Skyrocketing numbers of invasive, methicillin-resistant Staphylococcus aureus (MRSA) disease originating not only in health care facilities but in the community as well are cause for enormous concern. A new study from the CDC estimates the 2005 incidence of invasive MRSA in the United States at an astounding 31.8 per 100,000, a rate higher than the combined rate for invasive disease caused by pneumococcus (14.1/100,000), group A streptococcus (3.6/100,000), meningococcus (0.35/100,000), and Haemophilus influenzae (1.4/100,000).^1,2 Most of these infections were associated with health care facilities, whether the actual onset of the infection occurred in a hospital or nursing home or after discharge. But about 14% were classified as community-associated, occurring in previously healthy individuals with no history of hospitalization. For many, the infection was lethal: The 18,650 deaths projected country-wide in 2005 exceeds the number of deaths attributed to HIV/AIDS in the United States for that year.

The capacity of this pathogen to strike previously healthy adults outside health care settings is particularly troubling. Infections caused by community-associated (CA) MRSA strains are seen in emergency departments more frequently than any other infection and can cause severe disease resulting in necrotizing fasciitis or even death. Now a group of researchers from the Laboratory of Human Bacterial Pathogenesis in Montana have come up with what may be the key to understanding how this organism goes about its lethal business.³ They have found within the CA-MRSA strain a class of secreted peptides that have a remarkable ability: They can lure the victim's neutrophils to the disease site and dissolve them, thereby eliminating the body's main cellular defense against S aureus infection. In animal models, the researchers have found high concentrations of these peptides in standard CA-MRSA strains, where they contribute significantly to the strains' ability to cause disease. These peptides, the researchers believe, account at least in part for the enhanced virulence of CA-MRSA. Understanding this mechanism may point investigators in the right direction for developing new drugs to treat these infections.

1. Klevens RM, Morrison MA, Nadle J, et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA. 2007:298(15): 1763-1771.
2. Bancroft EA. Antimicrobial resistance: it's not just for hospitals [editorial]. JAMA. 2007;298(15):1803-1804.
3. Wang R, Braughton KR, Kretschmer D, et al. Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA [published online ahead of print November 11, 2007]. Nat Med.