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Updated: Jul 5, 2026
Subcutaneous Infection of Methicillin Resistant Staphylococcus Aureus (MRSA)
Published on: February 9, 2011
A population-based study of the incidence and molecular epidemiology of methicillin-resistant Staphylococcus aureus
Catherine Liu1, Christopher J Graber, Michael Karr
1Div. of Infectious Diseases, University of California, San Francisco, 513 Parnassus Ave., S-380, Box 0654, San Francisco, CA 94143, USA. catherine.liu@ucsf.edu
Background:
Methicillin-resistant Staphylococcus aureus (MRSA) infections have become a major public health problem in both the community and hospitals. Few studies have characterized the incidence and clonal composition of disease-causing strains in an entire population. Our objective was to perform a population-based survey of the clinical and molecular epidemiology of MRSA disease in San Francisco, California.
Methods:
We prospectively collected 3985 MRSA isolates and associated clinical and demographic information over a 12-month period (2004-2005) at 9 San Francisco-area medical centers. A random sample of 801 isolates was selected for molecular analysis.
Results:
The annual incidence of community-onset MRSA disease among San Francisco residents was 316 cases per 100,000 population, compared with 31 cases per 100,000 population for hospital-onset disease. Persons who were aged 35-44 years, were men, and were black had the highest incidence of community-onset disease. The USA300 MRSA clone accounted for 234 cases of community-onset disease and 15 cases of hospital-onset disease per 100,000 population, constituting an estimated 78.5% and 43.4% of all cases of MRSA disease, respectively. Patients with community-onset USA300 MRSA versus non-USA300 MRSA disease were more likely to be male, be of younger age, and have skin and soft-tissue infections. USA300 strains were generally more susceptible to multiple antibiotics, although decreased susceptibility to tetracycline was observed in both community-onset (P = .008) and hospital-onset (P = .03) USA300 compared to non-USA300 strains.
Conclusions:
The annual incidence of community-onset MRSA disease in San Francisco is substantial, surpassing that of hospital-onset disease. USA300 is the predominant clone in both the community and hospitals. The dissemination of USA300 from the community into the hospital setting has blurred its distinction as a community-associated pathogen.
Insights
Community-onset Methicillin-resistant Staphylococcus aureus (MRSA) infections are significantly more common than hospital-onset infections in San Francisco. The USA300 clone is the dominant MRSA strain in both settings, blurring its community-associated origins.
Area of Science:
- Infectious Diseases
- Epidemiology
- Molecular Biology
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat globally.
- Limited data exists on the population-level incidence and genetic makeup of MRSA strains causing disease.
- This study addresses the need for a comprehensive population-based analysis of MRSA epidemiology in San Francisco.
Purpose of the Study:
- To determine the incidence and molecular characteristics of MRSA disease in San Francisco.
- To compare community-onset versus hospital-onset MRSA infections.
- To identify the predominant MRSA clones and their clinical associations.
Main Methods:
- Prospective collection of 3985 MRSA isolates from 9 San Francisco medical centers (2004-2005).
- Associated clinical and demographic data were gathered for each isolate.
- Molecular analysis was performed on a random sample of 801 isolates.
Main Results:
- Annual incidence of community-onset MRSA disease was 316 cases/100,000 population, far exceeding hospital-onset at 31 cases/100,000.
- The USA300 MRSA clone was highly prevalent, accounting for 78.5% of community-onset and 43.4% of hospital-onset cases.
- USA300 infections were associated with younger age, male sex, and skin/soft-tissue infections, with decreased tetracycline susceptibility noted.
Conclusions:
- Community-onset MRSA infections are substantially more common than hospital-onset infections in San Francisco.
- USA300 is the dominant MRSA clone circulating in both community and hospital settings.
- The widespread presence of USA300 in hospitals challenges its classification solely as a community-associated pathogen.
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