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Monday, February 24, 2014

Unclumping a killer

Staphylococcus aureus is an extremely common bacteria that about one third of people routinely have on their skin with no ill effects. Unfortunately, S. aureus can turn deadly, especially if it takes a hold in someone’s bloodstream.

One of the steps in going from benign hitchhiker to pathogenic killer is for the S. aureus to combine into masses of cells. Like other types of bacteria, S. aureus can form a nearly indestructible biofilm layer across a surface. However, unlike many other microbes, S. aureus can also form free-floating balls of cells by attaching to each other and to proteins within blood. This means that understanding exactly how and why the cells clump together might give us a way to defeat this increasingly antibiotic-resistant bug.

The good news is that Alexander Horswill and his colleagues from the University of Iowa have discovered a type of S. aureus mutant that is unable to stick together. Unlike normal bacteria, the surfaces of these mutants are coated with ‘Giant Staphylococcus Surface Protein (GSSP)’. These are spiky proteins that protrude from the outer membrane of the microbe. The more of them that are present, the less that cell can clump together with other cells.

Caption: Images taken with a scanning electron microscope show wild-type bacteria (left) forming tight aggregates or clumps in the presence of blood proteins. In contrast, cells of the mutant strain (right) over produce a giant surface protein, have a spiky appearance, and do not clump tightly together. This clumping defect makes the mutant strain less deadly in an experimental model of the serious staph infection, endocarditis.
Credit: Alexander Horswill, University of Iowa

This is particularly significant because when S. aureus is unable to combine in these clumps, it's also non-pathogenic. Thus, finding a way to increase production of GSSP may keep this microbe from becoming a killer.

Walker JN, Crosby HA, Spaulding AR, Salgado-Pabón W, Malone CL, Rosenthal CB, Schlievert PM, Boyd JM, & Horswill AR (2013). The Staphylococcus aureus ArlRS Two-Component System Is a Novel Regulator of Agglutination and Pathogenesis. PLoS pathogens, 9 (12) PMID: 24367264.