Outbreak of skin infections sheds light on risk factors for bacterial resistance

Scientists identify two major risk factors for community-acquired skin infections caused by methicillin-resistant Staphylococcus aureus (MRSA) in a report published in the May 1 issue of The Journal of Infectious Diseases: previous antibiotic use and a genetic predisposition.

S. aureus is a common bacterium found on human nasal mucous membranes and skin, and strains that are resistant to methicillin can cause disease. Until recently drug-resistant strains were considered to be acquired almost exclusively in hospital settings, but reports of MRSA acquired in the community are increasing, and are most often associated with skin and soft-tissue infections such as furunculosis and cellulitis. Risk factors for infection by community-onset MRSA have to date been poorly understood by public health officials and other scientists.

To address this issue, Dr. Thomas W. Hennessy and colleagues of the Centers for Disease Control and Prevention and other institutions investigated a 1999 outbreak of furunculosis in rural southwestern Alaska.

The researchers conducted a case-control study in one village located in the region of the outbreak. They identified 34 persons with a laboratory-confirmed skin infection caused by community-onset MRSA, and 94 persons with no history of skin infection in the previous year who were the study “controls.” Infected persons and controls were similar in age and sex. Dr. Hennessy and colleagues found that persons with MRSA skin infections had received significantly more courses of antibiotics in the year before the outbreak than had the controls (a median of 4 vs. 2 courses, p=0.01).

The researchers sought in particular to determine whether a virulence factor previously associated with skin and soft tissue infections, Panton-Valentine leukocidin, could account for the high rates of MRSA skin infections in the region. To address the question of the virulence factor, the scientists studied clinical specimens of disease-causing S. aureus from a medical center in the outbreak region. Of these specimens, 113 were methicillin-resistant and 81 were methicillin-susceptible. Genes for the potently cytotoxic Panton-Valentine leukocidin virulence factor were present in 110, or 97%, of the isolates of MRSA, compared with none of the methicillin-susceptible isolates (p<0.001). Dr. Hennessy and colleagues concluded that use of antibiotics facilitated the genetic selection of methicillin-resistant strains of S. aureus expressing the Panton-Valentine leukocidin virulence factor. In other words, the combination of methicillin resistance and Panton-Valentine leukocidin may have provided the strain with characteristics favoring a furunculosis outbreak. In response to the outbreak, the researchers educated local health care providers about the emergence of MRSA as the predominant skin pathogen in their community and developed treatment guidelines that emphasize reserving antibiotics for severe infections only. The researchers believe their findings on risk factors for MRSA infections may be generalized to other parts of the United States and the world where incidence of community-onset MRSA is increasing, and highlight the ongoing challenge of antibiotic-resistant pathogens to public health.
Founded in 1904, The Journal of Infectious Diseases is the premier publication in the Western Hemisphere for original research on the pathogenesis, diagnosis, and treatment of infectious diseases; on the microbes that cause them; and on disorders of host immune mechanisms. Articles in JID include research results from microbiology, immunology, epidemiology, and related disciplines. JID is published under the auspices of the Infectious Diseases Society of America (IDSA), based in Alexandria, Va., a professional society representing more than 7,500 physicians and scientists who specialize in infectious diseases.