MRSA superbug evades immune defences with a jumping gene

A gene identified in MRSA (Methicillin-resistant Staphylococcus aureus) may contribute to the spread and virulence of the superbug.

The gene - sasX - is located in a segment of DNA called a mobile genetic element and is capable of jumping from one bacterium to another. Researchers in China and the USA found that sasX has increased in prevalence and has spread from one MRSA strain to others. The presence of sasX in MRSA means that a person is more likely to develop an infection as the gene protects the bacterium from our immune system.

MRSA epidemics appear in waves, particularly affecting hospitals where the infection can spread rapidly between patients whose immune system is already weakened. In the USA, deaths from MRSA infection outnumber those due to HIV (human immunodeficiency virus). This study, published in Nature Medicine, is one of the first to investigate both the frequency and biological function of a gene in MRSA, providing insight into one mechanism by which these waves occur.

Researchers examined samples from over 800 patients with MRSA at three hospitals in China between 2003 and 2011. In samples taken between 2003 and 2005, sasX was almost exclusively found in a single strain of MRSA but in samples collected from 2009 to 2011 its frequency in other strains increased from five to 28 percent.

'This is direct evidence of horizontal gene transfer from one group of MRSA strains to another', study leader Dr Michael Otto of the National Institute of Allergy and Infectious Disease told The Scientist.

SasX function appears to be important at several stages during MRSA infection. Crucially, it helps the bacteria aggregate tightly and evade attack from the immune system. 'Our idea is that when they stick together, it is more difficult for a white blood cell to ingest the bacteria and kill them', Dr Otto told Nature.

The study focused on a strain of MRSA most prevalent in Asia, but the spread of the infection means that sasX is likely to spread beyond the region.

Understanding how outbreaks of MRSA spread is crucial in treating them and preventing their spread. Dr Frank Lowy, an infectious disease physician at Columbia University Medical Centre in New York, who was not involved in the study, told Nature that 'the holy grail is to identify a gene that confers virulence and to develop a vaccine against it'.

This has not yet been achieved for MRSA, he went on to explain, but combining several virulence proteins in a single vaccine may succeed where others have failed. 'SasX presents another good target', he said.