Methicillin-resistant S. aureus is a variant of S. aureus which is considered to be resistant to all beta-lactam antibiotics (including penicillins, cephalosporins, and cephamicins). It may also be resistant to one or more other classes of antibiotics. By definition, MRSA must be resistant to one of the following semi-synthetic penicillins: methicillin, oxacillin, or nafcillin. Treatment of MRSA infections should be based on the susceptibility results from the patient culture. MRSA strains have been identified as a major source of healthcare-acquired infections and outbreaks in the U.S. For over four decades, MRSA has presented a challenge for infection control departments of hospitals attempting to control and eradicate this organism. In recent years, long-term acute care hospitals, long-term care facilities, rehabilitation centers, and small community hospitals have seen increasing numbers of cases. These facilities experience continuous reintroduction of resistant organisms due to the recurrent admissions and transfers of patients within these settings.
More recently, MRSA has also been increasing in the community in individuals without healthcare-associated risk factors. In a 2005 study of S. aureus in Florida outpatient settings, 49.7% of S. aureus isolates were reported to be MRSA (Kolar and Sanderson, 2007). The strains of these CA-MRSA infections are genetically distinct from the typical HA-MRSA commonly encountered in healthcare settings.
1. Healthcare-Associated MRSA (HA-MRSA) – Infection and colonization are typically seen in older individuals with one or more of the risk factors outlined later in this newsletter series. Resistance to multiple classes of antimicrobial agents is common.
2. Community-Associated MRSA (CA-MRSA) – Community-Associated MRSAc ases are frequently seen in younger persons and involve skin and soft tissue infections. Outbreaks of these infections have been described in numerous populations including people found in correctional facilities (jails and prisons), sport teams, men who have sex with men, commercial fishermen, and minority populations. Resistance to multiple classes of antimicrobials is uncommon. The most common CA-MRSA strain in the United States, the USA300 strain, is routinely resistant to erythromycin. Many of the CA-MRSA infections may be effectively treated with good wound care with or without oral antibiotics, while more resistant strains may require intravenous vancomycin. Frequently, these community-associated cases have initially been misdiagnosed as spider bites. This misdiagnosis prevents timely treatment which may result in a progression of the infection and increased chance of transmission to others. Although genetic variation exists between the types of MRSA, the community-associated variant has been found in healthcare settings and is capable of causing invasive infections and serious complications. HA-MRSA has also been demonstrated in community populations. Since distinction requires laboratory testing, the two variants are most often characterized by their operational definitions.