Medicine hailed the introduction of methicillin in 1959 to treat penicillin-resistant Staphylococcus aureus. But the jubilation was short-lived. Two years later reports appeared in Europe that some strains of S. aureus were immune to methicillin. That resistance soon spread to Asia and the United States and became known as methicillin-resistant S. aureus (MRSA), usually occurring during or after a hospital stay.
Then in the 1990s a new strain of MRSA appeared in healthy individuals who had not been anywhere near a hospital. This became known as community-acquired MRSA (CA-MRSA) and, at its worse, carries its own set of horrific symptoms such as toxic shock syndrome and blood and bone infections.
- 95,000 cases of invasive MRSA infections occur annually in the U.S.
- MRSA causes 19,000 deaths each year
- 86 percent of MRSA infections are healthcare associated
- 14 percent of MRSA infections are due to community-acquired MRSA
- In 2012 it cost $3.2 billion to $4.2 billion to treat hospitalized MRSA patients
(Sources: CDC, Pew Trust Health Initiatives)
While treating and preventing the spread of MRSA has been a challenge, testing and analysis of samples have also challenged laboratorians and researchers. What tests best identify resistant strains? What speed can be achieved in new tests? What cost is justified for testing? Can tests pinpoint which antibiotic an infection will resist? Is sequencing the answer?
As time passed, new tests kept up with ever-changing MRSA strains. Despite the progress, the battle is ongoing, requiring newer, faster, better, more specific assays.
Setting Up a System
One question, handled differently in individual institutions, is how to detect MRSA. Some hospitals take nasal swabs from every patient upon admission. Others only test patients coming from specific settings, such as long-term care, or who have been hospitalized recently or will be admitted to the intensive care unit.
As varied as how and who is tested, is how they are treated. Some hospitals practice universal decolonization. Others screen and isolate all suspected cases; while still others practice a combination of all of these, depending on the circumstances.
"There are different approaches," said Duane W. Newton, PhD, D(ABMM), clinical associate professor, department of pathology, and director, clinical microbiology and virology laboratories, University of Michigan Health System, Ann Arbor, Mich. "You can universally screen everyone being admitted or you can screen higher-risk patients coming from other healthcare settings.
"The approach taken here at the University of Michigan Health System is more narrowly focused," Newton explained. "We sample high-risk patients, those going to the ICU, and immunocompromised patients on specific units." He referenced a 2006 study that found MRSA was more likely to occur in elderly, in women, those with diabetes and patients coming from long-term care facilities.1
"The biggest issue is that clinicians are continually challenged in how to manage these infections and the availability of good treatments is challenging as well," Newton told ADVANCE. "With new antibiotics coming out, our role is to be able to keep up with that and perform susceptibility testing."
Newton pointed out more and more rapid diagnostics are aimed at detecting MRSA: "PCR-based assays; microphage assays for rapid detection, primarily in blood cultures; and other assays with application for blood cultures, not just nasal swabs."
Testing of these samples varies, "from culture-based to molecular methods. Many places are doing culture-based chromogenic media, using chromogenic substrates, which takes about 24 hours," Newton said.
Last year, a British hospital isolated a MRSA outbreak and tracked new infections using rapid whole-genome sequencing. According to Cambridge University, who participated in the study, this was the first time DNA sequencing had been used to bring an end to an infectious disease outbreak.2
Brandi Limbago, PhD, deputy chief of the clinical and environmental microbiology branch in the CDC's division of healthcare quality promotion, explained the CDC uses sequence analysis for MRSA strain typing but not whole-genome sequencing. "Most of the time it's unnecessary. It's a powerful tool, but still pretty resource intensive," she said. "The cost is coming down but the interpretive side of it is still very cumbersome.
"There are other tests that will do the job," she concluded.
Some of these tests, used in labs like University of Michigan Health System's, include: BD's GeneOhm MRSA; Cepheid's GeneXpert, including Xpert MRSA and Xpert SA Nasal Complete, and Roche's LightCycler MRSA Advanced.
Newton said all these tests offer speed, taking from 1 to 2 hours to get results.
That speed/time becomes money if you look at the steps in the process. "That speed could be important," Newton said. "If you're testing everyone who comes into your hospital, all those people have to be on contact precautions until you know they are negative for MRSA. If your hospital is all-private rooms, then any patient can be put in precautions.
"Testing is around $150, varying from hospital to hospital and platform to platform," he continued. "But relative to other hospital costs, the lab is significantly cheaper than imaging, chemotherapy regimens, and surgery."
Limbago believes a future approach to combating MRSA could include a vaccine. "I know people are working on it, but I don't know that anyone is close."
Gail O. Guterl is a frequent contributor to ADVANCE.
- Graham, PL. A U.S. population-based survey of Staphylococcus aureus colonization. Ann Intern Med. 2006;144(5);318-325.
- BioOptics World. DNA sequencing controls MRSA outbreak for the first time. 2012. http://www.bioopticsworld.com/articles/2012/11/dna-sequencing-controls-mrsa-outbreak-for-the-first-time.html. Accessed January 10, 2013.