Tiny magnetic beads produce a visual signal that can be used to quickly detect pathogens

Getting blood test results can take anywhere from one day to a week, depending on what the test is for. The same applies to tests for water contamination and food contamination. In most cases, the waiting time is associated with the time-consuming steps of sample processing and analysis.

Now, MIT engineers have identified a new optical signature in a widely used class of magnetic beads, which can be used to quickly detect contaminants in a variety of diagnostic tests. For example, the team has shown that the signature can be used to detect signs of salmonella in food.

So-called Dynabeads are microscopic magnetic beads that can be coated with antibodies that bind to target molecules, such as specific pathogens. Dynabeads are commonly used in experiments in which they are mixed into solutions to capture particles of interest. But from there, scientists have to take additional, time-consuming steps to ensure that the molecules are actually there and bound to the beads.

The MIT team found a faster way to confirm the presence of pathogens associated with Dynabead, using optics and, specifically, Raman spectroscopy. This optical technology identifies specific molecules based on their “Raman signature,” or the unique way the molecule scatters light.

The researchers found that Dynabeads have an unusually strong Raman signature that is easily detectable, like a fluorescent tag. And they found that this signature can act as a “reporter”. If detected, the signal could be a quick confirmation, in less than an hour, that the target pathogen is indeed present in a given sample. The team is currently developing a portable device for the rapid detection of a range of bacterial pathogens, and has published its findings in an article now available on www. arXiv Prepress server It is to be published in a special issue of the journal Journal of Raman Spectroscopy.

“This technology may be useful in situations where the clinician is trying to narrow down the source of infection in order to provide better information on prescribing antibiotics, as well as to detect known pathogens in food and water,” says the study co-author. Marissa McDonald is a graduate student in the Harvard-MIT Program in Health Sciences and Technology. “In addition, we hope that this approach will eventually expand access to advanced diagnostics in resource-constrained environments.”

Among the MIT study co-authors: postdoctoral assistant Gongjuan Lee; Visiting Researcher Nikiwe Mhlanga; research scientist Jeon Wong Kang; Professor Tata Rohit Karnik, who is also Associate Director of the Abdul Latif Jameel Water and Food Systems Lab; and Assistant Professor Loza Tadesse of the Department of Mechanical Engineering.

Oil and water

Looking for diseased cells and pathogens in fluid samples is an exercise in patience.

“It’s a needle-in-a-haystack problem,” says Tadesse.

The numbers present are so small that they must be grown in controlled environments in sufficient numbers, their cultures stained, and then studied under a microscope. The whole process can take several days to a week to get a reliable positive or negative result.

Karnic and Tadesse’s labs have independently developed technologies to speed up different parts of the pathogen testing process and make the process portable using Dynabeads.

Dynabeads are commercially available microbeads made of a ferromagnetic core and a polymer shell that can be coated with antibodies. Surface antibodies act as hooks to bind specific target molecules. When mixed with a liquid, such as a vial of blood or water, any particles present will clump onto the Dynabeads. Using a magnet, the scientists can gently push the beads to the bottom of the flask and filter them out of the solution. Karnick’s lab is looking at ways to separate the beads further into those bound to the target molecule, and those not. “And the challenge remains: How do we know we have what we’re looking for?” says Tadesse.

The beads themselves are not visible to the naked eye. This is where Tadesse’s work comes into play. Her lab uses Raman spectroscopy as a way to fingerprint pathogens. I found that different cell types scatter light in unique ways that can be used as a signature to identify them.

And in the team’s new work, she and her colleagues found that Dynabeads also have a unique and strong Raman signature that can serve as a surprisingly clear beacon.

“We were initially looking to identify the fingerprints of the bacteria, but the Dynabeads’ signatures were actually very strong,” says Tadesse. “We realized that this signal could be a way to tell you whether you have these bacteria or not.”

test beacon

As a demonstration, the researchers mixed Dynabeads into bottles of water contaminated with salmonella. They then magnetically isolated these beads on microscope slides and measured the way the light propagates through the liquid when exposed to laser light. Within half a second, they quickly detected the Dynabeads’ Raman signature—confirmation that the Dynabeads and, by inference, Salmonella, were present in the liquid.

“This is something that can be used to quickly give a positive or negative answer: Is there a contaminant or not?” says Tadesse. “Because even a handful of pathogens can cause clinical symptoms.”

The team’s new technique is much faster than traditional methods and uses elements that can be adapted into smaller, more portable forms, a goal the researchers are currently working towards. The approach is also very versatile.

“The salmonella is the proof of concept,” says Tadesse. “You can buy Dynabeads with anti-E.coli antibodies, and the same thing will happen: it will bind to the bacteria, and we’ll be able to detect the Dynabead signature because the signal is so strong.”

The team is especially keen to apply the test to conditions such as sepsis, where time is of the essence, and where the pathogens causing the condition are not quickly detected using conventional laboratory tests.

“There are many cases, such as in sepsis, where it is not always possible to grow disease-causing cells on a plate,” says Lee, a member of Karnick’s lab. “In this case, our technology can quickly detect these pathogens.”