Community-associated MRSA: a dangerous epidemic.

Although a relatively unspectacular, nonmotile coccus, Staphylococcus aureus is a dangerous pathogen and a major public health concern. In addition to being a common food-poisoning agent, it can cause serious skin and soft tissue infections and life-threatening diseases. Furthermore, resistance to multiple antibiotics, most notably to penicillin and methicillin, is common in S. aureus and makes treatment especially difficult. However, the biggest current threat from this human pathogen is the pandemic spread of community-associated methicillin-resistant S. aureus (CA-MRSA), a crisis affecting many aspects of our social life and second only to HIV/AIDS in scope and importance. Infections with MRSA have traditionally been limited to healthcare settings and individuals with risk factors for infection. Therefore, it came as quite a shock when MRSA infections unrelated to hospitals were reported in healthy individuals beginning in the late 1990s: first in children, later in professional football players and other sports teams, prisoners and men who have sex with men (MSM). The strains that cause these infections are remarkable because they combine antibiotic resistance with exceptional virulence and transmissibility, a phenomenon unprecedented with S. aureus. Moreover, while most S. aureus infections were previously thought to originate from nasal colonization, transmission of CA-MRSA includes body-tobody and sexual contacts. Within a very short period of time, CA-MRSA have become not only the most frequent causes of softand skintissue infections in the community, but are also replacing traditional MRSA strains in hospitals on a large scale. Where did CA-MRSA come from so suddenly? It is generally agreed that CA-MRSA have evolved from highly pathogenic methicillin-susceptible S. aureus (MSSA) strains in the community by the acquisition of methicillin-resistance elements. Interestingly, all CA-MRSA strains have the same type of resistance element that was probably acquired from Staphylococcus epidermidis, indicating an important role for this less virulent skin commensal as a reservoir for the horizontal transmission of genetic factors to S. aureus.