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Editor's Note:
Robert S. Daum, MD, reviews the epidemiology, clinical presentation, and molecular characterization of community-acquired methicillin-resistant Staphylococcus aureus infections. In this report, Dr. Daum reprises his lecture, "Community-Acquired MRSA: What, Why, and Where, " presented in the Scientific Session:"Resistance in Gram-Positive Cocci: Staphylococci," at the 5th International Symposium on Antimicrobial Agents and Resistance (ISAAR); April 27-29, 2005; Seoul, Korea, as well as other recent data on this important problem.
Staphylococcus aureus is an important human pathogen and can be a transient component of the nasal and skin flora. It has been well known as a nosocomial pathogen, where available data rank it among the leading causes of nosocomial infections. It also is a major cause of community-acquired infections, although the magnitude of this problem is unknown due to the paucity of data.
The semisynthetic antimicrobial methicillin was introduced into clinical practice in the early 1960s. Resistance to it was recognized at the time of its introduction and is commonly referred to with the acronym methicillin resistant S aureus or MRSA. The term MRSA remains in wide use, despite the attrition of methicillin from the antimicrobial formulary. The MRSA acronym suggests cross-resistance to all other beta-lactam antimicrobials, including cephalosporins and carbapenems. Attempts to invoke oxacillin as the designated class compound with consequent use of the acronym ORSA to indicate oxacillin-resistant S aureus , have met with limited success.
For many years, MRSA infections were largely nosocomial. MRSA isolates were epidemic and endemic in hospital environments and largely confined to individuals who acquired the infection in hospitals and to others engaging in certain behavior risks such as intravenous substance abuse. Traditional "risk factors" for MRSA infection have been formulated based on this healthcare exposure paradigm.
A change in the epidemiology of MRSA infections was signaled by a study done at the University of Chicago Children's Hospital, Chicago, Illinois, in the mid-1990s. An investigative team began to recognize MRSA infections that had their onset in the community in children who lacked the traditional risk factors for MRSA colonization or infection. A 25-fold increase in the prevalence of infection was documented. Moreover, these community-acquired MRSA (CA-MRSA) infections had several features that distinguished them from the healthcare-associated MRSA infections (H-MRSA). First, although the term MRSA suggests resistance to all beta-lactam antibiotics, the CA-MRSA isolates tended to be resistant to fewer classes of antibiotics than their H-MRSA counterparts beyond the pan-beta-lactam resistance they shared. Additionally, the clinical syndromes manifest by CA-MRSA isolates resembled those associated with methicillin-susceptible isolates from the community and differed from those caused by H-MRSA isolates.
Since the publication of that initial report in 1998, CA-MRSA disease now appears to be epidemic at many medical centers in the United States. MRSA rates in excess of 75% have been documented for S aureus isolates from children presenting in pediatric emergency departments, such as those at the University of Chicago Children's Hospital and Texas Children's Hospital. (Sheldon Kaplan, MD, Texas Children's Hospital, Houston, Texas, Personal communication, 2005 and Robert S. Daum, et al, unpublished data) Increasingly, the MRSA isolates we still call "community-acquired" are being found to be transmitted in healthcare environments as well, suggesting that these strains are unusually fit pathogens in terms of their adaptability to the human host.
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