial pathogens.

Studies in this area have tended to focus on recording the relatively weak Raman signals from the captured target cells or molecules, according to the project's published paper. But the Dynabeads themselves provide a strong Raman signature of their own fingerprint, and so indirectly signal the presence of the target species too.

"We were initially seeking to identify the signatures of bacteria, but the signature of the Dynabeads was actually very strong," commented Loza Tadesse. "We realized this signal could be a means of reporting to you whether you have that bacteria or not."

Detection of sepsis where time is critical

In proof-of-concept trials, the project used Dynabeads coated with anti-salmonella antibodies for indirect detection of Salmonella enterica bacteria, a leading worldwide cause of food-borne illness.

The researchers mixed Dynabeads into vials of water contaminated with salmonella, then magnetically isolating these beads onto microscope slides and measuring the Raman scatter signal detected when they were exposed to laser light.

Within half a second the team detected the Dynabeads' characteristic Raman signature, with major Raman peaks from the beads' polystyrene coating and from their magnetic iron cores, a confirmation that bound Dynabeads and by inference salmonella were present.

This approach effectively uses the Dynabeads as dual-function materials, to both magnetically isolate cells in the sample and provide strong Raman signal for a rapid, sensitive, and specific approach to bacterial detection. The same principle could be adapted to probe for other targets using commercially available pathogen-specific Dynabeads, according to MIT, avoiding the need for other staining steps or substrate engineering.

"Salmonella is the proof of concept," said Loza Tadesse. "You could purchase Dynabeads with E.coli antibodies, and the same thing would happen. It would bind to the bacteria, and we would be able to detect the Dynabead signature because the signal is super strong."

The team is particularly keen to apply the test to conditions such as sepsis, where speedy diagnosis is critical and where pathogens that trigger the condition are not rapidly detected using conventional lab tests.

"There are a lot of cases like sepsis, where pathogenic cells cannot always be grown on a plate," said Jongwan Lee, a member of Rohit Karnik’s lab. "In that case, our technique could rapidly detect these pathogens."