Community Associated Methicillin-Resistant Staphylococcus Aureus: A Review

Michael J. Rybak, PharmD; Kerry L. LaPlante, PharmD


Pharmacotherapy. 2005;25(1):74-85. 

In This Article

Abstract and Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) is a common bacterial pathogen responsible for a variety of infections in both children and adults. Treatment of infections caused by this organism is problematic due to its resistance to many drugs. Recent reports of community-associated MRSA (CA-MRSA) infections in patients with no known risk factors have serious public health implications. Therapeutic options for these infections are untested; therefore, the potential exists for high morbidity and mortality. Recently, clinical definitions have been established, and new molecular approaches have allowed investigators to distinguish CA-MRSA more easily from traditional nosocomial-derived MRSA strains. Identifying potential risk factors for CA-MRSA acquisition and fully characterizing the epidemiologic, clinical, and molecular properties of these strains are necessary to provide effective therapeutic guidelines.

Methicillin-resistant Staphylococcus aureus (MRSA) is a formidable bacterial pathogen responsible for a variety of infections commonly seen in patients of all ages.[1,2,3] Acquisition of this organism is typically associated with particular settings (health care institutions, such as hospitals and long-term care facilities) and patient groups (patients with prolonged hospitalization, past antimicrobial use, indwelling catheters, decubitis ulcers, postoperative surgical wounds, and use of intravenous drugs or treatment with enteral feedings or dialysis).[4,5,6,7] Infections due to MRSA present a considerable dilemma to clinicians, since therapeutic options are limited and suboptimal dosing contributes to heightened mortality and increased length of hospital stay.[6,8,9]

Although alteration of target penicillin-binding proteins is the primary mechanism of resistance to β-lactam antibiotics, over the years MRSA strains have gained multiple mechanisms of resistance to several classes of antimicrobials, including macrolides, aminoglycosides, fluoroquinolones, tetracyclines, and lincosamide antibiotics such as clindamycin. For the past several decades, glycopeptide antibiotics, such as vancomycin, have been considered the only agents to which MRSAs have not developed resistance. Unfortunately, due to overuse of glycopeptide antibiotics, MRSAs have emerged with reduced susceptibility to these agents as well.[10,11]

In recent years, there have been several reports of community-associated MRSA (CA-MRSA) infections throughout the world, including several outbreaks in the United States.[12,13,14,15] Most of these outbreaks have been associated with a single-clone strain. Transmission has occurred by close physical contact in situations involving children in day-care centers, children and adults on Indian reservations, athletes, military personnel, correctional facilities, and men having sex with men.[16,17,18] Of concern, these patients are otherwise healthy individuals with no known risk factors for MRSA acquisition.[3,7,13,15,19,20,21,22,23,24,25,26]

The prevalence of MRSA infections is increasing in the community.[27,28,29] Recent investigations have revealed several characteristics that differentiate CA-MRSA from health care-associated MRSA (HA-MRSA) strains. Community isolates tend to be susceptible to a variety of non-β-lactam antibiotics, whereas HA-MRSAs are typically resistant to multiple antibiotics. Other differences are that genotypes of community isolates are not the same as those of health care-derived isolates, community strains harbor a novel methicillin resistance cassette gene element not identified to date among strains that are endemic to health care setting, and community isolates occur in patients lacking typical risk factors for MRSA.[1,2,3,4,5] Finally, community isolates are more likely than health care-derived isolates to encode putative virulence factors, such as Panton-Valentine leucocidin, a cytotoxin virulence factor that has been associated with severe pneumonia in children and with skin and soft tissue infections in adults.

The Centers for Disease Control and Prevention (CDC) has established criteria to distinguish CA-MRSA from HA-MRSA isolates.[30] According to these criteria, the diagnosis of CA-MRSA must be made in an outpatient setting or by culture showing MRSA within 48 hours after admission to the hospital. The patient must not have experienced any of the following during the year before infection: hospitalization; admission to a nursing home, skilled nursing facility, or hospice; dialysis; or surgery. In addition, the patient must be without permanent indwelling catheters or medical devices that pass through the skin into the body.


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