MRSA Gene that Enhances Superbug's Virulence Is On the Rise

http://www.scientificamerican.com/article.cfm?id=mrsa-gene-that-enhances-superbugs-virulence-is-on-the-rise

Researchers have identified a gene that makes some strains of an antibiotic-resistant bacterium more virulent, and have found that the gene is becoming more prevalent.

Epidemics of infection with methicillin-resistant Staphylococcus aureus (MRSA) come in waves. To uncover the molecular basis of MRSA's virulence, Michael Otto, a molecular microbiologist at the US National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, and his colleagues focused on a rare genetic element within a strain of MRSA, ST239, that is predominant in Asia.

When the researchers ascertained that the gene, sasX, encodes a protein anchored on the surface of S. aureus, they suspected that they had found something that might enhance the bug's infectious prowess. Sure enough, strains with sasX clung more tightly to the nasal linings of mice than did otherwise identical bacteria that lacked sasX. In addition to respiratory infections, S. aureus can also cause boil- and rash-producing skin infections. In these types of infections, sasX was a problem as well, leading to larger lesions in mice than strains without the gene. The results are published today in Nature Medicine.

One reason for such virulence, suggest the authors, is that sasX-carrying bacteria aggregate tightly. "Our idea is that when they stick together, it is more difficult for a white blood cell to ingest the bacteria and kill them," says Otto.