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Probe detects staph infection faster, more cheaply

Feb 3rd 2014 at 8:04 PM

Currently, to test if patients are infected with Staphylococcus bacteria, commonly known as staph, doctors have to take a biopsy and send it for analysis. Now, researchers from the University of Iowa have developed an ingenious noninvasive chemical probe that can detect the presence of a common species of staph in less than an hour.

The probe specifically targets Staphylococcus aureus, a species of staph that is common both inside hospitals and out in the general community. This species causes skin infections, and it can spread to the joints and bones and is potentially fatal, particularly in patients whose immune systems are already weak.

To design the probe, the team made use of the fact S. aureus shreds DNA. They exploited this feature to make the probe emit a light, which signals to doctors that the bacterium is present.

Speaking about their study, published in the journal Nature Medicine, first author and post-doctoral researcher Frank Hernandez says:

"Every year in the US half a million people become infected by S. aureus bacteria, and 20,000 of those who become infected die. We believe that we are significantly improving the actual methods for detecting bacteria with a simple approach, which we expect to be cheap, fast and reliable."

Corresponding author James McNamara, assistant professor in internal medicine, adds:

"We've come up with a new way to detect staph bacteria that takes less time than current diagnostic approaches. It builds on technology that's been around a long time, but with an important twist that allows our probe to be more specific and to last longer."

Currently, it can take days for doctors to find out if a patient is infected with staph, as they have to wait for lab results of biopsies. Prof. McNamara says they are "flying blind," and that it is "the state of medicine at this time."
Probe made of two molecules

The ingenious design of the probe hinges on a unique feature. It is a particle made of two molecules. One molecule gives off light under certain conditions, and the other molecule blocks that light.
illustration of DNA helix
Staph shreds DNA, so the researchers exploited this feature to make the probe emit a light, which signals to doctors that the bacterium is present.

As long as the particle stays whole - that is with the light-giving and the light-canceling molecules attached to each other - then no light is given off.

But if the particle is split, thus allowing the molecules to go their separate ways, then the light-giving molecule is not blocked and starts to give off light.

This unique feature is what makes it so useful. When the staph bacterium encounters the particle, it starts slashing at it like it does with DNA.

It cleaves it in half, releasing the light-emitting molecule from its light-blocking partner, allowing it to shine and give away the fact staph is present in the tissue.

The idea is doctors could administer the probe and, with the right equipment, see if it lights up in the infected tissue, to test if staph is present.
Probe uniquely and quickly detects staph

The researchers are not sure why staph behaves as aggressively as it does. They think it could be because it has developed this as the only way to tackle the sticky environment that DNA creates when it leaks out of infected, dying cells.

The idea of the chemical probe is not new, but what is new is that this team has produced one that lasts longer and identifies staph quickly, as Dr. Hernandez explains:

The "sword" that staph wields to shred the DNA is an enzyme called nuclease. Normal healthy tissue cells have a type of this enzyme as well, but the team has developed the probe so that only the staph nuclease attacks it and not the normal tissue cells.

They have so far tested it in human serum and mice with muscle infections and found it worked as they expected. Healthy normal cells did not cleave the probe particle.

Prof. McNamara says this showed the key feature of their chemical probe:

"If the probe gets cleaved by serum nucleases, then our probe would be lit up all over the bloodstream. But since it's split only by staph nucleases, then we can pinpoint where the staph bacteria are active."

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