Asian MRSA Epidemic Tied to a Single Gene

A single gene appears to play a crucial role in nasal colonization, immune evasion, and virulence in the methicillin-resistant Staphylococcus aureus (MRSA) infection in Asia, according to a new study.

Researchers at Fudan University (Shanghai, China), the US National Institutes of Health (NIH; Bethesda, MD, USA), and other institutions have identified the sasX gene--which encodes for a surface protein that only occurs in three of the 43 known predominant strains of the Asian MRSA clone--as an important factor in the spread of MRSA in Asia. The researchers succeeded in demonstrating that sasX holds a key role in MRSA colonization and pathogenesis, substantially enhancing nasal colonization, lung disease, and abscess formation and promotes mechanisms of immune evasion.

To determine the prevalence of the gene, the researchers looked at sasX-positive clones in about 800 S. aureus isolates from infected patients at three large hospitals in eastern China from 2003 to 2011. They found that the frequency of the gene rose from 21% to 39% of isolates during that time, but remained rare among community isolates, suggesting that the new clones spread predominantly in the hospital setting. The researchers also observed that sasX is quickly migrating via horizontal gene transfer from the original MRSA sequence type 239 (ST239) strain to invasive clones belonging to other sequence types. The study was published early online on April 22, 2012, in Nature Medicine.

“The surface protein promotes nasal colonization, as bacteria with a mutant form of the gene had impaired adhesion to human nasal epithelial cells in vitro,” noted lead author associate professor Min Li, PhD, of the department of laboratory medicine at Fudan University, and colleagues. “It also appears to cause intercellular bacterial aggregation and biofilm formation, which leads to marked enhancement of immune system evasion mechanisms.”

A recent report by the Swedish Institute for Infectious Disease Control (SMI; Solna) warns that China now threatens world health by unleashing waves of MRSA superbugs on every other country, due to blatant overuse of antibiotics that has caused a recent doubling of untreatable MRSA infections in Chinese hospitals. Similarly, overuse of antibiotics in farm animals is causing superstrains of MRSA to be shipped around the world.

“We have a lot of data from Chinese hospitals and it shows a very frightening picture of high-level antibiotic resistance,” said Andreas Heddini, MD, of the SMI. “Doctors are daily finding there is nothing they can do, even third and fourth-line antibiotics are not working. There is a real risk that globally we will return to a preantibiotic era of medicine, where we face a situation where a number of medical treatment options would no longer be there. What happens in China matters for the rest of the world.”

Related Links:
Fudan University
US National Institutes of Health
Swedish Institute for Infectious Disease Control