In The Field

ASM: Superbugging

MRSA (Methicillin resistant Staphylococcus aureus, to use its full name) is rife in hospitals, prisons and pretty much everywhere else.

But after a crash course in Staphylococcus aureus, it seems that the picture is far more complicated (when is it not?). Not all S. aureus are created equal, as Vance Fowler, of Duke University, told me.


There are actually all kinds of different strains of S. aureus spreading around the world – and some are far more fearsome than others. A harmless type of S. aureus is hitchhiking around in the noses of one third of us, with no ill effects. Other strains can kill – and scientists want to know what distinguishes the nasal companion from the killer.

To this end, Fowler has been collecting samples of S. aureus from patients since 1994, along with a DNA sample and a 17-page medical history. He now has a bank of around 2000 bacterial strains, one of the most extensive collections of this bacterium in the world.

He and his colleagues are homing in on the genetic differences between these strains. They first divided up a bunch of patients into three groups according to the severity of their symptoms. Some were perfectly healthy (these samples. Fowler told me, were partly collected from undergraduate students willing to have their noses scraped while they lined up to watch a basketball game). A second group had mild symptoms; a third had the most dreaded of symptoms such as bone or joint infections.

Fowler’s team next analysed seven genes in each patient’s S. aureus, and placed them into groups according to how similar they are. They found that all the strains fall into eight different families – and that two of these groups appear to be linked to more dangerous infections. One called CC5, for example, caused severe infections 60% of the time, compared with some 15% in another group.

Fowler is working with collaborators to find the exact genetic alterations that make these strains nastier in the human body. They examined 419 genes in more detail and homed in on four regions which seem to be more common in the nastier strains. These regions – which are known to jump from one bacteria to another – contain several genes, such as ones that make immune-provoking molecules.

The idea now is pinpoint more precisely which changes in these genes might transform an innocuous strain of S. aureus into a destructive one.

Armed with such information, doctors might be better equipped to battle infections: perhaps by testing someone’s strain in hospital and decide whether to treat early and hard. Another hope is to identify people who are most susceptible to infection – and perhaps offer them a vaccine, should one ever be made.

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