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Study exposes superbug defences (2009)

An international study between scientists from New Zealand and Denmark has revealed exactly how the bacterium Staphylococcus aureus evades the human body’s key immune defences.

Superbug defenses

Maurice Wilkins Centre scientists led by Professor John Fraser at The University of Auckland and a scientific team led by Professor Gregers Anderson at The University of Aarhus, Denmark, published a paper in the journal Proceedings of the National Academy of Sciences (PNAS) in January 2010 describing how a protein from S. aureus interferes with the human immune system. 

In New Zealand, as with most other countries, S. aureus is the most common cause of hospital-acquired infection. It also causes serious outbreaks in the community where antibiotic resistant strains such as Methicillin-resistant S. aureus (MRSA) are proving very difficult to treat. 

Professor Fraser, Deputy Director of the Maurice Wilkins Centre, says the study focuses on a small protein that the bacterium produces called SSL7 (Staphylococcal Superantigen- Like protein 7). The team has shown how this protein binds to Immunoglobulin A (IgA), a special defence antibody in our gut and lungs. The SSL7 protein also binds to complement C5, one of a series of proteins that “complement” the work of antibodies in destroying bacteria.

 “We’ve created a structural model of the complex formed when SSL7 binds to IgA and C5,” says Professor Fraser. “The model has enabled us to see how SSL7 cleverly uses IgA as a ‘scaffold’ to capture two molecules of C5. By doing this, the reaction that brings complement proteins together to destroy the bacteria’s cell wall can’t take place.” 

“By binding to IgA and C5 at the same time, the SSL7 protein simultaneously blocks several crucial parts of the body’s immune defence against bacterial infection.” 

Professor Fraser says that by knowing how S. aureus works to block the immune system, scientists can begin to develop therapeutic drugs that directly target proteins like SSL7. 

“Remarkably, SSL7 also highlights exactly where to pin-point drugs designed to prevent the unwanted over-activation of C5 that normally leads to a serious inflammatory disorder."


Image: Representation of the 3D structure of the SSL7 protein (blue) in complex with IgA (green) and C5 (grey) proteins. Image courtesy of Professor John Fraser.