A new strain of MRSA that infects healthy people in the community as opposed to vulnerable and sick people in hospitals appears to be gaining a foothold because of certain clever tricks the bacteria has learned about the human immune system that it uses to its advantage.
This is one of the findings of a new US study by researcher Dr Michael Otto of the National Institute of Allergy and Infectious Diseases’ (NIAID) Rocky Mountain Laboratories in Hamilton, Montana, and colleagues. The study is published in the early online issue of the journal Nature Medicine.
MRSA is a highly resistant form of the common bacteria Staphylococcus aureus. It does not respond to treatment with conventional antibiotics. Community Associated MRSA, or CA-MRSA is emerging as a more virulent strain that may be even harder to treat.
Traditionally, people were more likely to catch MRSA in a hospital than in the community, because it typically infected those whose immune system was already compromised. But the emergence of CA-MRSA outside of the hospital environment is causing the medical profession to become increasingly concerned.
In fact, the researchers note that in the US, CA-MRSA is the most common reason people with infections seek help in the emergency room.
CA-MRSA appears to be more virulent than hospital MRSA and can cause severe infections and even death in otherwise healthy people as the highly resistant bacteria eats away at flesh (necrotizing fasciitis), undeterred by the attempts of a healthy immune system to try and stop it.
Scientists were baffled at how the bacteria managed to do this, so Otto and colleagues decided to investigate.
They found that CA-MRSA has developed an ability to trigger the immune system to attack it with white blood cells but it then seeks them out and destroys them in large numbers. In effect, the bacteria has learned to flush out the enemy and mount an attack while defences are weak.
Otto and colleagues discovered that CA-MRSA secretes a class of peptide (the molecule that makes up protein) called PSMs. These staphylococcal peptides recruit, activate and neutralise (by lysing, or bursting) human neutrophils (a common type of white blood cell), thus eliminating the main line of defence against infection by Staphylococcus aureus.
They found that the CA-MRSA is able to produce high concentrations of PSMs, which they found by developing lab versions of CA-MRSA and testing them on mice.
Otto and colleagues also pinpointed the genes on the CA-MRSA bacterium that produces PSMs. They said these may not be the only ones involved, but they were probably the most important ones.
Also, when they compared strains of CA-MRSA with strains of MRSA from hospitals, they found that the CA-MRSA bacteria produced far more PSMs.
it is hoped that this discovery will give a useful new direction to the development of drugs against this virulent new strain of MRSA, which is fast becoming a serious public health issue.
“Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA.”
Rong Wang, Kevin R Braughton, Dorothee Kretschmer, Thanh-Huy L Bach, Shu Y Queck, Min Li, Adam D Kennedy, David W Dorward, Seymour J Klebanoff, Andreas Peschel, Frank R DeLeo & Michael Otto.
Nature Medicine Published online: 11 November 2007.
doi:10.1038/nm1656
Written by: Catharine Paddock