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Methicillin-resistant Staphylococcus aureus (MRSA) now causes more deaths per year in the U.S. than AIDS. MRSA’s tendency to resist treatment by common antibiotics has predictably earned it the nickname “Superbug” in the lay press. But beyond the press hype is the foundation of a true epidemic. Staphylococcus species of bacteria are fairly benign and are commonly found on the skin of healthy people. It only becomes a problem when it enters the bloodstream – through an open cut for example – at which point it can cause an infection. These infections are usually minor and if the body’s immune system can not take care of it, then a short course of antibiotics will do the trick. Herein lies the problem with MRSA, it’s resistance to antibiotics means that if the body’s immune system can not respond then it is very difficult to get rid of the bugs. Symptoms of infection can range from boils and abcesses to pneumonia.

A paper published this week in Nature Medicine by researchers from the U.S. National Institutes of Health elegantly describes the identification of the Superbug’s super power. Michael Otto and colleagues look at members of a protein family that are expressed specifically in the community-associated MRSA strain and implicate them in the bacteria’s virulence.

From the mid 70’s when MRSA was first identified, infection was typically associated to exposure to the bacteria during a hospital stay. Hospital-associated MRSA (HA-MRSA) has been estimated to infect up to 100,000 British hospital patients a year resulting in nearly 5,000 deaths. Recently, however, attention has shifted to community-associated MRSA (CA-MRSA) which is not restricted to hospitals. CA-MRSA has been found in homeless shelters, prisons, schools, daycare centers, sports venues and on household pets. Both it’s ubiquitousness and and ferocity makes CA-MRSA a particularly nasty customer, though it is resistant to fewer antibiotics than its hospital associated cousin.

The NIH researchers focus their attention to the CA-MRSA strain, asking what makes it a more successful disease agent than the hospital strain. The researchers identify a group of proteins known as phenol-soluble modulin peptides (PSM) that differ in size in both types of MRSA when compared to the benign Staphylococcus epidermidis. More PSMs are produced in CA-MRSA when compared to HA-MRSA, suggesting that this may be an important difference between the two strains. As a functional test of the role of the PSMs, the researchers created strains CA-MRSA with the genes encoding the proteins deleted. When mice were infected with these deletion strains, disease symptoms were much less severe than mice infected with unaltered CA-MRSA. This result indicates that PSMs play an essential role in diseases induced by CA-MRSA. Finally, the researchers asked how the PSMs promote symptoms in CA-MRSA infections. They focused on neutrophils – the most common type of white blood cell and the one typically involved in immune response to bacterial infections. PSMs activated neutrophils but were also able to destroy the cells. Thus, infection with CA-MRSA triggers the immune system to respond, but white blood cells are destroyed when they attack the invading bacteria, effectively leaving the infected organism immunocompromised.

The research described in this paper is soundly reasoned and well conducted. Their results are solid and provide genuine insight into virulence in CA-MRSA. This is the first step in developing a strategy to improve control and treatment of what is rapidly becoming an epidemic. Otto and his group propose developing antibodies to CA-MRSA specific PSMs as the next logical step in the fight against the Superbug.

As a final note, the best way to control the spread of MRSA whether it be hospital or community associated is washing your hands. It all goes back to what our moms’ taught us, doesn’t it. One of the podcasts that I listen to regularly is The Dawgcast – my source for Georgia Bulldogs news. According to the Dawgcast guys, the drought in Georgia is so bad that not only is Governor Sonny Perdue publicly praying for rain, but fans attending the recent Georgia – Auburn game were discouraged from flushing the toilets and washing their hands in the bathrooms. Sports venues are a great place to pick up CA-MRSA, if UGA officials aren’t careful they will have a bigger problem on their hands than a drought.

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