New method may also improve efficiency of antibiotic prescriptions
Wednesday, June 22, 2016

Half a million people are infected with staph bacteria each year, leading to skin infections, as well as more dangerous and deadly diseases, especially when the infection enters the bloodstream.

But diagnosing Staphylococcus aureus bacteremia (SAB) in the blood can take days—doctors draw a blood sample for testing, and results aren’t generally available for 24 to 48 hours. While waiting, a patient is placed on antibiotics to treat an infection that may or may not exist—and if it does, more testing and waiting follows to determine if the antibiotic treatment is working.

Using a probe created for a previous study, University of Iowa researchers detect nuclease activity. This is the final step in the assay prior to measuring fluorescence, which indicates the presence of micrococcal nuclease (MN).

Theoretically, a patient could be on an antibiotic for three days before learning it isn’t effective and having to be prescribed something different.

Now, UI researchers have identified a procedure that may reduce the wait for diagnosing a staph infection from a few days to just three hours, as well as the amount of time patients may have to wait to determine their medicine’s efficacy.

When a patient tests positive for Staphylococcus aureus bacteremia, they typically have it for more than a day,” says James McNamara, associate professor in internal medicine at the UI Carver College of Medicine and senior author of a study to be published in PLOS ONE ( in June.

The more immediate test results could be especially beneficial, McNamara says, for patients suspected of having MRSA, a form of staph bacteria that has been linked to life-threatening infections in the bloodstream, lungs, and heart.

One of the biggest reasons for delayed detection of SAB, McNamara says, is the exceedingly low levels of the bacteria in the blood of infected patients. Currently, blood sample testing is used to not only determine whether bacteria are in the blood, but to also determine the type of bacteria present.

In their study, McNamara and his team show that by measuring micrococcal nuclease (MN), a specific enzyme secreted by staph, in a blood sample, doctors can detect and diagnose a staph infection within three hours. Based on its fast turnaround time, this enzyme test may also enable doctors to monitor whether a prescribed antibiotic is doing what it’s supposed to be doing, and change medications if it isn’t—all in the same day.

Research done more than 50 years ago showed that heating up enzymes would cause them to unravel; when they cooled, most were unable to regain their original form and, thus, lost their enzymatic properties. MN, however, regains its form once it’s been cooled.

By first heating blood plasma samples from patients and then cooling them, researchers were able to essentially “kill off” the nucleases they didn’t want. The low levels of MN that remained were then concentrated and their enzymatic activity was detected with highly sensitive molecular probes. When combined, these procedures provide a foundation for a faster and less expensive diagnosis.

For this study, McNamara and his team collected samples from 17 patients who had already tested positive for staph, as well as various samples that tested negative for staph or positive for different bacteria. The next step, he says, is a larger study involving patients who have not already been diagnosed.

“We’re in the early stages of development, so we need to push forward,” he says.

Other UI authors on this study include Elliot Burghardt; Katie Flenker; Karen Clark; Jeff Miguel; Dilek Ince; Patricia Winokur, executive dean of the UI Carver College of Medicine; and Bradley Ford, medical director of Clinical Pathology.

Funding: National Institutes of Health grants [AI101391 and AI106738 to J.O.M.] and [P30CA086862].