Light shed on common superbug

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

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, have described how a protein from S. aureus interferes with the human immune system. Their findings have been published in the journal Proceedings of the National Academy of Sciences USA (PNAS).

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 MRSA are proving very difficult to treat.

Professor Fraser, Deputy Director at the Maurice Wilkins Centre, says the study, a culmination of six year’s work, focuses on a small protein from the superbug 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 lung. 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 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."

(Source: University of Auckland: Proceedings of the National Academy of Sciences USA: April 2010)