PSMs contribute to viciousness of CA-MRSA, study shows

Key Points

National report - Much of the virulence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is caused by phenol-soluble modulin (PSM) peptides, a family of proteins that had not previously been studied in the pathogen.

The discovery, published in Nature Medicine, opens the door to a better understanding of the infection as well as potential new targets for intervention, researchers say.

Epidemiologic association had led to the belief that Panton-Valentine leukocidin (PVL) was the principal toxin that contributed to the lethalness of CA-MRSA, but that had never been demonstrated conclusively.

Researchers exposed mice to different strains of S. aureus containing the PVL gene and variants where that gene had been deleted. They saw no difference in virulence of the infections and concluded that PVL does not play a significant role in CA-MRSA skin and soft tissue infections.

RML researcher Michael Otto, Ph.D., decided to search for other toxins produced by CA-MRSA that might explain why the infection can be so deadly to otherwise normal, healthy young adults.

He hypothesized that the unknown toxin worked by forming pores in the membrane of neutrophils, the immune system's first line of defense against S. aureus, killing those cells.

Link with PSMs

Dr. Otto tells Dermatology Times he looked for structural features in peptides consistent with this process of lysing cells, and focused on the phenol-soluble modulin (PSM) family of proteins. They were first identified in 1999 in Staphylococcus epidermidis, but had not been studied in S. aureus.

Once Dr. Otto had identified the PSM proteins, he linked them back to the genes that produced them in S. aureus.

Working in vitro, Dr. Otto says, "The PSM genes are present in all of the strains of MRSA that we have looked at; there is just a difference in their expression.

"The CA-MRSA strains produced much higher levels of PSM than the HA (healthcare-associated)-MRSA strains."

He identified PSM-alpha-3 as the most virulent of the six PSM proteins produced by S. aureus through a series of experiments that knocked out individual PSM genes in the USA300 and US400 strains of CA-MRSA, then exposed mice to the pathogens.

Within five minutes of exposure to PSM-alpha-3, neutrophils began to exhibit signs of priming, such as flattening, and had formed structures indicating that the integrity of the plasma membrane had been compromised. Many of the cells were completely destroyed within an hour.

"The pore-forming process is a common one that is not specific to a specific cell type. PSMs produced by S. aureus likely kill other types of cells," Dr. Otto says, but neutrophils are the ones crucial to the immune response reining in the infection.

Probing further, Dr. Otto found that PSMs are pro-inflammatory at lower concentrations, which attracts neutrophils that can destroy the bacteria.

However, CA-MRSA uses a quorum-sensing mechanism that upregulates expression of the PSM genes "only when the bacterial colony grows large enough to produce a high level of PSMs sufficient to kill the neutrophils."

Dr. Otto says PSMs are a major factor in CA-MRSA virulence, but probably not the only factor.

HA-MRSA generally thrives in patients who are already immunocompromised from disease or associated treatment, as the bacteria face lower barriers to colonization and expansion.

"But the CA-MRSA strains need to be aggressive enough to attack healthy individuals," Dr. Otto says. He believes the more virulent forms of CA-MRSA evolved through this process of normal selective pressure.