As mentioned in the course introduction, MRSA infections fall into two general types:
- HA-MRSA Infections that occur in people who are, or have recently been, hospitalized.
- CA-MRSA Infections that are apparently acquired in the community
There are a number of factors that distinguish HA-MRSA from CA-MRSA isolates. These factors are summarized in the table below.
Origin of strains | Nosocomial infections Five isolates associated with healthcare settings: USA100, -200, -500, -600, -800 USA100 is the predominant isolate while USA 200 is the second most common isolate. USA700 has been isolated in both healthcare and community settings.
| Evolved from endemic methicillin-susceptible S. aureus (MSSA) strains
Two clones, USA300 and USA400, are associated with the majority of CA-MRSA infections in the United States. USA300 has emerged as the most prominent clone and is not found among hospital strains.
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Genetic lineage | Isolates usually carry large SCCmec types I, II or III (34-67 kb) The larger size of SCCmec II and III permits the inclusion of other non-beta-lactam resistance genes so that HA-MRSA strains tend to be multi-drug resistant | Isolates carry a smaller SCCmec variant, predominantly type IV (24 kb), less often type V or variant VT. SCCmec IV (except for mecA) does not permit the inclusion of other non-beta lactam resistance genes so that CA-MRSA isolates exhibit resistance to only methicillin and erythromycin and are more often susceptible to other non-beta lactam antibiotics (e.g., trimethoprim/sulfamethoxazole (SXT) and clindamycin).
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Affected population | Largely affects older adults and people with weakened immune systems; those who have undergone surgical procedures are at increased risk. | Healthy persons in the general population without established risk factors for MRSA acquisition |
Clinical syndromes | Found at multiple sites, most commonly bloodstream infections, urinary tract infections (UTI) and respiratory tract infections | Predominantly skin and soft tissue infections (SSTIs), such as abscesses, cellulitis, folliculitis and impetigo and a serious form of pneumonia Genes for Panton-Valentine leukocidin (PVL) are associated with SCCmec IV; the clinical spectrum of infections caused by CA-MRSA is directly related to the presence of PVL genes, coding for the production of a cytotoxin that causes tissue necrosis and leukocyte destruction.
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